• FINE FOCUS, VOL. 3 (2)

PREDICTIONS OF 
FUTURE GEOGRAPHICAL 
DISTRIBUTION OF TWO 
VECTORS OF AMERICAN 
TRYPANOSOMIASIS: 
IMPLICATIONS FOR 
ENDEMIC CHAGAS 
DISEASE IN TEXAS, USA
HELEN G. SCOTT AND CATHERINE A. WAKEMAN* 
DEPARTMENT OF BIOLOGICAL SCIENCES, TEXAS TECH 
UNIVERSITY, LUBBOCK, TX

Copyright 2017, Fine Focus. All rights reserved.

AMERICAN TRYPANOSOMIASIS • 129

MANUSCRIPT RECEIVED 30 MARCH 2017; 
ACCEPTED 09 JUNE 2017

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It is known that climate has a direct effect on vector-
borne and zoonotic diseases, and in the face of climate 
change, understanding this link has become more 
urgent. Many such vector-borne diseases primarily 
afflict impoverished populations and have therefore 
been previously understudied. One major focus of our 
research is to understand the influence that climate has 
on the distribution of disease causing microorganisms 
and their vectors, especially those in relation to American 
trypanosomiasis (Chagas disease). Chagas disease is caused 
by the hemoflagellate protozoan parasite, Trypanosoma 
cruzi. For this study, we hypothesized that the increasing 
prevalence Chagas in the state of Texas is due to 
expanding distributions of vectors. To test this hypothesis, 
historical data on vector distribution and climate was 
used to determine the probable locations of prevalent 
vectors in Texas. Predictions for the future distributions 
were made using environmental niche models for 
bioclimatic variables with a maximum entropy algorithm. 
Of the two Triatominae species studied, the range and 
concentration of both decreased under a global warming 
scenario, a finding that is consistent with the current 
research of risk of Chagas disease in Venezuela. In future, 
this same procedure will be used on more Chagas vectors 
to better understand if there is a northward shift for 
vectors, or if Texas is becoming more inhospitable to all 
vectors of Chagas.

ABSTRACT

CORRESPONDING 
AUTHOR

Catherine A. Wakeman 

Department of Biological 

Sciences, Texas Tech University, 

2901 Main

Lubbock, TX 79409-3131. 

helen.scott@ttu.edu

KEYWORDS
• T. cruzi
• Chagas disease
• Triatomine
• Climate Change
• Vector

It has long been understood that the Earth’s climate 
has wide ranging impacts on human health (9). One 
such impact that has been previously understudied is 
the effect climate has on microbial diseases. Changes 
in temperature and rainfall, as we are currently 
seeing in anthropogenic climate change, may affect 
the distribution and abundance of disease vectors (9). 

INTRODUCTION

These vectors may carry microbial agents 
of disease, such as viruses (causative agents 
of diseases such as Chikungunya, Dengue, 
and West Nile Virus), bacteria (causative 
agents of diseases such as Lyme Disease), 
and protozoans (causative agents of diseases 
such as Leishmaniasis) (7).  The focus of this 
study is the vector-borne disease American 
trypanosomiasis, or Chagas disease.  

Chagas disease is a tropical infectious 
disease that is estimated to have affected as 
many as 18 million people worldwide, with 
more conservative estimates placing the 
number in the 8-10 million range (18, 12, 13, 7, 
20). The majority of human infections occur 
in Mexico and Central and South America 
(1, 18). The variable estimates of disease 
prevalence are caused by underreporting 
of infection, often due to the fact that many 
people do not know they are infected and 
do not seek treatment (12, 5). This is, in part, 
due to the dual phased nature of the disease 
(1). The acute and the chronic phase are often 
separated by several decades (18). The first 
phase (the acute phase) may last anywhere 
from a few weeks to several months after 
initial infection (12, 18). The acute phase is 
asymptomatic or characterized by mild flu-like 
symptoms not unique to Chagas, including 
fever, fatigue, body aches, headache, rash, loss 
of appetite, diarrhea, and vomiting (12, 18). 
The chronic phase, which develops in 30% 
of all people infected, may exhibit cardiac or 
gastrointestinal (GI) complications and can be 
life threatening (12, 18). Common cardiac signs 
include cardiomyopathy, heart failure, altered 
heart rhythm, and cardiac arrest/sudden death 
(12, 18). Less commonly, chronic GI symptoms 
include enlarged esophagus (megaesophagus) 
and enlarged colon (megacolon), both of which 
can lead to difficulties with eating or passing 
food (18). While there are cures for Chagas, 
they are most effective in the acute phase (18). 
Symptom management and treatments in the 

chronic phase may inhibit the progression of 
cardiomyopathy and lower the fatality rate, but 
the evidence for their efficacy is weak (18).

Chagas disease is caused by the 
hemoflagellate protozoan parasite 
Trypanosoma cruzi (12, 20). Infection may 
occur via organ transplants and blood 
transfusions from infected donors, and the 
consumption of uncooked food contaminated 
with feces from infected bugs and congenital 
transmission (12, 22). However, the most 
common route is vector transmission (12). T. 
cruzi lives in the gut of a variety of insects 
in the trypomastigote form, and is shed in 
the feces of the insect vector (4, 2, 4, 23). The 
parasite is then able to enter the bloodstream of 
a host via the wound left by the insect vector’s 
bite. Once in the host cells, T. cruzi transforms 
from a trypomastigote, a mature flagellate form 
of the trypanosome, to an amastigote, when 
no external cilia or flagella are present, and 
multiplies within the tissue (4). Progeny are 
released as trypomastigotes into the blood, to 
spread the infection to other tissues within the 
current host (4). While trypomastigotes are in 
the blood the host can serve as a reservoir, and 
any uninfected insect vectors that take a blood 
meal from this host may become infected, thus 
perpetuating infection to other organisms (4). 
The insect vectors, all from the Triatominae 
family, are commonly called kissing bugs, 
due to their tendency to bite around the 
lips and eyes (1, 22, 7). In the endemic areas 
of the disease range, approximately 50% 
of permissive vectors (vectors capable 
of transmitting disease to humans) carry 
detectable burdens of T. cruzi (5, 20). The most 
common species that transmit Chagas are 
Triatoma sanguisuga, Triatoma gerstaeckeri, 
Triatoma protracta, and Rhodinus prolixus (5, 
7, 20). T. gerstaeckeri and T. sanguisuga were 
chosen to be the focus of the study as they are 
among the most common Chagas vectors in 
Texas, and are commonly linked with human 

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and canine T. cruzi infection. T. gerstaeckeri has 
the highest T. cruzi infection rate in Texas, with 
58% of all  T. gerstaeckei individuals testing 
positive, making it a clinically relevant vector 
(5, 7).

Historically, endemic Chagas disease has 
been limited to rural areas of Mexico, Central 
America, and South America (1, 18). This 
range has been maintained due to the suitable 
environment for the reservoirs, Triatominae 

vectors, and the T. cruzi parasite (12, 20). 
Relevant reservoirs in Texas include dogs, 
feral swine, woodrats, and armadillos (12). 
Ideal climate parameters for the transmission 
of the parasite are tropical conditions, with 
mild winters, high humidity, and warm 
nights (1).  With changing climates and global 
warming, there has been a recent shift in 
the distribution of vectors, including those 
transmitting diseases such as malaria, lyme 
disease, leishmaniasis, as well as Chagas disease 

(1, 6, 20). 

The southern United States are already 
suitable habitats for the kissing bug vectors, 
as seen in recent autochthonous cases (cases 
acquired within the United States) of Chagas 
disease (12). As the disease is underdiagnosed 
and unreported, estimates of the true 
prevalence of Chagas in the southern United 
States vary wildly. In Texas alone, estimates 
of infection range from 4 to 267,000 total 
locally acquired cases (1,15). The large range of 
suspected cases exists because Chagas is often 
underdiagnosed and untreated. The two phase 
course of the disease, nonspecific symptoms, 
and possibility of totally asymptomatic 
infection makes diagnosis difficult (12). 
Surveillance of donated blood supply in Texas 
has highlighted that there are a number of 

Chagas disease cases that were never treated. In 
Texas, 0.01% of donated blood tested positive 
for T. cruzi. Texas leads the United States in 
autochthonous cases, reported mainly in the 
southern regions of the state (12). The first 
autochthonous case of Chagas disease in the 
United States was in southern Texas in 1955, 
and since then the number of locally acquired 
cases have continued to grow. However, most 
studies use seroprevalence, which does not 
differentiate between locally acquired cases 
and those that were contracted in traditionally 
endemic countries. No recent publications 
specifically assessed locally acquired infection 
in a larger setting than individual case reports. 
While human data is far from complete, 
the use of sentinel species, including dogs, 
also shows an increasing number of cases 
over time since the mid 20th century. This 

Figure 2. Maxent model of distribution of T. 
gerstaeckeri in A) Present Day, B) 2050, C), 2070, 
white dots are training locations, warmer colors 
indicate higher ecological suitability for vectors.

AMERICAN TRYPANOSOMIASIS • 133

Figure 1. Maxent model for 
distribution under current 
conditions (average of 1960 
to present). White dots are 
training locations, warmer colors 
indicate higher risk of vectors 
being present for A) Triatoma 
gerstaeckeri and B) Triatoma 
sanguisuga

A A B

C

B

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trend is expected to continue, with more 
and more cases occurring in the U.S. with 
the highest risk in Texas (14). The main goal 
of this project is to assess the impact climate 
change is projected to have on bioclimatic 
variables in the years 2050 and 2070, and if 
this will change the ecological suitability of 
Texas for T. gerstaeckeri and T. sanguisuga. To 
explain the increase in locally acquired Chagas 
disease cases, we predicted that the ranges 
and concentrations of our species of interest 
would increase. However, our results do not 
agree with this prediction, indicating that the 
future Chagas cases in Texas are more likely to 
be caused by species that are coming to Texas 
from Latin and South America in response to a 
changing climate worldwide.

METHODS

DATA

Data of the occurrence and distribution 
of specific triatomine vectors was assembled 
from previous collections dating from present 
day to 1960. Records of previously collected, 
identified, and T. cruzi tested specimens 
were obtained from museum collections 
(Texas A&M University, College Station; 
and University of Texas Brackenridge Field 
Laboratory, Austin), and published peer-
review journal articles (2, 5, 6, 7, 10, 11, 17, 
20, 23, 14). Only collections post-1960 were 
used because the WorldClim (Version 1.4, 
http://www.worldclim.org/) data used to in 
the environmental layers of analysis was an 
average of information since that year. 

MODEL CONSTRUCTION

A maximum entropy algorithm was 
used to construct distribution models for 

each of the species of interest. The collected 
Triatominae occurrence points and bioclimatic 
environmental layers were used with Maxent 
software (version 3.3.3k) for a historical average 
of species distributions across Texas. Maxent 
was used as it is standard in the literature 
for use in constructing species distributions 
from a large number of collected specimens, 
a presence-only record (10). Bioclimatic 
parameters were obtained for the state of 
Texas from WorldClim and are as follows: 
annual mean temperature, mean diurnal range, 
isothermality, maximum temperature of 
the warmest month, minimum temperature 
of the coldest month, temperature annual 
range, annual precipitation, precipitation of 
the wettest month, precipitation of the driest 
month, precipitation of the wettest quarter, 
precipitation of the driest quarter, precipitation 
of the warmest quarter, precipitation of the 
coldest quarter. The technical specifications 
of the Maxent run include, a convergence 
threshold of 1.0 x 10-5, and use of the threshold 
and hinge features, without duplicates. 

FUTURE MODELS

Bioclimatic predictions under the AIM 6.0 
pathway, RCP6.0, from the version 1.1 of the 
Beijing Climate Center Climate System Model 
(BCC_CSM1.1) GCM were retrieved in a 30 
second spatial resolution from WorldClim.

RESULTS
Principle component analysis indicated 

that of 19 bioclimatic variables, the first two 
principal components (minimum temperature 
of the coldest month, and annual mean 
diurnal temperature for T. gerstaeckeri, 
and  precipitation seasonality, followed by 
precipitation of the coldest quarter for T. 
sanguisuga) account for 44.9% and 47.2% of 

the explained variance for T. gerstaeckeri and T. 
sanguisuga, respectively. 

The most important bioclimatic factors 
for each species (after application of the 
Jackknife procedure) differed between the 
two at all time points. For T. gerstaeckeri, the 
most important variable was the minimum 
temperature of the coldest month, followed 
by annual mean diurnal temperature, annual 
precipitation, and precipitation seasonality. For 
T. sanguisuga, the most important predictor 
variable was the precipitation seasonality, 
followed by precipitation of the coldest 
quarter, and minimum temperature of the 
coldest month. In 2050, the most important 
variable for T. gerstaeckeri changed to annual 
temperature range. Annual temperature range 

remained as the most important variable for T. 
gerstaeckeri in 2070.

For T. gerstaeckeri, estimates of suitability 
across the state ranged from 0.00 to 0.92 for 
the current conditions, between 0.00 and 
1.00 for 2050, and between 0.00 and 0.77 for 
2070. For T. sanguisuga, suitability ranged 
from 0.00 to 0.85  for the current conditions, 
between 0.00 and 0.75 for 2050 and between 
0.00 and 0.68 for 2070. These both show an 
overall decrease in suitability. Areas of current 
high suitability for T. gerstaeckeri, such as in 
South Texas along the USA-Mexico border, 
saw a decrease in suitability by 85% by 2070. 
Whereas current areas at medium risk saw a 
dramatically localized increase of suitability, 
such as the 46% increase in the Houston and 
Galveston area, near Trinity bay along the 

AMERICAN TRYPANOSOMIASIS •135

Figure 3. Maxent model of distribution of T. 
sanguisuga in A) Present Day, B) 2050, C), 2070, 
white dots are training locations, warmer colors 
indicate higher ecological suitability for vectors.

A B

C

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Gulf coast. Areas of current high suitability 
for T. sanguisuga decreased by 33% by 2070 
in central Texas where the highest suitability 
is currently. No areas saw an increase in 
suitability for T. sanguisuga.  Additionally, both 
species show unchanged low suitability in the 
Texas panhandle in the northwest part of the 
state.

DISCUSSION
The current distribution of vectors of 

Chagas disease is based on bioclimatic 
variables.

Of the bioclimatic variables studied to 
determine vector distribution, as listed above, 
different variables were more important 
to some species of vector than to others. 
Triatoma gerstaeckeri’s niche was most heavily 
influenced by the minimum temperature 
of the coldest month, indicating a threshold 
temperature, below which the vectors die, 
most likely while in egg or nymphal stages. 
The niche of Triatoma sanguisuga was 
determined by the precipitation variation 
throughout the year, favoring areas with 
constant and dependable rainfall. This 
requirement results in a distribution more 
toward the gulf coast region of Texas and 
the east of the state, which tend to have more 
stable precipitation levels than the west and 
immediately along the coast. (Figure 1A and 1B).

Presence of T. gerstaeckeri and T. sanguisuga 
will decrease and shift in response to climate 
change across the state of Texas. 

Under the RCP6.0 climate pathway, “Aim 
6.0”, the densities of both studied vectors 
will decrease and the distribution will shift 
towards the east. By 2050, T. gerstaeckeri 
will have a highly restricted range with an 
epicenter in central Texas, as well as a large 
density on the gulf coast near Houston. Both 
trends are continued in 2070 (Figure 2). In 

2070, distribution of T. gerstaeckeri is restricted 
further, with the exception of a large increase 
of ecological suitability in the southern region, 
which may be caused by the presence of 
microclimates along rivers in the area or that 
the rural area is suited for the vectors than 
further north in more populous areas (Figure 
2). The trends observed for T. gerstaeckeri 
contrasts somewhat with T. sanguisuga which 
will have a much broader range in 2050 and 
2070, comparable to its present day range, 
but with a much lower concentration (Figure 
3). The distribution for T. sanguisuga will be 
centered in central and east Texas (Figure 3). 
The presence of both vectors decreases along 
the Texas-Mexico border and southernmost 
regions of the state. This indicates that there is 
also a maximum suitable temperature being 
exceeded in these climatic scenarios. The 
consensus in the field is that Chagas disease 
rates will increase in America, with Texas 
being the first state affected. The finding of 
decreased vector range and density was thus 
very surprising and did not agree with our 
hypothesis that vector ranges will increase. 
However, the vectors studied do not represent 
all Chagas competent vectors. And while the 
species studied have been major sources of 
concern in Texas in the past, they may not 
be major vectors for disease in the future (14, 
20). Both of these species have been endemic 
to Texas for many years, and are thus likely 
highly adapted to its current climate (10.1). 
Future Chagas infections may be caused 
more by invading species coming from Latin 
America as the climate warms and becomes 
too hot to be suitable, and find refuge in Texas 
with a warmer climate to match that of the 
previously inhabited endemic areas. Therefore, 
for a better understanding of risk of infection, 
further research would open up this study to 
newer triatomine species to Texas, including: 
T. indictiva, T. lecticularia, T. protracta, and T. 
rubida. This future direction would look to 
see if these species show the same northward 
trend over time. Anthropogenic effects on 

the environment increase the risk of Chagas 
transmission.

The risk of Chagas infection increases 
in cities, specifically in the largest cities of 
Texas: Houston, Dallas, and Austin due to 
the high human population in each of the 
cities. A high human population favors the 
establishment of a local infection cycle because 
the triatomine bugs are attracted to areas of 
human activity and the presence of infected 
reservoirs increases the risk of transmission to 
humans (20). While bugs are often found in 
rural areas, they are attracted to gaseous CO2 
which is emitted at higher levels in urban 
areas than rural ones (23). Larger cities also 
tend to have high immigration rates; people 
raised where Chagas is already endemic often 
are chronically infected without knowing it 
and can serve as a reservoir to infect naive 
vectors and perpetuate infection into their new 
area (4). Larger cities also exhibit an urban 

heat island, where the inner city is warmer, 
specifically overnight than the outlying rural 
or suburban areas (9). This affects the mean 
diurnal temperature (the average temperature 
range for each day in any given month), which, 
as seen with T. gerstaeckeri, can have a strong 
effect on vector distribution (17). Because of 
this a strong urban heat island, yields a stable 
overnight temperature, making cities more 
suitable than other areas for some vectors (20).

AMERICAN TRYPANOSOMIASIS • 137

The authors would like to thank the Center for the Integration of STEM Education and 
Research at Texas Tech University for the undergraduate financial support. We are grateful to 
Nancy McIntyre (Texas Tech University) for MaxEnt training and support, to Jennifer Vanos 
(University of California- San Diego) a geographical information systems consultant, and the 
Texas Tech Writing Center for language editing and proofreading.

ACKNOWLEDGEMENTS

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