Covid-19 and Canines Q&A

Can the SARS-CoV-2 virus that causes COVID-19 infect canids like wolves, coyotes, or dogs?

There is weak evidence that dogs can become infected with SARS-CoV-2, the virus that causes the disease COVID-19. Before we get into the specifics, it is important to understand the components of the infection process.

To become infected, an organism (dog, human, bacteria, or any living thing) must become exposed, meaning it must be in close contact with active, or “live”, virus. The virus then must enter the organism’s cells and begin to replicate – this is where the infection begins. Once that organism becomes infected, it is considered a host. Viruses require a host because they cannot survive or replicate on their own. This is where most transmission events fail: the virus either cannot enter host cells or cannot replicate itself with host cell machinery. SARS-CoV-2, for example, has a specific structure that allows it to enter human respiratory cells. Other animal species may lack the specific cells, cell structures, or cell machinery that allow for viral invasion and replication, and therefore exposure may not lead to infection. Finally, once the virus is replicating in the host, it must be able to leave that host and infect a new one – this is called transmission. Transmission allows the virus to persist. Typically, viruses are shed from an infected host, for example by a sneeze or cough, and subsequently a new infection arises when the virus is successfully transmitted to another organism (typically the same species as the original host), without any previous exposure. As you can see, it is typically difficult for an infection to propagate through a population.

Bats are the ancestral wildlife host for SARS-CoV-2 [1-4], not canines. SARS-CoV-2 is thought to have spilled over from bats into an intermediate species or directly into humans associated with a market in China that sold wildlife [5]. To date, there have been only two potential cases of dogs producing antibodies against the virus causing COVID-19, indicating that dogs were exposed. In fact, we know these dogs had very close contact with infected people as their owners were diagnosed with COVID-19 (see Question #2). Therefore, we cannot say with complete certainty that canids cannot become infected with SARS-CoV-2, but we can say that it is highly unlikely (especially for free-ranging wild canids like wolves and coyotes). Preliminary research shows that dogs are a poor host for the virus [5]. It is important to emphasize that there is no evidence that dogs become ill when they are exposed to SARS-CoV-2, and no evidence that dogs can transmit virus to other dogs or back to humans.

The CDC guidance on domestic dogs and COVID-19 can be found here [7].

Sources:

Li, W., Shi, Z., Yu, M., Ren, W., Smith, C., Epstein, J.H., Wang, H., Crameri, G., Hu, Z., Zhang, H. and Zhang, J., 2005. Bats are natural reservoirs of SARS-like coronaviruses. Science, 310(5748), pp.676-679.
https://www.sciencedaily.com/releases/2020/03/200317175442.htm
https://www.nature.com/articles/s41591-020-0820-9
https://www.biorxiv.org/content/10.1101/2020.03.30.015008v1
https://doi.org/10.1016/S0140-6736(20)30251-8
https://www.biorxiv.org/content/10.1101/2020.03.30.015347v1.full.pdf
https://www.cdc.gov/coronavirus/2019-ncov/daily-life-coping/animals.html

I heard about a case in China where a dog caught SARS-CoV-2 from a human, did that happen?

There were multiple news sources that published articles about a Pomeranian dog in Hong Kong testing positive for SARS-CoV-2 (the virus that causes the disease COVID-19), the same virus that has caused the human pandemic [1-5]. Shortly after, a Germen shepherd also in Hong Kong reportedly tested positive for the virus as well [6-8]. These two cases are not as clear cut as the news articles present them, so we will try to clarify what we know about these dogs.

The Pomeranian’s owner was infected with SARS-CoV-2 and hospitalized on February 25. The dog was given a serological test on February 28 that returned a subjective result – there was some antibody activity, but overall, the test was inconclusive. Then, the World Health Organization and the University of Hong Kong took samples to identify any live virus particles in the dog (meaning the virus was actively infecting the dog and replicating); this test was negative, indicating that the dog did not have an active infection. Finally, on March 3, another serological test was positive, meaning that the dog had mounted an immune response to SARS-CoV-2. Since the dog appeared healthy, it was returned home [9]. The Pomeranian died on March 16 from underlying health issues associated with its old age (17 years old).

The German shepherd’s owner was infected with SARS-CoV-2 but neither the shepherd nor the other dog in the house showed symptoms. Still, they were tested for SARS-CoV-2 and the shepherd tested positive, while the other dog was negative [10]. We could not find information on whether or not scientists tested for SARS-CoV-2 antibodies or live virus in the shepherd.

Taken together, it is probable that these dogs were exposed to SARS-CoV-2 from close contact with their infected owners, not vice-versa. The initial positive SARS-CoV-2 tests only detected the presence of the viruses’ genetic material, and the inability to detect live virus suggest the virus was not replicating in the Pomeranian. As of March 26, thousands of domestic dogs have likely been exposed, no dogs have shown clinical signs of COVID-19, and only these two dogs in Hong Kong tested positive out of 17 dogs from COVID-19-confirmed households or close contacts. It is important to not overinterpret these results: none have been rigorously analyzed and the sample size is small. Nonetheless, it seems that dogs can mount at least an immune response to the virus and it is advisable for infected people to avoid close contact with pets [11] and to follow the CDC guidelines [12].

Sources:

Wolves and dogs carry coronaviruses though, right?

Coronaviruses are a huge group of viruses that infect many, many mammal and bird species. In fact, coronaviruses can also cause the common cold in humans. Most of these coronaviruses are specific to their host, or the species it infects. Wolves and dogs can be infected by canine coronavirus. Although most dogs do not display any symptoms of canine coronavirus, some will have gastrointestinal problems like diarrhea, loss of appetite, and lethargy [1,2]. Canine coronavirus (for which there is a puppy vaccine) is common where dogs are kept in crowded conditions; in wild wolf populations, canine coronavirus ranges from rare (Yellowstone National Park: 12% positive by PCR, 19/158 [3,4]), to high seasonal spikes (Alaska: 70% antibody prevalence in spring [5]). A canine respiratory coronavirus has been described [6] and it appears to be less common than canine coronavirus (0% positive by PCR, 0/131 in wolves captured in Yellowstone National Park [4]).

Canine coronavirus is an Alphacoronavirus, while SARS-CoV-2 (the virus that causes COVID-19) is a Betacoronavirus, both from the family Coronaviridae. The proteins on the outside of the Alpha– and Beta–coronaviruses are different, which determines which species the viruses can infect. In fact, Alpha– and Beta-coronaviruses are different genera, meaning they are genetically quite different. For comparison, within the family Hominidae, humans are from the genus Homo and chimpanzees (our closest living relative) are from the genus Pan – canine coronavirus and SARS-CoV-2 are as taxonomically distinct as humans and chimpanzees.

Sources:

https://vetmed.illinois.edu/pet_column/coronavirus-pets/
https://www.sciencedirect.com/topics/veterinary-science-and-veterinary-medicine/canine-coronavirus
Almberg, E.S., Cross, P.C., Hudson, P.J., Dobson, A.P., Smith, D.W. and Stahler, D.R., 2016. Infectious diseases of wolves in Yellowstone. Yellowstone Science, 24(1), pp.47-49.
Yellowstone Wolf Project, unpublished data.
Zarnke, R.L., Evermann, J., Ver Hoef, J.M., McNay, M.E., Boertje, R.D., Gardner, C.L., Adams, L.G., Dale, B.W. and Burch, J., 2001. Serologic survey for canine coronavirus in wolves from Alaska. Journal of Wildlife Diseases, 37(4), pp.740-745
Priestnall, S.L., Brownlie, J., Dubovi, E.J. and Erles, K., 2006. Serological prevalence of canine respiratory coronavirus. Veterinary microbiology, 115(1-3), pp.43-53.

In April there was a story on the news about a tiger that tested positive for SARS-CoV-2 at the Bronx Zoo in New York. What can you tell us about how COVID-19 impacts pet cats and other felines?

Two Malayan tigers, two Amur tigers, and three African lions at the Wildlife Conservation Society’s Bronx Zoo are doing well after showing symptoms of COVID-19. A 4-year-old female tiger and the other big cats began showing symptoms of wheezing, a dry cough, and lack of appetite in late March. The 4-year-old tested positive for the virus on April 2nd and the other symptomatic animals were assumed to be infected as well. Fortunately, recent reports indicate all the big cats are recovering well, with no worsening symptoms, and the virus has not spread to any other zoo animals. At this time it is thought that an asymptomatic zoo employee unknowingly and unintentionally infected the animals [1-5].

There are a few cases of domestic cats in China, Belgium, and Hong Kong testing positive for specific antibodies against SARS-CoV-2 [6-9]. In New York state, two domestic cats tested positive for SARS-CoV-2 [10-11]. These cats (from separate households) displayed mild respiratory symptoms and are expected to make a full recovery. Currently, there is only preliminary research about SARS-CoV-2 infections in animals, so there is limited information about how felines are affected by this specific virus. We do know that feline infections can result from close contact with humans with COVID-19 [6]. There is preliminary evidence suggesting that this virus is better able to replicate in cats than dogs, but this needs additional rigorous testing [12]. The notion that cats may be a better host is also supported by research on the coronavirus that caused the 2003 SARS pandemic, SARS-CoV-1. SARS-CoV-1 and SARS-CoV-2 are closely related, and cats are effective hosts for SARS-CoV-1 [13].

The take home is that cat-to-human transmission of SARS-CoV-2 has not been documented, but it is always advisable to practice basic good hygiene when interacting with household pets (i.e. wash your hands). People exposed to the virus should avoid close contact and follow the CDC guidelines on how to protect pets from being exposed [14].

Sources:

Can pathogens like coronaviruses pass between wild animals and humans?

Pathogens (viruses, bacteria) can sometimes transmit between wild animals and humans. Most often, the wild animal host is naturally infected with the pathogen and it rarely and unpredictably transmits to a human. If the human is a good host where the virus can readily enter its cells and replicate, then the infection can take off within the human population. Scientists call the natural wild animal host the reservoir for the pathogen, and when it infects a new host (like a human) it is called spillover or zoonosis. These spillover events are most likely to occur where humans have very high contact rates with live wildlife – for example, poultry and meat markets, areas where bushmeat is a major food source, and the wildlife/pet trade. As described in Questions 1 and 6, the infection process is complex and overall the risk of successful spillover is very low; however, as we have witnessed with the SARS epidemic and now the COVID-19 pandemic, even one spillover event can wreak havoc in countries across the globe.

Why are diseases like COVID-19, SARS, and Ebola appearing more regularly?

Spillover events, when a virus or pathogen transmits from its natural wildlife host to a human, are becoming more frequent [1]. HIV/AIDS, SARS (severe acute respiratory syndrome), MERS (Middle East respiratory syndrome), Swine Flu (H1N1), Ebola virus disease, and now COVID-19 are all diseases caused by viruses originally from wildlife that are now transmitted among humans. For a pathogen to spillover, many rare events must take place within a short time span (figure below). A figure from Plowright et al. (2017) diagrams how a ‘successful’ pathogen needs to overcome numerous barriers. These include barriers within the wildlife host population, the barrier of cross-species transmission, the barrier of evading the new host immune system, and finally, the barrier of effective individual-to-individual transmission in the new host population.

In general, spillover is very unlikely. Then why is it happening more? Historically when wildlife was used as a primary protein source, human populations were more dispersed and isolated. Human-wildlife contact is increasing primarily as a function of human population growth and ecosystem degradation which results in increased human encroachment on wild habitats, expanding wildlife trade, and financial incentives to bring wild animals to urban markets. More humans in contact with wildlife creates more opportunities for spillover [reviewed in 3]. Every human-wildlife contact is an opportunity for pathogen spillover. As just one example, wild animal products for sale in a Lao PDR market were found to be touched, on average, 7 times each hour [4]. As we have seen play out with the global spread of COVID-19, our ‘be-anywhere-in-24-hours’ global connectivity greatly amplifies the risks of even one extremely rare spillover event. Once these pathogens become established in a human host and are able to transmit among individuals, they can spread into pandemics. Policy efforts are now underway by lawmakers in China and Vietnam [5] to phase out wildlife trade and eliminate a wildlife-human interface now linked to both COVID-19 and the SARS pandemics. On other fronts, international wildlife organizations and Germany’s Ministry of the Environment are calling for new One Health⁺ approaches, which recognize the interconnection of animal, human and environmental health, and greater protection of nature to prevent the spillover of new infectious diseases [6-8].

Barriers to spillover. This figure was adapted from Plowright et al. 2017, Nature Reviews Microbiology [2]. Click here to see a movie of this diagram (https://static-content.springer.com/esm/art%3A10.1038%2Fnrmicro.2017.45/MediaObjects/41579_2017_BFnrmicro201745_MOESM11_ESM.gif).

Sources:

http://dx.doi.org/10.1038/nature06536
Plowright, R.K., Parrish, C.R., McCallum, H., Hudson, P.J., Ko, A.I., Graham, A.L. and Lloyd-Smith, J.O., 2017. Pathways to zoonotic spillover. Nature Reviews Microbiology, 15(8), p.502.
Keusch, G.T., Pappaioanou, M., Gonzalez, M.C., Scott, K.A., Tsai, P. and National Research Council, 2009. Drivers of Zoonotic Diseases. In Sustaining Global Surveillance and Response to Emerging Zoonotic Diseases. National Academies Press.
http://dx.plos.org/10.1371/journal.pone.0150666
https://news.mongabay.com/2020/04/chinese-ban-on-eating-wild-animals-likely-to-become-law-qa-with-wcss-aili-kang/
https://www.bmu.de/en/pressrelease/minister-schulze-global-nature-conservation-can-reduce-risk-of-future-epidemics/
https://wwf.panda.org/wwf_news/?362052/EU-economy-must-bounce-forward-rather-than-back-after-Covid-19
https://www.wcs.org/one-planet-one-health-one-future

Where can I learn more about disease transmission between people and wild animals?

There is an abundance of literature about wildlife to human pathogen spillover, both in the popular press and science literature. Below are sources I find particularly useful, but there are many more as well.

COVID-19 and Canines Q&A