The COVID-19 pandemic has provided many opportunities to observe how scientists, clinicians, policy-makers, and the public respond to scientific uncertainty and changing research evidence. We have seen the triumphs, such as the impressively rapid development of effective vaccines for a novel pathogen, often using new technologies. We have also seen errors and shortcomings, including the early view among experts that masks were not likely to be very effective outside of the hospital setting,1 and the rapid uptake by clinicians of unproven therapies for COVID that turned out not to be effective, such as hydroxychloroquine.2 We have seen the dire consequences of the public and government officials failing or refusing to understand the complexities of preliminary and changing scientific evidence. One example of relevance to small animal clinicians is our understanding of the role of dogs and cats as potential hosts or vectors for SARS-CoV-2. We have moved in fits and starts over the last year and a half from essentially no information to a relative firm, though still limited, understanding of whether these common companion animals can be infected by or transmit this coronavirus. As always, veterinary medicine has had to make due with far less robust evidence than available for humans to support decisions about patient management. In some ways, this is actually an advantage in this type of situation, especially if we are accustomed to making thoughtful and critical decisions in the face of significant evidence gaps. As early as January 2020, the United States Centers for Disease Control and Prevention (CDC) and the World Small Animal Veterinary Association (WSAVA) issued statements about the potential for SARS-CoV-2 infection in dogs and cats. These indicated it was uncertain if pets could be infected or transmit the virus, but this was possible, especially given previous evidence that cats were susceptible to closely related coronaviruses. The goal of these statements was partly to raise awareness and encourage reasonable precautions among pet owners and veterinary workers. But there was also the concern people would panic if they thought dogs and cats could give them COVID-19, and this would lead to abandonment or destruction of pets.3 Early pandemic days Dogs and cats were obviously not a major focus of COVID-19 testing during the early days of the pandemic. However, reports of possible COVID infections in dogs and cats appeared quite early, in March and April of 2020. Dogs and cats tested positive, on PCR or with serology, in Hong Kong, China, Europe, and the United States in this timeframe. It was not clear at this point, however, whether these represented true infections or mechanical carriage of the virus, and whether dogs or cats could be clinically affected or transmit the virus amongst themselves or to humans. The lack of clear evidence was compounded by the lack of widespread and systematic testing. Public health authorities often actively discouraged testing dogs and cats. This was partly due to the need to conserve limited testing resources for humans, but also because of concerns positive tests (real or false) would cause unnecessary and unproductive anxiety in the public. More than a year into the pandemic, our testing of dogs and cats is still extremely limited and sporadic, so we may never have a clear picture of the extent or clinical consequences of infection in these species. There are now plans for testing of dogs and cats belonging to infected people in South Korea, though only if the pets show symptoms, and this might help fill some of this evidence gap. Eventually, enough positive results in dogs and cats emerged, predominantly those with owners who were known to be infected, for confidence to grow that these were true infections. The next question to be answered was whether pets would be clinically affected by coronavirus infection and whether they could transmit SARS-CoV-2. Both experimental and epidemiologic studies gradually revealed dogs are susceptible to infection, but at a very low level. Several studies have shown dogs can be positive on PCR testing, which indicates the virus is present in the area of the body sampled.4–6 They can also develop anti-SARS-CoV-2 antibodies, indicating true infection rather than simply mechanical carriage.7,8 However, not all studies of dogs exposed to the coronavirus have found evidence of infection, and it appears dogs are minimally susceptible and shed virus only very briefly.9–11 What is clear? Whether or not dogs with SARS-CoV-2 infection become ill is less clear. Given the limited testing, we cannot say for certain, but relevant clinical symptoms have not consistently been reported in dogs testing positive.5 There have been reports of dogs with concurrent respiratory illness in which COVID-19 was suspected to be a contributing factor, but it seems unlikely dogs regularly experience significant symptoms. Similarly, there is little direct evidence to determine whether or not dogs can spread COVID-19. Experimental studies haven’t found transmission between dogs.5,6 There have also been no confirmed reports of dog-to-human transmission. One study in Spain claimed walking dogs was a significant risk factor associated with COVID-19 in people, but the study had severe methodological issues and did not actually show dog ownership to be a risk factor, so the conclusion about walking dogs is not really supported by this research.12 The situation is a bit different with cats. Early on, it was considered likely cats would be susceptible to COVID-19 based on previous evidence of susceptibility to the related coronaviruses.13 Experimental studies in which cats are deliberately exposed to the COVID-19 virus and surveys of cats in households where people have had COVID-19 have consistently shown cats do get infected with this virus.5–7,9,11,14–17 How easily cats can be infected isn’t completely clear, partly because they only shed the virus for a very short time, making it easy to miss an infection with PCR testing. Serologic studies suggest 10-50 percent of cats living with infected humans will acquire the infection.7,8,15,18 That is a pretty broad range, but clearly infection is not uncommon. Humans are undoubtedly the primary source of infection for cats, though experimental studies show they can spread the infection to other cats in close quarters, and the source of infections seen in some stray cats isn’t known. It is unclear how commonly cats show clinical symptoms associated with COVID-19 infection. Respiratory and gastrointestinal symptoms have been reported in both experimental studies and in cats exposed to infected humans, but there are also plenty of reports of infected cats not showing any signs of illness.16 As in humans, the clinical manifestations of COVID-19 infection probably vary among individuals. Finally, there is not yet any evidence cats can spread COVID-19 to humans. Shedding appears to occur for a very short time. Of course, pet cats infected by humans with active COVID-19 are unlikely to spend time in the company of uninfected humans outside the household, so there are practical and biological impediments to feline-mediated spread of the virus. While it is not impossible cats could be a source of infection for humans, it has not yet been demonstrated and seems unlikely to be a significant risk. There is a lot less information regarding the risk of SARS-CoV-2 infection in other companion species. Rabbits appear, like dogs, to be minimally susceptible and rarely show clinical signs or shed significant amounts of virus.19 Birds are not susceptible. Ferrets are definitely susceptible and do show clinical symptoms, and they can potentially infect other animals. While there are still significant gaps in our knowledge about how SARS-CoV-2 can affect different pet species, we have steadily improved our understanding through the course of the pandemic. The early “best guesses” about the issue mostly proved accurate. While some companion species can be infected, experience symptoms, or even theoretically spread the infection to other animals, overall the role of these species in the pandemic has been negligible, and the risk to humans still appears extremely small. Brennen McKenzie, MA, MSc, VMD, cVMA, discovered evidence-based veterinary medicine after attending the University of Pennsylvania School of Veterinary Medicine and working as a small animal general practice veterinarian. He has served as president of the Evidence-Based Veterinary Medicine Association and reaches out to the public through his SkeptVet blog, the Science-Based Medicine blog, and more. He is certified in medical acupuncture for veterinarians. Columnists’ opinions do not necessarily reflect those of Veterinary Practice News. References 1 Brooks JT, Butler JC. Effectiveness of Mask Wearing to Control Community Spread of SARS-CoV-2. JAMA. 2021;325(10):998. doi:10.1001/jama.2021.1505 2 Singh B, Ryan H, Kredo T, Chaplin M, Fletcher T. Chloroquine or hydroxychloroquine for prevention and treatment of COVID-19. Cochrane Database Syst Rev. 2021;(2). doi:10.1002/14651858.CD013587.pub2 3 Parry NMA. COVID-19 and pets: When pandemic meets panic. Forensic Sci Int Reports. 2020;2:100090. doi:10.1016/j.fsir.2020.100090 4 Sit THC, Brackman CJ, Ip SM, et al. Infection of dogs with SARS-CoV-2. Nature. 2020;586(7831):776-778. doi:10.1038/s41586-020-2334-5 5 Bosco-Lauth AM, Hartwig AE, Porter SM, et al. Experimental infection of domestic dogs and cats with SARS-CoV-2: Pathogenesis, transmission, and response to reexposure in cats. Proc Natl Acad Sci U S A. 2020;117(42):26382-26388. doi:10.1073/pnas.2013102117 6 Shi J, Wen Z, Zhong G, et al. Susceptibility of ferrets, cats, dogs, and other domesticated animals to SARS-coronavirus 2. Science. 2020;368(6494):1016-1020. doi:10.1126/science.abb7015 7 Fritz M, Rosolen B, Krafft E, et al. High prevalence of SARS-CoV-2 antibodies in pets from COVID-19+ households. bioRxiv. September 2020:2020.09.22.307751. doi:10.1101/2020.09.22.307751 8 Patterson EI, Elia G, Grassi A, et al. Evidence of exposure to SARS-CoV-2 in cats and dogs from households in Italy. bioRxiv Prepr Serv Biol. July 2020. doi:10.1101/2020.07.21.214346 9 Patterson EI, Elia G, Grassi A, et al. Evidence of exposure to SARS-CoV-2 in cats and dogs from households in Italy. bioRxiv. July 2020:2020.07.21.214346. doi:10.1101/2020.07.21.214346 10 Temmam S, Barbarino A, Maso D, et al. Absence of SARS-CoV-2 infection in cats and dogs in close contact with a cluster of COVID-19 patients in a veterinary campus. One Heal (Amsterdam, Netherlands). 2020;10:100164. doi:10.1016/j.onehlt.2020.100164 11 Ruiz-Arrondo I, Portillo A, Palomar AM, et al. Detection of SARS-CoV-2 in pets living with COVID-19 owners diagnosed during the COVID-19 lockdown in Spain: A case of an asymptomatic cat with SARS-CoV-2 in Europe. medRxiv. May 2020:2020.05.14.20101444. doi:10.1101/2020.05.14.20101444 12 Rodríguez-Barranco M, Rivas-García L, Quiles JL, et al. The spread of SARS-CoV-2 in Spain: Hygiene habits, sociodemographic profile, mobility patterns and comorbidities. Environ Res. 2021;192:110223. doi:10.1016/J.ENVRES.2020.110223 13 van den Brand JMA, Haagmans BL, Leijten L, et al. Pathology of experimental SARS coronavirus infection in cats and ferrets. Vet Pathol. 2008;45(4):551-562. doi:10.1354/vp.45-4-551 14 Sailleau C, Dumarest M, Vanhomwegen J, et al. First detection and genome sequencing of SARS‐CoV‐2 in an infected cat in France. Transbound Emerg Dis. 2020;67(6):2324-2328. doi:10.1111/tbed.13659 15 Zhang Q, Zhang H, Gao J, et al. A serological survey of SARS-CoV-2 in cat in Wuhan. Emerg Microbes Infect. 2020;9(1):2013-2019. doi:10.1080/22221751.2020.1817796 16 Hosie MJ, Hofmann-Lehmann R, Hartmann K, et al. Anthropogenic Infection of Cats during the 2020 COVID-19 Pandemic. Viruses. 2021;13(2):185. doi:10.3390/v13020185 17 Hosie MJ, Epifano I, Herder V, et al. Detection of SARS‐CoV‐2 in respiratory samples from cats in the UK associated with human‐to‐cat transmission. Vet Rec. 2021;188(8):no-no. doi:10.1002/vetr.247 18 Barrs VR, Peiris M, Tam KWS, et al. SARS-CoV-2 in Quarantined Domestic Cats from COVID-19 Households or Close Contacts, Hong Kong, China. Emerg Infect Dis. 2020;26(12):3071-3074. doi:10.3201/eid2612.202786 19 Mykytyn AZ, Lamers MM, Okba NMA, et al. Susceptibility of rabbits to SARS-CoV-2. Emerg Microbes Infect. 2021;10(1):1-7. doi:10.1080/22221751.2020.1868951
