key: cord-299432-lbv69du4
authors: Franklin, Alan B.; Bevins, Sarah N.
title: Spillover of SARS-CoV-2 into novel wild hosts in North America: A conceptual model for perpetuation of the pathogen
date: 2020-05-12
journal: Sci Total Environ
DOI: 10.1016/j.scitotenv.2020.139358
sha: 
doc_id: 299432
cord_uid: lbv69du4

Abstract There is evidence that the current outbreak of the novel coronavirus SARS-CoV-2, which causes COVID-19, is of animal origin. As with a number of zoonotic pathogens, there is a risk of spillover into novel hosts. Here, we propose a hypothesized conceptual model that illustrates the mechanism whereby the SARS-CoV-2 could spillover from infected humans to naive wildlife hosts in North America. This proposed model is premised on transmission of SARS-CoV-2 from human feces through municipal waste water treatment plants into the natural aquatic environment where potential wildlife hosts become infected. We use the existing literature on human coronaviruses, including SARS CoV, to support the potential pathways and mechanisms in the conceptual model. Although we focus on North America, our conceptual model could apply to other parts of the globe as well.

J o u r n a l P r e -p r o o f 2 infected humans to native wildlife that could subsequently serve as new reservoir hosts for the virus. In this way, the virus may become entrenched in areas outside of its region of origin and available for future outbreaks. However, for this to occur there are a number of hurdles that the virus would need to overcome to infect a novel wildlife host. Here, we propose a plausible mechanism where SARS-CoV-2, the pathogen causing the disease COVID-19, could spillover from infected humans into novel wildlife hosts in North America. We developed a conceptual model for this hypothesized mechanism (Fig. 1) , which we outlined as five premises supported from the existing literature on shedding of coronaviruses by humans, survivability of these viruses, and documented pathways for movement of coronaviruses into the natural environment.

Although we focused on North America, our conceptual model could also be applied to other parts of the globe.

For our conceptual model to be plausible, we assume that coronaviruses can be shed in the feces of infected humans. This assumption has been supported by research on other coronaviruses (Yeo et al. 2020) . For example, MERS coronavirus was detected in 14.6% and 2.4% of fecal and urine samples, respectively, from 37 infected individuals (Corman et al. 2015) , while SARS CoV was shed in the feces of 56 infected individuals for a median of 27 days, with 4 individuals shedding for over 100 days (Liu et al. 2004 ). In addition, 38.4% of SARS-infected patients experienced diarrhea, SARS-CoV RNA was detected in 16─73% of the stool samples, and cultured from the small intestines of 5 patients (Leung et al. 2003 , Yeo et al. 2020 ). In addition, SARS-CoV remained infectious in diarrheal fecal samples at room temperature for up to 4 days (Lai et al. 2005) . More recently, a U. S. patient with COVID-19 also shed SARS-CoV-J o u r n a l P r e -p r o o f 3 59% of 96 infected patients for up to 31 days (Zheng et al. 2020 ). Thus, a portion of infected humans are capable of shedding novel coronaviruses, including SARS-CoV-2, in their feces, with some for extended (1-4 months) periods. Although there is uncertainty whether SARS-CoV-2 remains infectious in feces, similar coronaviruses, such as SARS-CoV, are infectious in feces, and ACE2 receptors for SARS-CoV-2, are abundant in gastric, duodenal, and rectal epithelia cells in humans (Xiao et al. 2020) . This, coupled with the positive detection of SARS-CoV-2 RNA from feces in a large portion of infected patients (Wu et al. 2020b , Xiao et al. 2020 , Zheng et al. 2020 suggests that infectious SARS-CoV-2 virions are secreted from gastrointestinal cells into feces (Xiao et al. 2020 ).

If fecal shedding is substantial in terms of number of infected individuals at focal locations, such as in hospitals, airports, or concentrated housing, then substantial amounts of virus could be deposited into municipal sewage systems (pathways 1 and 2 in Fig. 1 ). Human coronaviruses (HCoV) can survive for extended periods in the environment, especially aqueous environments (Geller et al. 2012) . Survival rates of HCoV in saline ranged from 80-100% for 3 days and 30-55% for six days (Geller et al. 2012 ). Using two surrogate HCoVs for SARS CoV, Casanova et al. (Casanova et al. 2009 ) found that the viruses remained infectious in water and sewage for 17-22 days at 25° C and <1 log 10 reduction in infectivity at 4° C. In addition, SARS-CoV-2 RNA has been detected in untreated wastewater and sewage in sufficient quantities to J o u r n a l P r e -p r o o f

Once HCoV arrives at waste water treatment plants (WWTP) in sewage (pathway 2 in Fig. 1) , it needs to maintain its infectivity through the treatment process to be of concern in the effluent that is ultimately introduced into the natural environment. In general, the transit time for sewage to reach a WWTP is less than one day (Wigginton et al. 2015) . While WWTP do reduce virus levels, infective virus is still detected in the effluent from these plants (Wigginton et al. 2015) . Based on metagenomics, coronaviruses were detected in 80% of the samples from effluent class B sewage sludge from 5 WWTP in the U.S., which is typically applied to agricultural lands as a soil amendment (Bibby and Peccia 2013) . Although data is lacking for coronaviruses, wastewater treatment prior to disinfection results in 0─2 log 10 reduction of infective enteroviruses and 2 ─ >3-log reduction of infective adenoviruses, with infectious virus still being detected in the final effluent from the waste treatment process released into the environment (Simmons and Xagoraraki 2011, Wigginton et al. 2015) . Thus, there is evidence that infectious coronaviruses could pass through the wastewater treatment process and enter natural aquatic systems via WWTP (pathway 3 in Fig. 1 ).

Even if WWTP completely or mostly eliminated SARS-CoV-2 from their effluent when Once released into natural aquatic systems, coronaviruses would be available for uptake in wildlife hosts (pathway 4 in Fig. 1 ). Insectivorous bats (order Chiroptera) are the logical reservoir host for COVID-19 in North America because they were considered the reservoir host for SARS CoV (Banerjee et al. 2019) and are suspected as a host, such as Rhinolophus spp., for SARS-CoV-2 (Lu et al. 2020 ). In addition, coronaviruses have been detected in feces of North American bats with viral RNA prevalence in shed feces of 17-50%, depending on the species of bat (Dominguez et al. 2007 ). Other North American wildlife strongly associated with aquatic environments are raccoons (Procyon lotor), which have also been infected with coronaviruses (Martin and Zeidner 1992) . Interestingly, raccoons also feed on aquatic organisms, such as mollusks, that are also potential candidates to bioaccumulate viruses. where they increased their foraging activity downstream from WWTP relative to upstream (Vaughan et al. 1996 , Abbott et al. 2009 ). This suggests a potential pathway for bats to become infected with HCoV, and presumably SARS-CoV-2, from water sources contaminated with these pathogens from WWTP.

America through a wildlife host is whether spillback from wildlife hosts to humans could occur (pathway 5 in Fig. 1 ). Spillover into wildlife and subsequent spillback into the human population represents an unmanaged source of SARS-CoV-2. For example, if bats are considered the likely reservoir host for COVID-19, spillback of coronaviruses in North America could occur where bats use buildings for nocturnal roosts or hibernation sites and deposit their feces where humans might encounter them, such as in attics of residences (Voigt et al. 2016) . North

American bats have been shown to shed coronaviruses in their feces, often at high (up to 50%)

prevalence (Dominguez et al. 2007) . Airborne transmission of SARS CoV from human fecal matter was considered the primary route of infection of 187 human cases in a housing complex (Yu et al. 2004) , indicating that aerosol transmission of HCoV from bat feces to humans is possible.

Although there is still considerable uncertainty about which wildlife species can serve as reservoir hosts for SARS-CoV-2, current research on domestic animals indicates potential J o u r n a l P r e -p r o o f 8 candidates in wild cats and mustelids . Because domestic cats can efficiently replicate SARS-CoV-2 and transmit the virus to naïve cats , domestic cats represent another mechanism for spillover transmission from wild cats to humans. Another risk associated with the establishment of SARS-CoV-2 in a wildlife host population is the potential for mutation in novel hosts that results in a variant virus (Menachery et al. 2017) . Of particular concern, is whether there is the potential for recombination of SARS-CoV-2 with other bat coronaviruses should spillover of SARS-CoV-2 occur in bat populations.

While the primary risk associated with the current COVID-19 outbreak appears to be humanto-human transmission of SARS-CoV-2, we believe the existing evidence also supports the plausibility of novel coronaviruses, such as SARS-CoV-2, spilling over to new wildlife hosts through fecal shedding by infected humans and introduction to the natural aquatic environment via the waste water treatment system. In addition, we posit that spillback of novel coronaviruses from the new wildlife hosts is also possible. Thus, considering the current COVID-19 pandemic, wastewater treatment plants and their surrounding environments should come under increased scrutiny for serving as a potential areas where spillover into wildlife hosts could occur. This was recently conducted in the Netherlands where SARS-CoV-2 RNA was found in sewage from WWTPs servicing 6 cities and an airport (Medema et al. 2020) . Such scrutiny could integrate surveillance of key wildlife species, such as bats and raccoons, which have the potential for acquiring coronaviruses from their aquatic environments. An enhanced surveillance program of identifying SARS-CoV-2 spillover into wildlife would ideally involve 1) identifying "hotspot" municipalities with high human caseloads of COVID-19, 2) identifying areas downstream from WWTP effluent releases, and 3) sampling target wildlife species, such as bats, mustelids, and J o u r n a l P r e -p r o o f 9 raccoons, for SARS-CoV-2 antibodies and viral presence. Such an effort could be conducted in conjunction with sampling WWTP effluent as proposed by Medema et al. (2020) .

At this stage, a probabilistic risk assessment model would probably not be very informative because of the uncertainties of the mechanisms described here and the need to parameterize the pathways we propose. For example, most detection of coronavirus in fecal and environmental samples have focused on RNA detection through PCR and has, generally, not documented infectivity. We argue that further research into our proposed pathways is warranted, especially because outbreaks of novel, zoonotic pathogens have epidemiological implications beyond just human-to-human transmission. 

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The findings and conclusions in this publication are those of the authors and should not be construed to represent any official USDA or U.S. Government determination or policy. This 
