Bulletin


BULLETIN
of the

ILLINOIS NATURAL HISTORY SURV]
HARLOW B. MILLS, Ckitf

Tularemia, Weather,

and Rabbit Populations

RALPH E. YEATTER
DAVID H. THOMPSON

Print**! br Authority •{ the

STATE OF ILLINOIS
ADLAI B. STEVENSON, Govtrmor

DEPARTMENT OF REGISTRATION AND EDUCATION
C HOBART ENGLE, Dlrtel^r





STATE OF ILLINOIS
Adi.ai E. Stevenson, Governor

DEPARTMENT OF REGISTRATION AND EDUCATION
C. HoBART Enci.e, Director

NATURAL HISTORY SURVEY D I \M S I O N

Harlow H. Mii.is, Chief

Volume 25 BULLETIN Article 6

Tularemia, Weather,

and Rabbit Populations

RALPH E. YEATTER
DAVID H. THOMPSON

Pnntiii hy Autlinrity of tlic State 0/ Illinois

U R B A N A, ILLINOIS

J,n,f 1952



STATE OF ILLINOIS
Adlai E. Stevenson, Governor

DEPARTMENT OF REGISTRATION AND EDUCATION
C. HoBART Engle, Director

A. E. Emerson, Ph.D.
L. H. TiFFAN-Y, Ph.D
Roger Adams, Ph.D

BOARD OF NATURAL RESOURCES AND CONSERVATION
C. HoBART Engle, Chairman

Biology Walter H. Newhouse. Ph.D., Geology
Forestry L. R. Howson, B.S.C.E., C.E.,
D.Sc, Chemistry Engineering

George D. Stoddard. Ph.D., Litt.D., L.H.D., LL.D.. President of the University of Illinois
Delyte W. Morris, Ph.D., President of Southern Illinois University

NATURAL HISTORY SURVEY DIVISION
Urbana, Illinois

SCIENTIFIC AND TECHNICAL STAFF
HARLOW B. MILLS, Ph.D., Chief

Bessie B. East, M.S., Assistant lo the Chief

Section of Economic Entomology

George C. Decker, Ph.D., Entomologist and Head
J. H. Bigger, M.S., Entomologist
L. L. English, Ph.D., Entomologist
C. J. Weinman, Ph.D.. Entomologist
S. C. Chandler, B.S., Associate Entomologist
Willis N. Bruce. M.A., Associate Entomologist
John M. Wright, M.A., Assistant Entomologist
W. H. LuCKMANN, M.S., Assistant Entomologist
H. B. Petty, M.A., Extension Specialist in
Entomology

Paul Suranyi, Ph.D., Laboratory Assistant
Massoud Varzandeh, M.S., Research Assistant
Norman C. Gannon, M.S., Special Research

Assistant
Jeane Warner. Laboratory Assistant
Sue E. Watkins. Technical Assistant

Section of Faunistic Surveys and Insect
Identification

H. H. Ross, Ph.D.. Systematic Entomologist and
Head

Milton W. Sanderson, Ph.D., Associate Taxonomist
Lewis J. Stannard, Jr., M.S.. Assistant Taxonomist
Leonora K. Glo\T). M.S., Laboratory Assistant
Philip W. Smith, M.S., Laboratory Assistant
William R. Richards, B.S., Laboratory Assistant
Vera Twitty, B.S.. Technical Assistant

Section of Publications and Public Relations

James S. Avars. B.S., Technical Editor and Head
Blanche P. Young, B.A.. Assistant Technical Editor
William E. Clark. Assistant Technical Photographer
James W. Curfman, B.S., Technical Assistant

Technical Library

Marguerite Simmons. M.A., M.S.. Technical
Librarian

Ruth Warrick, B.S., Assistant Technical Librarian

Section of Applied Botany and Plant Pathology

Leo R. Tehon, Ph.D.. Botanist and Head
J. Cedric Carter, Ph.D.. Plant Pathologist

J. L. Forsberg, M.S., Associate Plant Pathologist
Ralph W. Ames, Ph.D., Associate Plant Pathologist
Paul F. Hoffman. Jr., Ph.D., Associate Plant Pa-

thologist

G. H. Boewe, M.S., Assistant Plant Pathologist
G. J. Stessel. M.S., Assistant Plant Pathologist
B. M. Zuckerman, M.S., Assistant Plant Pathologist
Robert A. Evers, M.S., Assistant Botanist
E. A. Curl, M.S., Special Research Assistant
RovENiA F. Fitz-Gerald, B.A., Technical Assistant

Aquatic Biologist and

Associate Aquatic

Section of Aquatic Biology

George W. Bennett. Ph.D.,
Head

William C. Starrett, Ph.D.,
Biologist

Donald F. Hansen, Ph.D.. Assistant Aquatic Biologist
R. W. Larimore, Ph.D.. Assistant Aquatic Biologist
Robert D. Crompton. Field Assistant
Leonard Durham. M.S., Technical Assistant*
P. L. McNeil. B.S.. Technical Assistant*

Section of Game Research and Management

T. G. Scott. Ph.D.. Game Specialist and Head
Ralph E. Yeatter. Ph.D., Game Specialist
F. C. Bellrose, B.S., Associate Game Specialist
H. C. Hanson, M.S., Assistant Game Specialist

J. S. Jordan. M.F., Assistant Game Technician
William Nuess, Field Assistant
Lysle R. Pietsch, M.F.. Project Leader^

Section of Forestry

Willet N. Wandell, M.F.. Forester and Head
Lawson B. Culver, B.S., Associate in Forestry Ex-

tension
Ross J. Miller, M.S., Field Ecologist

Charlotte Aschbacher, B.A., Technical Assistant

Consultant in Herpetology: Hobart M. Smith. Ph.D., Associate Professor of Zoology. University of Illinois.
Consultant in Parasitology: Norman D. Levine, Ph.D., Associate Professor of Veterinary Parasitology and

of Veterinary Medicine, University of Illinois.

*Employed by the Illinois Department of Conservation and assigned to the Natural History Survey for ad-
ministrative and technical supervision.

tEmployed by the Illinois Department of Conservation under terms of the Federal Aid in Wildlife Restoration
Act and assigned to the Natural History Survey for administrative and technical supervision.

This paper is a contribution from the Section of Game Research and Management.

(48291—4M—4-52)



PORE W O R

D

IT IS commonly assumed, by people who
are not immediately associated with re-

search, that endeavors in this field ot ac-

tivity can be completed in a short time and

that findings of value will always accrue.

This assumption is often, but not by any

means always, valid. Further, short-term re-

searches almost universally have their base

in painstaking studies which have progressed

over a long period, some time previous to the

initiation of the short-term observations.

Values which result from research studies

are often so closely intertwined with time

that these values do not become significant

until observations have been continued over a

long period. This is the case in the present

important contribution, a study correlating

game populations, weather, and human
health, and utilizing observations covering

about a quarter of a century.

Dr. Thompson was zoologist for the Illi-

nois Natural History Survey from 192.? to

1944 and, since that time, has been Senior

Naturalist for the Forest Preserve District

of Cook County, Illinois. Because of his

earlier training in animal pathology and

statistics, he became interested in tularemia

soon after the first cases were reported in

Illinois. In 19.54 Dr. Yeatter, a specialist

in research and management of upland

game in the Middle West, joined our staff

and teamed up with Dr. Thompson to work
on the relation of human tularemia to

rabbit hunting in Illinois. Dr. Yeatter has

been principally responsible for completing

the project during the past 10 years and

for preparing this manuscript for publica-

tion. These men noticed the relationship

between mild autumn temperatures and the

severe outbreaks of human tularemia which

occurred in Illinois following the opening of

the rabbit-hunting seasons in 19.^8 and 1939.

Later they learned that Dr. Robert G. Green
of Minnesota had already foreseen this re-

lationship from a consideration of the life

history of the ticks which transmit the in-

fection from rabbit to rabbit and from a

study of the course of the disease in the

cottontail.

The following study sliould be of interest

to a great many people.

Harlow B. .Mills, dhirf

Illinois Natural History Survey



V

?i^

"/

/ /

. -v.**!^ *„--«-*^r <<? •

•-^-;'



C ON T E NTS

Acknowledgments .^52

History of Tularemia 352

Tularemia ix Wildlife 353

Tick and Insect Vectors 353

Tularemia in Man 356

Weather and Tularemia 366

Rabbit Populations and Tularemia 370

Tularemia and the Rabbit H unter 374

Recommendations 377

Summary 379

Literature Cited 380



Hunters in an uncultivated gray soil prairie area in southern Illinois. This part of the state,

which has a large cottontail population, has a relatively high human tularemia rate.



Tularemia, Weather,

and Rabbit Populations

RALPH E. Y E A T T E R *

DAVID H. THOMPSON*

TULAREAHA, a disease of rabbits

and hares (lagomorphs) , rodents,

and se\eral other animals, is

transmissible to man. In the period 1026-

1940. Illinois had more than 3,000 re-

ported cases of human tularemia, about

twice as many as any of the other states.

The great majority of these Illinois cases

were traceable to contact with cottontail

rabbits, fig. l.t Because these rabbits are

widely distributed on agricultural lands

throughout Illinois, and because they are

able to maintain themselves under heavy

hunting pressure, they rank among the

chief game animals of the state. At one

time, cottontail rabbits probably made up

about two-thirds of the total game bag

of llliniiis hunters, but, since the tularemia

hazard has become generally recognized,

rabbit hunting has lost some of its former

popularity. Thus, tularemia is of concern

to many thousands of people in Illinois not

only because of its relation to public health,

but also because of its adverse efifect on the

sport of hunting.

This paper deals with the relation of

human tularemia in different parts of the

state and in different \'ears to weather, to

the abundance of rabbits, and to some other

aspects of its epidemiology. In analyzing

the information on tularemia in Illinois,

•Dr. Yeatter holds the title of Game Specialist with

ifie Illinois Natural History Survey. Dr. Thompson.
r several years with the Illinois Natural History Sijrvey,

now Senior Naturalist of the Forest Preserve District

if Cook County, with headquarters at River Forest, Illinois.

tThc Mearns c<itlontjiI rabbit. SylzilnRui floruianiis

r'irarnti (.Mien), is the common rabbit in Illinois. South
1 the Ozarkian Uplift, in extreme southern Illinois, it-

: inge intergradcs with that of the Oklahoma cottonlall

r.ibbit. S. /. alacer (Bangs), according to Hamilton
I 1943). Other Lagomorpha in Illinois are the swatnp
rabbit, SvIi'ilaEUS atquaticui acquaticui (Bachman), dis-

tributed in wooded bottt)mland6 of a few southern counties,

and the white-tailed jack rabbit. Lfpus townsfnd'n cam-
puniui Hollister. which occurs in small numbers in the

northwestern part of the state.

the writers have made an effort to dctcr-

n)ine the methods of management which
would permit Illinois hunters to enjoy the

sport of rabbit bunting without undue risk

of infection. In a report by the writers

(Thompson iSc Yeatter 1941) at the

Seventh Midwest Wildlife Conference,

Des Moines, Iowa, December, 1941, and
in a Natural History Survey release

(.Anonymous 1941), the writers stated

that the human tularemia rate in Illinois

is related to weather at the time of the

opening of the hunting season. The\
added that in years in which the mean date

of the first 10 freezing nights of autumn
occurs before the opening of the rabbit-

hunting season, the tularemia rate among
humans in the state is much lower than in

other years. Green (1935, 1939) had

previously pointed out that in Minnesota

rabbits are free of tularemia during the

cold months of the year and had suggested

delaying the hunting of rabbits until

middle or late October as a way of reduc-

ing the hazard of tularemia in human
beings.

The Illinois records on tularemia and

weather for the decade following the

publication of the writers' first reports

appear to support the conclusion that the

tularemia rate and weather are closely

related. Also, as was pre\iously indicatetl

(Thompson & Yeatter 1941 ), it is evident

that the tularemia rate in Illinois may be

influenced strongly at times by fluctuations

of rabbit populations. The principal con-

clusion resulting from the present study

is that, because of certain characteristics of

the life history of the principal arthropod

vector, the incidence of human tularemia

in Illinois can be lowered significantly by

dela\ ing the opening of the rabbit-hunting

[351]



352 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

Fig. 1.—A Mearns cottontail rabbit. This rabbit is an important game animal throughout

its range from New York to Kansas. In Illinois, it is the reported source of more than 90

per cent of the human tularemia cases.

season until about December 1. Such
delay is urgent, particularlj' in the central

and southern parts of the state.

Acknowledgments

The writers are greatly indebted to the

Illinois Department of Public Health for

detailed records since 1926 on the dis-

tribution of tularemia cases in Illinois

and to the United States Public Health
Service for information on the prevalence

of tularemia in other parts of the country.

They are indebted also to the Wisconsin
Conservation Department for permission

to use its records of the annual kill of

cottontail rabbits in Wisconsin, to the

United States Forest Service for census

data on a southern Illinois area, and to Joe
L. Mote of Watseka, Illinois, for bag
records and field notes.

History of Tularemia

Tularemia was discovered in 1910 by

Dr. George W. McCoy of the United

States Public Health Service in the ground

squirrels of Tulare County, California

(McCoy 1911; Francis 1937). The
disease was named in 1920 by Dr. Edward
Francis (1921, 1937), also of the United

States Public Health Service, after he had
established the identity of the ground
squirrel disease with an outbreak of

"Deerfly fever" in man in Utah. The
essential facts about tularemia have been

worked out entirely by American investi-

gators. The causative organism is a bac-

terium, Pasteurella tularensis (McCoy &
Chapin) Bergey et al. (McCoy & Chapin
1912; Breed, Murray, & Hitchens 1948).

In spite of the short history of tularemia, it

is believed that this disease is not new but



1952 Yeatter & Tho.mi-son ; Tularemia 353

is an old one which was not identified until

comparatively recent years. Since 1925

it has been reported from all states except

\'ermont, as well as from Canada. Alaska,

Mexico, and several countries in Europe,

Asia, and Africa.

Tularemia in Wildlife

Burroughs et al. (1945) have listed 47

kinds of mammals and birds, distributed

chieH\- in North America, Europe, and

.Asia, in whicli natural infection with

tularemia has been shown by laboratory

tests. \'arious groups of lagomorplis and

rodents make up more than three-fourths

of their list. Also included are certain

gallinaceous birds, raptorial birds, and

carnivores, as well as a few domestic

animals.

A few vears ago, wildlife pathologists

of the University of Minnesota and the

United States Bureau of Biological Survey

(now the Fish and AV'ildlife Ser\ice),

co-operating with the Minnesota Depart-

ment of Conservation, studied the relation

of tularemia to the welfare of several wild-

life species in Minnesota. As a result of

these studies. Green (1939) reported that

cottontail rabbits are highl\- susceptible to

tularemia, and that infected cottontails in-

variably die, usual!)' within a week after

the onset of the disease. He found that

snowshoe hares are resistant to the disease

and that they seldom die from it. Jack

rabbits prove rather susceptible when ex-

posed, but, among wild jack rabbits in

Minnesota, tularemia is apparently- rare.

Green iS: Wade (1929) reported a fatal

tularemia infection in the bob-white, Col-

iiiiis virg'uiianus Linnaeus ; Green & Shil-

linger (1932) found the disease in the

ruffed grouse, Boiiasa umbellus togata, and

the sharp-tailed grouse, Pediocetes phasia-

nellus. Green (1939) stated that the

ring-necked pheasant is highly resistant to

infection, but that muskrats "suffer from

a highh- fatal infection." He reported that

a'l kinds of mice, squirrels, and other

small rodents in Minnesota are susceptible,

and suggested that mice may be an im-

portant reservoir of the disease in the wild.

Green & Wade (1928) found that

domestic cats contracted tularemia when
fed the carcasses of infected guinea pigs.

Green (1942) stated that dogs appear to

be almost entirely immune. Waller
(1940), however, reported agglutination
tests positive for tularemia in blood samples
of a dog whicli became sick after feeding

on a diseased rabbit. Downs et al. ( 1946)
were successful in infecting a number of

laboratory dogs by injection of virulent

tularemia organisms, but reported that

dogs were less susceptible to the disease

than most laboratory animals and that they

were often able to localize the infection.

Lillie & Francis (1936) reported as

follows on attempted experimental inocula-

tion of red foxes: "In 1934 four red foxes

(Viilpes fitlva) were obtained. One
(7484) was injected subcutaneously with

infected guinea pig spleen and culture

(Omo strain), and died 13 days later.

He refused to eat during the entire period.

There was a diarrhea during the last 2

days of life. Two other foxes died 2

months after the subcutaneous inoculation

of 7760 and after the first infectious feed-

ing of 7761. Both of these animals had
had noisy labored respiration ; the first

for some 4 weeks, the second for 3 or 4
days before death.

"B. tularense was recovered directly

from the blood of 7484, and inoculation of

various organs of all three reproduced

tularaemia in guinea pigs and cultures \\ere

recovered."

Human beings sometimes become in-

fected with tularemia through being bitten,

scratched, or clawed by dogs, cats, or other

flesh-eating animals that have become con-

taminated with the tissue or blood of in-

fected wildlife.

A considerable number of animal species

in which natural infection has not been

proved have been reported as probable

sources of human tularemia. Francis

(1937) mentioned as suspected carriers

the coyote, red fox, deer, ground hog,

tree squirrel, skunk, sage hen, bull snake,

and hog. Of domestic rabbits, Francis

stated, "Rabbits raised under domestic con-

ditions in rabbitries and hutches, although

highh' susceptible, have not been found

naturally infected, due probably to their

freedom from ticks.
'

Tick and Insect \'ectors

Tularemia is spread among wild animals

largely by the bites of ticks and insects.



354 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

Fig. 2.—Rabbit tick, Hacmaphysalis Icp-

oris-palustris (Packard). Top, male; center,

female
; bottom, female, engorged. The rabbit

tick is the chief agent in the spread of tulare-

mia among rabbits and is an important carrier
of the disease in other kinds of wildlife.

Infection in carnivorous mammals and
birds may, of course, come from eating

diseased prey. Also, it seems probable

that certain aquatic mammals, and possibly

other forms, sometimes contract the disease

from water that is contaminated with
Pasteurella tularensis. For example, the

findings of Jellison et al. (1942) suggest

that water-borne disease organisms were
the source of a tularemia outbreak among
beavers in Montana in 1939 and 1940.

Studies by Parker et al. (1951) indicate

that contamination of streams and mud
with tularemia organisms is widespread in

the northwestern United States, and that

fatal infections of muskrats and beavers

with tularemia probably have occurred

there in recent years.

The rabbit tick, Haemaphysalis leporis-

palustris (Packard), fig. 2, is the chief

arthropod vector of tularemia in the wild.

Francis (1937) stated, "The disease ....
is spread from rabbit to rabbit principally

by the rabbit tick, Haemaphysalis leporis-

palustris, but also by other blood-sucking

arthropods—ticks, lice, and fleas. The
rabbit tick, the rabbit louse, and the rabbit

flea do not bite man, and therefore they

are not a source of human infection."

Green (1942) reported that the rabbit

tick was principally responsible for the

spread of tularemia among wild animals

and birds in Minnesota.

Two other ticks, the American dog
tick, Dermacentor variabilis (Say), fig. 3,

and the Rocky Mountain wood tick, D.
andersoni Stiles, fig. 4, are known vectors

of tularemia. Both are sources of the dis-

ease in man as well as in various species of

wildlife. The American dog tick, D.
variabilis (also called wood tick), is dis-

tributed in the eastern United States, in the

Mississippi River valley, and in some of

the Plains states as well as in western

California, Canada, and Alaska (Coolev

1938). Green (1942) reported that the

most common mode of human tularemia in-

fection in Minnesota appeared, at the time

he wrote, to be the bite of a "wood tick"

(American dog tick) and the second most
common the cleaning of an infected rabbit.

He found that the adult dog ticks that

carry tularemia are those that have be-

come infected during immature stages

while feeding on diseased mice or other

small rodents.



June. 1952 Yeatter & Thompson- : Tularemia 355

^k

"»-*:fi».j(

Fi^. 3.—American liog tick, Dermacenlor
'var'iahWis (Say). Top, male; center, female;

bottom, female, engorged. This tick is widely

distributed in North America; its principal

range is east of the Rocky Mountains. It is

a source of tularemia in humans and probably

in various wild and domestic animals.

Fig. 4.—Rocky Mountain wood tick,

Dermacentor andersoni Stiles. Top, male;

bottom, female. This tick is a source of tula-

remia in humans and probably in domestic

animals and wildlife. It is found in western

parts of the North American continent.

Bell (1945) reported that a natural

barrier, apparently a bactericidal action of

the ticks' guts, keeps infection among dog

ticks at a relatively low level. Green

(1*542) reported a maximum of only 1

infected to about 2,500 uninfected dog

ticks in numerous tick sainples collected

in Minnesota. He noted that dog ticks

in Minnesota go into a resting period in

July and remain dormant until the follow-

ing spring, and that they cause infection

principally in May and June, just previous

to becoming dormant.

The Rocky Mountain wood tick is dis-

tributed in southwestern Canada and in

the western United States from the arid

zone east of the Rocky Mountains \vcst-

ward to central Washington, central Ore-

gon, and eastern California. Both im-

mature and adult stages have been found



356 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

on a wide variety of mammals and birds

(Cooley 1^38).' The adult stage has

been reported as a source of human tula-

remia in several western states. Wood
ticks are believed to have caused a tula-

remia outbreak among range sheep in Mon-
tana (Philip, Jellison, & VVilkins 1935).

Several kinds of insects are known to

be carriers of tularemia. Prince &
McMahon (1946) stated that the disease

has been transmitted under experimental

conditions by the bites of several insects,

including deer flies, Chrysops discalis

Williston, stable flies, Stoinoxys calcitrans

(Linnaeus), bed bugs, Cimex lectularis

Linnaeus, and mosquitoes, Aedes aegypti

(Linnaeus).

Lice and fleas, mentioned as vectors

by Francis (1937), appear to be poten-

tially important in the dissemination of

tularemia in the northern states because

they remain active in winter when most

arthropod vectors are dormant. Ecke

(1948) found that flea populations on

Illinois cottontails were higher in winter

than at other seasons. Evidence apparently

is lacking, however, that either fleas or lice

frequently transmit tularemia. Prince &
McMahon (1946) reported that guinea

pigs used in their transmission experiments

remained negative for tularemia after

exposure during 32 days to infected rat

fleas, Xenopsylla cheopis (Rothschild),

and California ground squirrel fleas,

Diamanus montanus (Baker). They con-

cluded that the two species of fleas tested

do not play an important role in the spread

of the disease. Green (1942) stated,

'Tleas are found on rabbits in southern

Minnesota during the winter; but,

although these insects can transmit tula-

remia, they appear to do so rarely."

Hopla (1951) found that tropical rat

mites, Bdellonyssus bacoti (Hirst), re-

tained tularemia organisms for consider-

able periods after feeding on infected mice.

Normal mice became infected when they
crushed infected mites orally, but not when
they were bitten by the mites.

Tularemia in Man

Francis (1937) stated that wild rabbits

and hares are the source of more than
90 per cent of all human tularemia cases

in the United States. Humans generally

become infected from contact of the bare

hands with the flesh or blood of infected

rabbits or from eating infected rabbit flesh

that is insufiiciently cooked. Usually in-

fection from handling a diseased rabbit

develops at the site of a scratch or puncture

in the skin, but occasionally it develops

in the eye as a result of contact with hands

or of spattering of washings during clean-

ing. There is considerable evidence that

the disease organism also is able to pene-

trate the unbroken human skin.

Francis (1937) described a typical

tularemia case as follows: "About 3 days

after exposure to infection, illness begins

with headache, chilliness, vomiting, aching

pains all over the body, and fever. The
patient thinks that he has the 'flu' and
goes to bed. The sore on the hand devel-

ops into an ulcer. The glands at the elbow
or in the armpit become enlarged, tender,

and painful, and later may develop into

an abscess. There is sweating, loss of

weight, and debility. Illness lasts about

3 weeks and is followed by a slow con-

valescence covering a period of 2 or 3

months. Most patients recover without
any bad after effects, but about 5 percent

die, especially if the case is complicated by

pneumonia." Although there is no evi-

dence of a natural immunity in man,
persons who have recovered from the

disease are permanently immune.
In regard to the diagnosis of the disease,

Francis (1937) wrote: "The history of

tick-bite, fly-bite, or wild rabbit contact

especially, or contact with other animals,

when coupled with fever, an ulcer on the

skin, and regional lymph-node enlarge-

ment, should call attention to tularaemia.

Diagnosis is made conclusive by obtaining

agglutination of Bacterium tularense

[Pasteurella iularensis] by the patient's

serum or by obtaining a culture of the

organism from the patient's ulcer or lymph
nodes following guinea pig inoculation, or

by obtaining a positive skin reaction using

an antigen prepared by Foshay of Cincin-

nati for intradermal injection."

As will be discussed later, treatment of

tularemia in man has been facilitated in re-

cent years by the use of antibiotics.

Distribution in the United States.

—It will be evident from fig. 5, showing

the distribution of 23,921 cases of tulare-

mia reported in the United States during

<



June, 1952 Yeatter & Thompson : Tularemia 357

Fig. 5.—Total numbers of reported human tularemia cases in the various states during the

period 1924-1949. Data from records of the United States Public Health Service.

the period 1924-1949, that the largest

number of cases occurred in states lying

in the central and south central portions

of the eastern half of the country. Jellison

5c Parker (1945), who emphasize the

importance of cottontail rabbits as a source

nf tularemia in humans, pointed out that

Table 1.—States that reported a total of

500 or more cases of tularemia, 1924-1949.*

State



358 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

Fig. 6.—Four general regions of the United States which exhibit distinctive trends and

seasonal distribution of human tularemia.

when tularemia declined in Illinois, Indi- States may be divided conveniently into

ana, and Iowa, there was a moderate in- four major regions, namely, Northern,

crease in the states to the north. Central, Southern, and Western, fig. 6.

With respect to human tularemia rates Trends in the annual tularemia rates dur-

and seasonal distribution, the United ing the period 1935-1949 in these four

28-

24

z
o

20

12

U) 8

-CENTRAL

SOUTHERN-

— -J/*'

. ^NORTHERN

1936 1938 1940 1942 1944 1946 1948

F'fi- 7.—Trends in human tularemia rates during the period 1935-1949 in the four regions
of the United States shown in fig 6. Data on which this graph is based are from records of the
United States Public Health Service.



June, 1952 Yeatter & Thompson : Tularemia 359

NORTHERN CENTRAL SOUTHERN WESTERN

Fig. 8.—Distribution of reported human tularemia cases by quarters of the year during

the period 1939-1949 in the four regions of the United States shown in fig. 6. Data on which
this graph is based are from records of the United States Public Health Service.

regions are shown in fig. 7. The seasonal

tlistribution of tularemia in these regions

is shown in table 2 and hg. 8.

In the Northern and Central regions

the inajorit}' of cases occur during late

autumn and early winter. Rabbits, which

are hunted at that time, cause most of the

cases. As indicated by table 3, furbearers,

squirrels, and upland gaine birds contribute

a few cases. During other seasons, these

regions show moderate numbers of cases

that, presumably, result from bites of ticks

or insects, from chance infections by a

variety of mammals or birds, both wild

and domestic, or from a small amount of

\ear-round hunting.

In the Southern Region, the human
tulareinia rate is fairly uniform through-

out the year, indicating several sources of

infection, the most important of which are

rabbits, ticks, and insect vectors. Francis

(1937) recorded 65 cases of tularemia in

the southern states, chiefly from March
through July, due to contact with dog

ticks, Dermacentor variahilis. Pullen &
Stuart (1945) reported that 176 (92.1

per cent) of 191 tularemia cases for which

tlie apparent source was recorded in Loui-

siana during a 16V-;-year period gave a

history of contact with rabbits. In this

region, year-round hunting probably playf

an important part in transmission of tula-

remia to luiman beings.

Tile human tularemia rate in the Wes;
ern Region is highest in summer. Because

the principal insect and tick vectors in this

region are most active from late spring to

late suminer, it appears that arthropods

may play a relatively greater part in trans-

mitting tularemia to humans there than

in the Northern and Central regions.

Wood ticks, Dermacentor aridersunl, were
reported by Francis (1937) to have caused

more than 50 cases in Montana and sur-

rounding states. According to the United

States Public Health Service (Anonymous
1940), sheep handlers have occasionall\' be-

come infected from contact with wood
ticks or tick feces in the wool. Jellison

(1950) listed, for the western states, 158

cases of tularemia probably caused b\- deer

fly bites, and showed that the distriiiution

of these cases corresponded roughly with

the distribution of the deer fly species,

Chrysops discalis Williston. Jellison et til

(1950) reported contamination with Pax

teurella tularensis organisms of certain

natural waters in Montana and also a

few cases of human tularemia caused by a

contaminated domestic water supply.



360 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

^o



June, 1952 Yeatter & Thompson : Tularemia 361

Jack rabbits, Lepus spp., are the reported

source of a number of cases in the \Vestern

Region. These rabbits also are important

hosts of ticks and probably of biting flies

that carry the disease ( Jellison & Parker

1945). jellison (letter of April 24, 1951,

from Hamilton, Montana) states, "Cot-

tontails ha\e not been as important in this

area as other sources of infection."

Distribution in Illinois.— Records of

the Illinois Department of Public Health

indicate that cottontail rabbits are re-

sponsible for the great bulk of human
tularemia in Illinois. According to Sharp

( 1939) all but 2 of 459 reported cases in

the state in 1938, during a severe outbreak

of the disease, were traceable to contact

with cottontail rabbits. McDaniels (1943)

stated that cottontails cause more than

95 per cent of the human tularemia cases

in Illinois. Other mammals, birds, ticks,

and blood-sucking insects probably serve

as the source of infection for most of the

other cases, as numerous infections have

been traced to them elsewhere in the

United States.

Recent studies by Jackson (1946) in

Indiana and Morgan (1949) in AViscon-

sin, table 3, show somewhat higher per-

centages of cases caused by vectors other

than rabbits reported for these states than

for Illinois. As has already been shown,
the proportion of tularemia cases caused

by the several kinds of known vectors

varies with different sections of the

country.

The mortality rates from human tula-

remia cases reported in Illinois during the

period 1926-1951 averaged 5.7 per cent,

table 4. Considerable variation in the

mortality rates was apparent from year to

year. It is of interest in this regard that

Philip & Davis (1935), Green (1943),

and others isolated several strains of tula-

remia from naturally infected animals.

These strains differed widely in virulence.

Green's work indicated that the virulence

of the disease depended largely on the kind

of animal from which it came. For

example, he found that the virulence of

tularemia from infected grouse was con-

siderably lower, when measured by the

Table 3.—Reported sources of human tularemia in Wisconsin (379 cases in which the

cause was reported) and Indiana (123 cases). Data from Morgan (1949) and Jackson (1946).



362 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

Table 4.—Data on human tularemia re-

ported by the Illinois Department of Public

Health 1926-1951.



June, 1952 Yeatter & Thompson: Tularemia 363

CASES PER

100,000

rn 1.1 - 5.0

I I 5 1-20.0

I } 20.1 -40.0

Figs. 9-12.-Distribmion of human tularemia cases per lOO.nOO population in Illinois for

the years 1936-1939.



364 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

Figs. 13-16.—Distribution of human tularemia cases per 100,000 population in Illinois for
the years 1940-1943.



June, 1952 Yeatter & Thompson : Tularemia 365

0.0- I

Q I.I - 5.0

5.1 -20.0

E23 20.1 -40.0

Fi^s. 17-20.— DislributitMl of human tularemia cases per 100,000 population in Uli:

the years 1944-1947.

for



366 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

CASES PER

100,000

0.0- 10'

d] I.I- 50

^ 5.1 -20.0

E53 20.1 -40.0

Figs. 21-22.—Distribution of human tularemia cases per 100,000 population in Illinois for

the years 1948-1949.

Here, there are scattered wood lots, over-

grown stream banks, hedges, and, during
periods of low agricultural prices, many
fields that are allowed to lie fallow for one

or more years at a time. The second region

is dissected by a number of small streams

draining into the Illinois and Mississippi

rivers. As a result, it has a high percentage

of forested and uncultivated land.

The tularemia rates in the above-named
regions are among the highest in the

United States. The prevalence of tulare-

mia in these regions probably is indirectly

related to the comparatively high rabbit

populations found there. Green, Evans,
& Larson's (1943) study of rabbit tick

populations on snowshoe hares in Minne-
sota showed that when hares were abun-
dant the average number of ticks per hare
was high, but when hares were uncommon
the average number was low. Possibly
both the relatively high rabbit populations
and dense ground cover in extensive areas
of southern Illinois favor the survival of
rabbit ticks, thus contributing to the
spread of tularemia.

Weather and Tularemia

The interrelationships of ticks and tula-

remia to the cottontail and snowshoe hare

in Minnesota were studied in detail for

a number of years by the late Dr. Robert
G. Green and his associates. They found
that rabbit ticks were the principal means
by which tularemia was spread from rabbit

to rabbit in Minnesota and that these

ticks left the rabbits by mid-autumn to go
into hibernation. Green, Evans, & Larson

(1943) reported that most hares and
rabbits were found free of ticks after

November 1, although occasional individ-

uals might carry ticks in December, Jan-
uary, and even in February.

Green (1935, 1939) reported that after

cold weather has caused the ticks to leave

the rabbits, and after an additional week
during which infected cottontails die,

hunting is comparatively safe. He recom-

mended that hunting in Minnesota be de-

layed until middle or late October.

Earlier in this paper it was pointed out

that in 1938 there was a marked decline



r
J

I

1952 Yeatter & Thompson : Tularemia 367

in human tularemia in the northern Great
Lakes states and a y;reat increase in Illinois

and states adjoining it on the east and west.

In Illinois there was a considerable increase

in the number of rabbits between the

autumn of 1937 and the autumn of 1938,

but it did not seem great enough to account

for the eightfold increase in the number
of tularemia cases reported in the 1938

hunting season. Inasmuch as there had

been no shifts in the date of opening of

the rabbit season in an\' of the North
Central states, it occurred to us that these

changes in tularemia rates might be due

to differences in temperatures about the

time of the opening of the season, as well

as to changes in rabbit abundance.

Although the percentage of infected rab-

bits apparently declines sharply throughout

the state soon after November 11 (the

opening date of the rabbit-hunting seasons

in recent years) the peak month for cases

reported to the Illinois Department of

Public Health is December, table 5. It

should be explained here that tularemia

cases are usually listed for the month in

which they are reported and not the month
of infection. Francis (1937) cites 45

Illinois tularemia cases for which the exact

month of onset is accurately recorded, as

follows : October, 1 case ; November, 23

cases; December, 18 cases; and January,

1 case. We suppose that the apparent dis-

crepancy arises from (a) delays between

killing and eating rabbits (cold storage or

freezing does not prevent infection), (b)

incubation period for the infection, (c)

Table 5.—Distribution of Illinois cases of

human tularemia, by months, 1926-1949.

Month



I

368 Illinois Natur.al History Survey Bulletin Vol. 25, Art. 6
,,

correlation than any of several other

methods that were tried.

Illinois, with a north-south length of

almost 400 miles, has striking differences

in climate, and for that reason the game

and fish codes recognize three zones of

about equal area : Northern, Central, and

Southern. For making comparisons of the

mean dates of the first 10 freezing nights,

we chose a representative weather station

located near the center of each of these

zones. The Sycamore, Springfield, and

McLeansboro stations have records cover-

ing long periods of years and are not in

deep valleys or near large bodies of water.

The three zones and the locations of these

weather stations are shown in fig. 23.

When the numbers of days between the

mean or average dates of these first 10

freezing nights and the dates of the open-

ing of the rabbit-hunting seasons are com-

pared with the numbers of cases of human

tularemia for 24 successive years, 1926—

1949, it is evident that there is a general

correspondence in rises and falls between

the two sets of data.

Table 6 shows the mean dates of the

first 10 freezing nights for each zone in

each of 24 autumns beginning in 1926,

when tularemia was first reported in Illi-

nois, as well as the opening dates of the

rabbit-hunting season, the number of days

between these dates in each year (the three

zones averaged), and the numbers of tula-

remia cases in the November-February
periods. The correspondence between the

two curves is most striking in recent years,

when recognition and reporting of the

disease has been general, fig. 24.

For each year of the period 1936-1940,

bracketing Illinois' greatest tularemia out-

break, data are available showing the dis-

tribution of cases by counties. These data

make possible a comparison of average tula-

%

Table 6.—Dates of rabbit-hunting seasons, average or mean dates of first 10 freezing
nights in autumn, and number of human tularemia cases reported in Illinois, 1926-1949. (N, C,
and S indicate respectively the Northern, Central, and Southern hunting zones in Illinois

as shown in fig. 23.)



June, 1952 Yeatter & Thompson'; Tularemia 369

350

150

1944

Fis. 24.—The number of cases of human tularemia per year iii Illinois and the average elate

if the first 10 freezing nights in each autumn, 1929-1949.

remia rates in each of the three zones in

each of those 5 years as they are related to

the number of days between the opening
date of the rabbit-hunting season and the

mean date of the first 10 freezing nights.

This 5-year period includes 1*^38 and 1939,

when there was an epidemic of tularemia

in Illinois, and 1936, 1937, and 1940, when
the tularemia rate was about average. A
summary is given in table 7.

Ecke's (1948) studies of ectoparasites

on cottontails collected in central Illinois in

the autumn of 1947 indicate that migration

of rabbit ticks to hibernating places was at

a peak in late October of that year and that

the great majority of these ticks had left

the rabbits to go into hibernation by the

time of opening of the hunting season in

early November. Temperatures were
slightly above average in October and

Table 7.—Days from opening of rabbit-huntini!, season to averajie or mean date of first 10

freezing nights in autumn, number of hunting-zone-years represented, and tularemia rate in

Illinois. 1936-1940. Figures on weather and human tularemia rate were obtained by averaging
data for each zone separately.

Days From Ope.sixg of
Rabbit-Hinting Season to Mean
Date of First 10 Freezing N'ights

• to 39 days before
I to 29 days before

to 19 days before
I to 9 days before
I to 9 days after

I to 19 days after

N'lmber of
Zone-Vears
Averaged

Tllare.mia Rate,
Cases per 100,000,

November- February

in

14 1



370 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

November of that year. It seems probable

that even though tick hibernation in central

Illinois is usually well advanced by No-
vember 11, the opening date of the rabbit-

hunting season in recent years, it is not

complete. It is apparent that this date is

too early to afford hunters the desired free-

dom from the danger of infection. Hiber-

nation of ticks may be delayed by recur-

rence or persistence of warm weather

;

presumably it occurs somewhat later in

southern Illinois, where the majority of

tularemia cases are found, than in the

central and northern parts of the state.

Portman (1944) reported finding rabbit

ticks feeding on cottontails in the southern

counties of Missouri during the winters of

1943 and 1944. He believed that southern

Missouri is the approximate northern limit

of the rabbit tick's year-round activity.

Since the extreme southern counties of

Illinois are close to and in approximately

the same latitude as the Missouri areas

where Portman did his collecting, it is

passible that a certain amount of year-

round activity of ticks occurs there also.

Although a fair number of Illinois

hunters now delay their rabbit hunting

until cold weather, apparently a great

many still begin to take rabbits as soon

as the season opens, believing that the

animals will soon be thinned out and made
wary by other hunters. The initial rush

greatly emphasizes the importance of the

date of opening of the season because this

is the critical period for tularemia. No
statistics are available in Illinois showing
how much hunting is concentrated in the

early days of the season, but we estimate

that between one-third and one-half of the

year's crop is taken in the first week.
In Ohio, Benjamin (1940) reported the

daily kill of rabbits in the season of 1939
on 180,000 acres of state-supervised hunt-
ing areas scattered through 35 counties. A
quarter of the season's total was killed on
the opening day (November 8), and by
the end of the fourth day (November 11)
more than half had been killed. Inciden-
tally, he found that the average daily kill

per hunter was about twice as great on the
opening day as it was 3 or 4 weeks later.

Burroughs & Dayton (1941) reported
the daily kill of cottontails on 9,000 acres
in Saginaw County, Michigan, in 1938
and 1939. In both seasons combined,

more than a third of the annual kill was
made on the opening day and more than a

half by the end of the first week.

All available evidence indicates that an
early rabbit-hunting season, such as is now
in effect in Illinois, results in the handling
by hunters of approximately half of the

rabbit kill at a time when the possibility

of contracting the disease is still com-
paratively great.

Although it is apparent that temper-

atures just preceding the opening of the

rabbit-hunting season play an important
part in determining the amount of human
tularemia in Illinois, the influence of

weather, as has already been suggested,

is frequently modified by another factor,

the abundance of rabbits.

Rabbit Populations and Tularemia

The tularemia rate for the whole Cen-
tral Region of the United States, fig. 6,

increased from 9.8 cases per million in-

habitants in 1937 to 27.3 cases per million

in 1938 and 28.1 cases per million in 1939.

The sharp rise in the tularemia rate during
the period 1937-1939 occurred simulta-

neously with a marked increase in the num-
bers of cottontail rabbits in the Central
Region, especially in the middle Mississippi

and the lower Missouri river basins. Sub-
sequently, following a general decrease of

rabbits, the tularemia rate declined, and,

by 1942, only 6.1 cases per million were
reported in that region. This apparent
relationship between the prevalence of

tularemia and the high peak of rabbit

populations emphasized the need for

further information on the behavior of

game populations.

Studies of animal populations in recent

years have shown that many kinds of ani-

mals are subject to more or less regular

variations in numbers. The snowshoe or

varying hare, Lepus americanus Erxleben,

is often cited as a classic example of an
animal that undergoes cyclic fluctuations.

MacLulich (1937) concluded from an
analysis of population data, including

records of the Hudson's Bay Company
covering more than a century, that the

cycles of the snowshoe hare in Canada
have varied in length from 8 to 11 years

and have averaged 9.6 years.

Although long-time population records,



1952 Yeatter & Thompson : Tularemia 371

such as have been used in studies of the

snowshoe hare, are lackinjj for cottontail

rabbits, records obtained by workers in

several northern states indicate that cotton-

tail populations tend to fluctuate rather

widely. This paper does not attempt to

arrive at final conclusions as to whether

Illinois cottontail rabbits have cyclic, and

thus possibly predictable, fluctuations.

Nevertheless, it seems advisable to discuss

briefly the a\ailable records of cottontail

population trends in Illinois and nearby

regions in recent years, because of the

apparent relationship of the populations to

the incidence of human tularemia.

Fig. 25 shows the annual kill of cotton-

tails in Wisconsin during the period I'^^l-

1*^49 as computed from hunters' reports to

the Wisconsin Conservation Department

(Anon\mous 1949 and personal communi-
cation from W. E. Scott). Cottontail

rabbits occur throughout W^isconsin except

possibly in a narrow zone along the north-

ern border, but they are generally most

common in the southeastern counties

(Morgan 1949). The cur\e in fig. 25

probably reflects the trend of the total

Wisconsin cottontail rabbit population

during the period l'^.? 1-1 949. The trend

of cottontail populations appears to have

followed rather closely the trends of snow-

shoe hare and grouse populations in Wis-
consin during the same period, as shown
by Grange (1948).

Although the Illinois cottontail rabbit

population data are not so nearly complete

as the data for Wisconsin, records from the

Natural History Survey and other sources

have made possible a partial reconstruction

of the fluctuations of cottontail rabbit pop-

ulations in the state during 25 recent years.

These data include, in addition to field

notes and published records, replies of

spcjrtsmen to questionnaires, estimates of

rabbit populations in southern Illinois by

the United States Forest Service, informa-

tion received annually by the Natural

History Survey on rabbit damage to crops

and orchards, and bag records and field

notes of qualified hunter-observers. Be-

cause these data refer to different regions,

they show something of the local aspects

of population fluctuations in Illinois. As
already indicated, local fluctuations are im-

portant in considering the relation of rabbit

abundance to human tularemia rates.

The most noteworthy feature of the

behavior of Illinois cottontail rabbit popu-
lations during the quarter century was the

sharp increase in numbers during the late

1930's, culminating in 1938 and 1939 in

the highest populations in many decades.

I he increase of rabbits in Illinois was most
evident in the southern half of the state,

where the main endemic centers of tula-

remia are found. As has been mentioned,

nearly 1,000 cases of human tularemia

were reported in Illinois during 1938 and
1939. Unusually high rabbit populations

during these years were reported also in

Iowa, Missouri, Kansas, Arkansas, and
some other central states, where the tula-

remia rate also showed a sharp rise. Dur-

Table 8.—Number of rabbits flushed per
square mile during pre-hunting-season jjame-

bird censuses on the .lasper County, Illinois,

study area, 4 square miles, 1936-1941.

Vear Number of Rabbits
PER Square Mile

1936 4.5
1937 3.8

1938 11 2

1939 14 2

1940 8 5

1941 3.3

Table 9.—Number of rabbits flushed per

square mile durinj^ pre-hunting-season censuses

made by the United States Forest Service on
sample areas of the Shawnee National Forest,

in southern Illinois, 1935-1949.

Year



372 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

ing 1938 and 1939, counts of rabbits killed

on highways in southern Illinois were

approximately six times as high as they

had been in immediately preceding years.

Table 8, showing numbers of rabbits

flushed during game-bird censuses made by

the Illinois Natural History Survey on a

4-square-mile study area in Jasper County

in southeastern Illinois, indicates the trend

of rabbit populations there during the

period 1936-1941.

Table 9 shows estimates of the number

of rabbits per square mile in the Shawnee

National Forest in extreme southern

Illinois during the period 1935-1949;

the estimates are based on counts made by

the United States Forest Service on

sample areas of the different forest types.

No censusing was done during 1942-1944.

The Forest Service data indicate that the

trend of rabbit populations in southern

Illinois was in many respects similar to

the trend in central Illinois, fig. 26, but

some differences are apparent. For

example, rabbit populations were high in

both central and southern Illinois about

1939 and 1945, but, in the central counties,

the population peak of the mid-1940's was
considerably smaller than that of 1939,

fig. 26. Also, rabbit numbers in central

Illinois increased in 1948 and 1949, while

in the Shawnee National Forest they ap-

parently remained low.

Population records for the dark soil

prairie region of east central Illinois cover

V. longer period of years than those for any
other section of the state. Fig. 26 indi-

cates the population trend during the

years 1925-1949. The curve is based on
highway "kill" records in central and east

central Illinois for several years, morning
roadside counts of live rabbits in and near
Champaign County, 1942-1949, and bag
records and field notes since 1925 of Joe L.

Mote of Watseka, Illinois.

Mr. Mote is a veteran hunter and a

competent observer of wildlife. His
rabbit-kill data are particularly valuable

because they cover a longer period than do
any other known Illinois records. For
years when he was unable to hunt, his

notes give information on the abundance
of rabbits. Although his records are from
a single locality in east central Illinois,

the indicated fluctuations of rabbit popula-

tions agree quite closely with those shown

by the records we have from other parts

of central Illinois.

Rabbit populations are usually not ex-

ceptionally high in the dark soil prairie

of central Illinois, and fluctuations are less

pronounced there than in the southern

counties. Nevertheless, it was observed

during 15 recent years that general trends

of rabbit populations in the central counties

approximated those in the gray soil prairie

of south central Illinois, where the cotton-

tail rabbit populations were relatively high

and where human tularemia was most
prevalent.

As fig. 26 indicates, in recent years

rabbit populations in central Illinois have

shown major increases at intervals of about

10 years, with minor fluctuations between.

It is of interest that in recent years popu-

lation peaks of the prairie meadow mouse,

Microtus ochrogaster (Wagner), in cen-

tral Illinois have occurred there at ap-

proximately the same times as those of

cottontail rabbits. Mohr (1947) mentions

meadow-mouse highs in the winters of

1939-40, 1942-43, and 1945-46. Accord-

ing to an annual report by G. C. Oderkirk
of the United States Fish and Wildlife

Service, prairie meadow mice were exceed-

ingly abundant in western and northern

Illinois in the winter of 1946-47. In

the autumn of 1949, members of the

Natural History Survey staff found a

recurrence of high meadow-mouse popu-

lations in the dark soil prairie of central

and north central Illinois. Grange (1949)
states, "Everything points to the fact, then,

that Illinois Cottontails are geared to the

Mouse cycle, and the cycle of Mouse-
Rabbit predators." He does not discuss in

detail the interrelationships of cottontail

and mouse populations.

Field records indicate that, in Illinois,

fluctuations of rabbit populations tend to

be regional rather than state-wide. Differ-

ences in population trends frequently have

been observed to be latitudinal. It may
be noted by comparing figs. 25 and 26 that

major population peaks in Wisconsin have

not occurred during the same years as in

central Illinois. Differences in the trends

of cottontail populations probably account

in large part for the previously mentioned

differences in the trends of the tularemia

rates in the Northern and Central regions

of the United States.



1952 "Veattkr & Thompson; Tui.arkmia 373

2^00-

2,000

m 1,000

500

1932 1934 1938 1940 1942 1944 1946

Fig. 25.—The calculateii kill of cottontail rabbits per year in Wisconsin, 1931-1949.
basic data for this graph are from the Wisconsin Conservation Department.

The

140

J I I I L _L J L J L

« HIGHWAY KILLS

PER 100 MILES

I I I I I L

ROADSIDE COUNTS,

, PER 1,000 MILESy

V
\

A

J I L

30

25

20

1926 1928 1930 1932 1934 1936 1938 1940 1942 1944 1948

Fig. 26.—Cottontail rabbit population trends in central Illinois. 1925-1949, as indicated by

bag records, highway 'kill" counts, and roadside counts. Highway "kill" records for the

years 1930-1932 are from Flint (1934) and for 1937 from Starrett (1938). The high rabbit

populations of 1938 and 1939 were accompanied by a severe outbreak of human tularemia,

fig. 2!. Comparison of fig. 25 with tig. 26 indicates that major population peaks of rabbits did

not oixur in the same vears in Wisconsin and central Illinois.



,?74 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

During periods of abnormally high

rabbit populations, warm weather pre-

ceding the opening of the hunting season

greatly increased the tularemia hazard in

Illinois. In years when rabbit populations

were unusually low, the tularemia rate

remained low even during warm autumns,

for example 1941, 1947, and 1948, fig. 24.

When rabbit populations were average,

the tularemia rate followed quite closely

the curve of the mean date of the first 10

freezing autumn nights. Moderately high

rabbit populations, such as occurred in

the middle and late 1940's and probably

in the early 1930's, brought increases in

the number of human tularemia cases, but

with these increases the disease did not

reach epidemic proportions even during

warm autumns.

Although there is evidence of more or

less regular fluctuations in numbers of

Illinois cottontails, our data do not indicate

that many of these fluctuations are violent.

The great population increase in 1938 and

1939 and the accompanying severe out-

break of human tularemia must be re-

garded as exceptional. Nevertheless,

hunters should bear in mind that a sus-

tained increase resulting in exceptionally

high rabbit populations in their hunting

areas may in turn favor a severe outbreak

of tularemia among the rabbits.

At times, tularemia may be a factor

that contributes to the decline of rabbit

numbers following population peaks.

Local tularemia outbreaks that were the

apparent cause of reduction of cottontail

populations from high to low levels have

been reported bv Waller (1940) and Hen-
drickson (1943) in Iowa, Hicks (1942)
in Ohio, and others. It seems safe to

assume, however, that tularemia is but
one of numerous factors that contribute to

population control among cottontails.

Tularemia and the Rabbit Hunter

Green (1942) has pointed out that

tularemia is in large part a hunter's prob-
lem. This observation seems applicable

to Illinois especiallv. At present, between
400,000 and 500,000 licensed sportsmen
hunt annually in Illinois. In addition,

many thousands of bo5's and men hunt on
their home farms without being required
to purchase hunting licenses. The great

majority of these people confine their

hunting entirely to upland game. Al-

though many of the Illinois hunters now
refuse to bag cottontails for fear of con-

tracting tularemia, most of them depend
largely on rabbit hunting for their sport.

The importance of the cottontail rabbit

as a game species in Illinois will be evident

when it is considered that rabbits can be

found on virtually every square mile of

farm land in the state and in most cities

and towns, and that they are usually

present in huntable numbers through the

open season, even in intensively cultivated

districts. All other resident game species

tend to be local in distribution. Because
cottontails can be found close to home by

most Illinois hunters, which makes hunting

them a relatively inexpensive recreation,

and because they have desirable sporting

and table qualities, these rabbits in recent

years have averaged more than one-half

of the number of game animals taken an-

nually by the hunters of the state.

Cottontail rabbits have usually increased

in numbers readily under wildlife habitat

improvement programs that have been con-

ducted in Illinois. This tendency was
illustrated on the Urbana Township Wild-
life Area by the marked rise of rabbit

populations, noted by the senior author,

that accompanied the growth of hedges

and blocks of shrubs and trees during the

years following planting on the area about

10 years ago. In favorable habitats, rab-

bits usually can be hunted heavily year

after year without permanent reduction of

their population levels (Pirnie 1949). In

fact, heavy hunting may be a desirable

game management measure to reduce

temporarily the population levels of rabbits

and thereby lessen the danger of epizootics.

It seems highly desirable for hunters

in Illinois and other states where rabbits

are important as game to have reliable in-

formation on how best to avoid contracting

tularemia while hunting. The medical

profession has recently made marked prog-

ress in successfully combating the disease

in man. Wildlife research workers and
game administrators in many states un-

doubtedly can contribute materially to the

solution of the tularemia problem by deter-

mining hunting practices and hunting

seasons likely to afford the greatest pro-

tection to hunters.



June, 1952 Yeatter & Thompson : Tularemia 375

Green (1942) observed that tularemia

in rabbits and hares in Minnesota was
cliiefly a spring and fall disease. He found
infection to be relatively uncommon in

tiiese animals during the summer months
and veri, rare during the winter. On the

basis of the observed decline of tularemia

in winter, he had previously ( 1935, 1939)
urged dela\" of rabbit liunting until after

the onset of cold weather.

In describing tularemia in rabbits, Green
( 1942) stated, "Tularemia is a bacterial

disease ; that is, it is caused by a microbe

which can be seen under an ordinary micro-

scope. The germ can be grown in the

laborator)' and is easily recognizxd by a

trained bacteriologist. When this germ
infects an animal, particularly a rabbit,

it grows in the blood stream and in all the

internal organs of the animal and produces

the disease which we call tularemia.

"\Vhen an infected rabbit is encountered

in the woods, its reactions are usually slow

and the animal appears somewhat tame.

Regardless of tularemia, it is alwa\s best

to take onlv rabbits or other wild animals

that show \igor and can be taken only with
difficulty.

"
1 he typical infection as found in a

rabbit can be recognized b)' a peppering
of tiny white spots on the liver and spleen.

The white spots are abscesses caused by
growth of the germ in those organs. The
infection is of relati\ely short duration in

rabbits and hares, usually lasting about

seven days. Cottontail rabbits arc very

susceptible and always die; snowshoe hares

are comparatively resistant and usually

recover.

"On cleaning a rabbit, if the reddish-

brown liver or spleen is seen to be peppered

with fine white spots, the carcass should be

burned, one's hands should be repeatedly

washed with soap and water and finally,

if possible, with a disinfectant. Any cuts

on the hands should be treated with

iodine."

Inasmuch as other conditions than tula-

remia may cause spotting of the liver or

other internal organs, figs. 27 and 28, this

symptom is not conclusive e\idence of the

Fig. 27.—Lesions of tularemia on liver and spleen of a domestic raliliit, followiriK experi-

mental infection. The small white spots become visible on the liver and spleen about the third

day of illness. Photo by Nick Kramis, photographer, Rocky Mountain Laboratory, United States

Public Health Service.



376 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

disease in rabbits. Fig. 27 illustrates the

characteristic appearance of the liver and

spleen of a rabbit infected with tularemia.

Tapeworm cysts are found rather fre-

quently in the muscles or attached to in-

ternal organs of Illinois cottontail rabbits.

These cysts are not related to tularemia

and are harmless unless eaten raw, but

it is better to discard an affected rabbit

rather than to try to cut out the cysts.

It should be borne in mind that rabbits

and other game may not exhibit visible

symptoms of tularemia in early stages of

the disease. Francis (1937) stated of

infected rabbits that "innumerable small

round spots [on the liver] become plainly

visible on the third or fourth day of illness,

but these spots are too small to be seen on

the first and second days of illness. There-

fore, if a rabbit is shot on the first or second

day of illness the liver, though diseased,

will appear healthy." Because it is possible

for infection to be present in apparently

healthy animals, thorough cooking of all

game is always a wise precaution. Tula-

remia contracted by humans as a result of

eating infected game that is insufficiently

cooked appears to be especially severe and

is associated with a high death rate. Cook-
ing of rabbits and other game until the

bone marrow and the meat surrounding

the bones are well done eliminates any

danger of tularemia infection.

A number of organizations have advised

hunters to wear rubber gloves when han-

dling cottontails. Yeatter & Thompson
(1943) have pointed out that this pre-

caution, although probably offering a de-

gree of protection, is not entirely practical

and may give a false sense of security to

hunters during tularemia epidemics. The
hunter usually is bare handed when han-

dling the gun, and during a day of hunting

receives scratches from briers or barbed

wire, or becomes chafed on the hands or

wrists. In addition, as has been mentioned,

considerable evidence indicates that in-

fection can take place through the un-

broken skin.

Although some hunters in recent years

have put on cotton or leather gloves before

picking up rabbits, they are probably in

the minority. Every shot rabbit is smeared

with blood, and, whether the animal is

Fig. 28.—Lesions of tularemia on lungs of a domestic rabbit, following experimental in-

fection. Photo by Nick Kramis, photographer, Rocky Mountain Laboratory, United States

Public Health Service.



June, 1952 Yeatter & Thompson: Tularemia .^77

carried in the hand or placed in a hunting
coat, it may come in contact with the skin.

Rubber gloves are not commonly used by

hunters in the field, and, e\en though an

attempt is made to use them while cleaning

rabbits, there are several points of danger.

The gloves may not be of sufficiently good

quality, but, more important, the average

hunter probably does not possess the train-

ing and skill required to use the gloves

without puncturing them or to remove and
sterilize them without contaminating the

hands or face. Many laboratory techni-

cians and medical workers have contracted

tularemia even when all available facilities

and known protective measures have been

employed. Therefore, it is unlikely that

rubber gloves alone offer the untrained

la\man much protection.

Although it would obviously be unwise

to minimize tularemia as a hunting hazard,

it should be pointed out that publicity given

the disease by newspapers and radio has

tended to instill in the public an exag-

gerated fear of becoming infected by han-

dling wild rabbits during nonepidemic

years or during late fall or winter months.

The writers (Yeatter ik Thompson 1^43)

have stated in this regard, "Tularemia is

a serious disease and it is not our intention

to underrate it, but it has been mostly a

mental hazard since the peak years. The
danger from car accidents and gun acci-

dents, which is considerably greater, does

not keep people from hunting. For ex-

ample, in 1942, Illinois had 67 cases and

one death from tularemia. In the same
\ear Wisconsin, with almost the same
number of licensed hunters had 109 gun
accidents and 36 deaths. Comparable
figures are not available for Illinois, but

they are probably about the same as those

for Wisconsin because 80 of the accidents

and 24 of the deaths came during the small

game season."

Hicks (1942), in recommending that

the rabbit-hunting season in Ohio be de-

la\ed until December, pointed out that

the delay in opening of the season would
minimize the frequency of human tulare-

mia and eliminate much of the psychologi-

cal hazard which then detracted greatly

from the pleasures of hunting in Ohio.

Hunters are not likely to follow elab-

orate precautions for handling rabbits skill-

fulh enough to protect themselves from

infection with tularemia. The writers be-

lieve that Illinois hunters can most effec-

tively reduce the tularemia hazard to a
minimum by delaying rabbit hunt.ng un.il

after the onset of siiarp, freezing weather.
This delay is urged especially for hunters
in the Central and Southern zones of the

state, where rabbit ticks presumably are

active later in the fall than in the Northern
Zone. It should be remembered that

periods of exceptionalh' high rabbit popu-
lations, such as occurred in Illinois in 1''38

and U'39, increase the danger of infection

and call for special measures on the part

of hunters.

Within the past decade, methods of

treating tularemia in humans have been

improved greatly, principally through the

use of antibiotics. The United States Fish

and Wildlife Service Wildlife LcaHet 271

(Anonymous 1948) stated in regard to

the use of streptom\cin in treating tulare-

mia, "Many remarkable recoveries with

few failures .... have been recorded

following the use of this agent." Recently

aureomycin and chloromycin ha\e largely

supplanted streptomycin in the treatment

of tularemia. Probably the 'ow death rate

in Illinois in recent years, table 4, reflects

the increasing use of antibiotics in treat-

.nent iif the disease.

Recommendations

Detailed analysis of weather and tula-

remia records indicates that delating tlie

opening of the rabbit season until Decem-
ber is the most practical method for re-

ducing tularemia in Illinois. Recommen-
dation is therefore made that the opening

date be changed to December 1 or, pref-

erably, December 8, at least in the Cen-

tral and Southern zones of the state. The
opening date in the Northern Zone might,

without serious hazard in most years, con-

tinue to be November 11. The fains to

be expected from standpoints of public

health and enjoyment of hunting seem to

far outweigh anv disadvantages that

might arise from this proposed change in

the game code. To compensate for the

dela\' of the opening date, the rabbit season

could be extended to Januarv 30.

Because of its adaptability to farm

habitats throughout Illinois, the cotton-

tail rabbit is undoubtedly our most impor-



378 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

Fig. 29.

—

Illinois Natural History Survey entomologist examining wild cottontail rabbit

for external parasites.

tant game animal. There are, however,

a number of questions that need to be

answered before the best use of this game
animal can be made. Studies of population

fluctuations should be continued, and rec-

ords of long-time censuses on areas that are

representative of the different sections of

the state should be analyzed to give better

methods for predicting population trends

and possible disease outbreaks.

Studies should be made to supplement
the investigations begun bj' Ecke (1948)
for the Natural History Survey—studies

involving the degree of infestation of rab-

bits by ticks and other tularemia vectors,

fig. 29, the per cent of diseased animals,

numbers of embryos per female, numbers
of broods of young per female, weights.

food, and other phases of the life cycle.

The relation of tularemia and its vectors

to rabbits, and to other wild and domestic

animals, should be further examined.

It seems evident from tularemia studies

already completed that much progress in

controlling the disease can be made by
enacting legislation for confining the

hunting season to the colder months of

the year, by educating hunters to take

adequate safeguards, and by warning the

public of any significant increases in the

tularemia hazard. In event that exceptional

conditions arise, such as those in 1938 and
1939, it is suggested that the Illinois De-
partment of Public Health, the Illinois

Department of Conservation, and the Illi-

nois Natural History Survey, working



I
June. 1952 Yeatter & Thompson: Tularemia 379

jointly, bring such an emergency to tlie

attention of the go\ernor of the state so

that he may by proclamation defer the

opening of the rabbit-hunting season until

after the principal danger has passed.

Summary

1. An analysis of the human tularemia

records of the United States Department
of Public Health indicates that, on the

basis of tularemia rates and seasonal in-

cidence of the disease, the United States

falls into four general regions, namely.

Northern, Central, Southern, and \\^est-

ern.

2. In a 24-year period beginning in

1926, Illinois had about twice as many
reported human tularemia cases as any

other state. The great majority of these

reported cases was among residents of the

southern half of the state.

3. The high incidence of tularemia

among hunters, their families, and others

in southern Illinois appears to be related

to the greater abundance of rabbits in that

region.

4. The human tularemia rate in any
year in Illinois seems to he determined both
by temperatures about the time of the

opening of the rabbit season and by the

abundance of rabbits. Very high rabbit

populations increase the tularemia hazard.

5. When rabbit populations were aver-

age, the human tularemia rates followed

the curve of the mean or average date of

the first 10 freezing autumn nights. W'lien

rabbit populations were low, the tularemia

rates remained low, even during warm
autumns. The most serious outbreak of

human tularemia known in the past half

century was in 1938 and 1939 when rabbit

populations were exceedingly high and the

autumns warm until well past the opening

date of the season.

6. Delay in the opening of the rabbit-

hunting season until December 1, partic-

ularly in the Central and Southern zones

of Illinois, is reconunended as the most

practical management method for reducing

the amount of human tularemia. The
Northern Zone may, without serious haz-

ard in most years, continue to open on

November 1 1.



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1945. The infection of ticks (Dermacentoy variahilis) with Pasicurelta tularensis. Jour.

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1940. State supervised hunting in Ohio in 1939. Ohio Wildlife Res. Sta. Release 136. 19 pp.

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Burroughs, A. L., R. Holdenried, D. S. Longanecker, and K. F. Meyer
1945. A field study of latent tularemia in rodents with a list of all known naturally

infected vertebrates. Jour. Infect. Dis. 76: 115-9.

Burroughs, R. D., and Laurence Dayton

1941. Hunting records for the Prairie Farm, Saginaw County, Michigan, 1937-1939.

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Case, H. C. M., and K. H. Myers
1934. Types of farming in Illinois: an analysis of differences by areas. 111. Ag. Expt.

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Cooley, R. A.
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Downs, Cora M., Lewis L. Coriell, Gifford B. Pinchot, Edward Maumenee, Alice Klauber,

S. S. Chapman, and Barbara Owen
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tularemia.) Jour. Immunol. 56(3) :2I7-28.

Ecke, Dean Hobert
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1934. The automobile and prairie wild life. III. Nat. Hist. Surv. Biol. Notes 3. 7 pp.

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1935. The periodic disappearance of game birds. Minn. Cons. no. 29:2-3, 19.

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r iQ[\ 1



June, 1952 Yeatter & Thompson : Tui.ARKMiA 381

Green, R. G., C. A. Evans, and C. L. Larson

1943. A ten-year population study of the rabbit tick llarmaphysiilis leporis-palustris. Am.
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Hamilton, William J., Jr.

1943. The mammals of eastern Tnited States. Vol. 11. Comstork Publishing Company,
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Hendrlckson, George O.

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Hicks, Lawrence E.

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)

Hopla. Cluff E.

1951. Experimental transmission of tularemia by the tropical rat mite. .\m. Jour. Trop.

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Jackson, James W.
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Jellison. William L.

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1942. Epizootic tularemia in the beaver, Castor lanadensis. anil the contamination of

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McDaniels, Herbert E.

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Morgan, Banner Bill

1949. Tularemia in Wisconsin. Wis. /Xcad. Sci., Arts, Letters Trans. 39:1-19.

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in beavers and muskrats in the northwestern United Stat-s. U. S. Pub. Health Serv.

Natl. Inst. Health Bui. 193. 61 pp.



382 Illinois Natural History Survey Bulletin Vol. 25, Art. 6

Philip, Cornelius B.. and Gordon E. Davis

1935. Tularemia. Observations on a strain of low initial virulence from rabbit ticks.

V. S. Pub. Health Serv. Reps. 50(28) : 909-11.

Philip, Cornelius B., Wm. L. Jellison, and H. F. Wilkins

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