Susceptibility of domesticated animals to Covid-19

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A Chinese study published Wednesday in the journal Science reported that tests of the new coronavirus on domesticated cats, dogs, pigs, chickens, ducks and ferrets found that both cats and ferrets are highly susceptible to the virus.

Cats can become infected through airborne transmission. Dogs, however, have a very low susceptibility to the virus.

Susceptibility means that a virus is able to enter a cell.

Specialists in veterinary medicine stressed that the study included very small numbers of cats and dogs, which were injected with much higher levels of the new coronavirus than they would likely encounter in the natural world.

They also said it is highly unlikely that a person would become infected from a pet.

“I don’t think that for most people cat-to-human transmission is the most likely way that they would be infected, but I’d be very surprised if this was impossible,” said David O’Connor, a professor of pathology and laboratory medicine at the University of Wisconsin-Madison.

“This is a human disease,” said Jeanette O’Quin, an assistant professor in the Department of Veterinary Preventive Medicine at Ohio State University.

“It’s being transferred from person to person. That is our greatest risk.”

Still, the new paper follows a scattering of reports of animals becoming infected with the new coronavirus. Less than a week ago the Bronx Zoo announced that a 4-year-old Malayan tiger, named Nadia, tested positive for COVID-19.

A separate Chinese study reported that after the outbreak began in Wuhan, 14% of cats in the area were found to have antibodies for the virus, O’Connor said.

In Hong Kong, a recent study examined 17 dogs and eight cats taken from households where a human had become sick with COVID-19 or had come in close contact with a confirmed patient.

In that group, two dogs tested positive, though one was deemed to be “a weak” positive. The cats were not positive at the most recent testing.

Jane Sykes, a professor of small animal medicine at the University of California, Davis School of Veterinary Medicine, cited a much larger report by a major veterinary laboratory, IDEXX, involving more than 4000 samples taken from dogs, cats and horses. None of the animals showed any evidence of the new coronavirus.

The samples were collected from animals in the U.S. and South Korea during a four-week period, starting on Feb. 14.

“There’s a lot of stress in the world and the human-animal bond is so important,” Sykes said. “We should be enjoying our pets, rather than being fearful of them.”

Sykes and O’Quin both recommended steps people can take to reduce the small risk of the virus passing between people and their pets.

Sykes said that people who have COVID-19 should probably keep the pet in another part of the house and have another person in the household care for it.

If there is no one else to care for the pet, the infected person should wear a mask when caring for the pet and wash their hands before and after feeding the animal. She also advised people not to allow a pet to lick them in the mouth.

Although the risk of the virus passing between pets and people “is really low, we should take precautions,” O’Quin said.

If pet owners are quarantining themselves, they should also quarantine their cats inside.

“If you’re not sick,” O’Quin said, “you can interact with them pretty much the way you would always.”


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Prof. Mick Bailey, Professor of Comparative Immunology, University of Bristol, said: 

“The fact that lockdown seems to have worked in China, and be working in Italy and Spain, suggests that reducing human-to human contact controls transmission. That is, human-to-human transmission is far more important than any other route, even if there are other routes.

“The occasional reports of infected cats belonging to patients diagnosed with SARS-2 does suggest that human-to-cat transmission may occur in the field, although likely to be rare. Transmission to pets did occur with SARS-1 but there was no evidence for pet-to-human transmission.

“This study in Science looks at experimental infections and clearly demonstrate that cats can be infected by experimental intranasal administration of virus. The caveats are that this was under experimental conditions and it isn’t clear how well the experimental dose of the virus represents a dose they might be exposed to from infected owners. 

“Cat-to-cat infection was also demonstrated in one cat, which suggests that aerosol droplets may be infectious. Again, though, exposure was continuous for an unspecified period of time and that cats may have spent some time spitting at each other.

“The serological study referenced in the Science study (https://www.biorxiv.org/content/10.1101/2020.04.01.021196v1) is a preprint and needs a significant amount of extra work before it can be interpreted as evidence of infection with SARS-2. As it stands, it’s best regarded as an opinion piece.

“My take on this is that human-to-cat infection can occur, but probably at low frequency: the antibodies found in Wuhan cats raises an interesting question, but needs to be confirmed by much better methodology. 

Even if true, it would neither prove infection or the ability to transmit.

Infected cats may be able to infect other cats, and I couldn’t completely exclude the possibility that they could infect humans, although there is no evidence for it. However, the main source of infections for humans is other humans.” 

Further information on the serological preprint: 

“Serological assays in humans are not well validated yet – several of the commercial tests aren’t performing as well as expected. Under those circumstances, we need to be cautious in interpreting a serological test in cats without adequate validation against a ‘gold standard’.

In this preprint, the serological assay isn’t validated at all.

The results are presented as OD450 at a single dilution: under normal circumstances you would only do that with a well-validated test where a single dilution has specifically been shown to distinguish between infected and uninfected cats.

The absence of reactivity in single serum samples from FIP-infected cats (a feline coronavirus) only shows that apparent seropositivity isn’t necessarily attributable to FIP.

However, remember that the FIP controls are single serum samples, and only 15 of the 102 post-outbreak cats were ‘positive’ – it’s entirely possible that FIP infections do result in cross-reactive antibody in some cats.“

Prof Richard Tedder, Visiting Professor in Medical Virology, Imperial College London, said:

“A recent report from the People’s Republic of China, shortly to be published in Science, examines the susceptibility of a number of different animals to infection by SARS CoV 2. Studies on the outcome of exposure of ferrets, cats, dogs, chickens and ducks showed that only ferrets, cats and dogs could be infected.

The level of virus growth in dogs was very low and exposure did not lead to infection in two of five challenged animals. No shedding of virus was found to occur in any of the dogs nor was infection passed on  neighbouring caged dogs. 

“Higher levels of viral growth were demonstrated in adult cats and infection was transmitted to a cat in an adjacent cage demonstrating aerosol spread. Juvenile cats were able to support the highest levels of virus replication with symptomatic systemic infection.

Just how much feline to human transmission occurs, if at all, is simply not known. Whilst it is known that shed virus can survive on plastic and firm plastic surfaces for quite extended times it does not do so on porous surfaces such as cardboard, quite how shed virus would survive on the pelt of and animal is open to speculation.

Viral shedding from an infected young feline could possibly represent a hazard to owners, but there is currently no evidence of cat to human transmission nor even evidence to show that cats in the community have ever been infected. The authors of the paper question to what extent surveillance of animal hosts, cats in particular, should be part of the control measures against this virus.”

Dr Rachael Tarlinton, Associate Professor of Veterinary Virology, University of Nottingham, said:

Is this good quality research?  Are the conclusions backed up by solid data?

“There are technical issues with the reporting of the qPCR data (the negative results should be negative and not set at a cut off point for this type of assay) that haven’t been resolved from the pre-print version but in general the papers methods and conclusions appear to have been done properly.

Have the authors accounted for confounders?  Are there important limitations to be aware of?

“It is really critical to remember that these studies were designed to establish animal models for testing of vaccines and drugs, not to establish what happens in everyday life with peoples pets.

“These studies use high doses of virus sprayed into an animals nose or injected into the trachea to induce initial infection, this is not what happens in a house with pets. Colleagues in Hong Kong have been closely monitoring pets there and have so far been a very tiny number of animals1 (2 dogs and one cat who were not sick) who appear to have contracted a mild infection from their owners.

These animals do not appear to have had enough of an infection to actually be able to infect other animals or people.

There has been one very similar case reported in one cat in Belgium2. This also appears to be what has happened in the well published case of the tigers at New York Zoo (who have contracted a mild infection from one of their keepers). One of the biggest Veterinary clinical pathology companies (IDEXX) have also been conducting qPCR testing on a large number of pets internationally, including in the USA and South Korea and have so far not detected any animals with infection3

“There has also been a Chinese study4 (still in pre-print so not reviewed yet but the methods appear to be robust and the study performed robustly) indicating that a very small number of cats in Wuhan (the centre of the initial COVID-19 outbreak) have antibodies to the virus and indicating they were exposed to it. In this study, 11% of cats  tested seem to have been exposed to the virus – note that carnivores sometimes test positive to pathogens from things they have eaten, this doesn’t necessarily mean they were actually infected just that they have had an immune response to it.

“This also happened in the SARs-1 outbreak where a very tiny number of domestic pets tested positive for the virus, probably after contracting it from their owners5. Similarly cats and ferrets were able to be experimentally infected with SARs-16 with very similar results to the current reports with SARs-2, and so these results are probably not therefore very surprising. There was no indication at all that pets spread SARs-1.

Does this mean cats can catch COVID-19 from humans or vice-versus?

“All the evidence we have so far indicates that domestic cats can in rare cases pick up COVID-19 infections from people but they don’t seem to get seriously ill and don’t appear to pass it back to people. You can never entirely say never, but cats really aren’t playing a role in the spread of the virus. There really isn’t need to be afraid for your cat, or of your cat, or to treat it any differently from any other member of your household during lockdown.

How does this work fit with the existing evidence?

“The findings in ferrets have been confirmed by a second group (the German federal reference lab) in a pro-med post release this morning7. The fact that two research groups are independently reporting very similar findings makes the results very much more reliable. 

“The two groups did not test exactly the same range of species, the Chinese study looked at dogs, pigs, chickens, ducks, ferret and cats whereas the German study looked at pigs, chickens, ferrets and fruit bats. However the results are very similar in the species tested in both studies (ferrets, pigs and chickens) so it seems robust.”

1. https://promedmail.org/promed-post/?id=7179945

2. https://promedmail.org/promed-post/?id=7151215

3. https://www.idexx.com/en/about-idexx/news/no-covid-19-cases-pets

4. https://www.biorxiv.org/content/10.1101/2020.04.01.021196v1

5. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3323218/

6. https://journals.sagepub.com/doi/full/10.1354/vp.45-4-551?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%3dpubmed

7. https://promedmail.org/promed-post/?id=7196506

Dr Andrew Freedman, Reader in Infectious Diseases & Honorary Consultant Physician, Cardiff University School of Medicine, said:

“This study provides convincing evidence that cats can be infected by SARS-CoV-2, both by direct inoculation into the nasal cavity and by droplet transmission from other infected cats in adjacent cages.  Dogs however were much less susceptible.

“It is important to note that this does not imply that cats are a significant cause of transmission to humans in the current COVID-19 pandemic or that cats are likely to come to any harm by catching the infection from their owners.  Further studies will be required to ascertain the true significance of these findings.”

Prof James Wood, Head of Department of Veterinary Medicine, and researcher in Infection dynamics and control of diseases, said:

“It has been clear for while that cats (and now tigers) are susceptible to infection, but there is no evidence that they can go on to infect humans. The experimental transmission study was under very artificial conditions, giving cats high doses, which did lead to some limited onward transmission between cats, although numbers were very small. These results are important but it is unlikely that these conditions would occur in real life.

“The preprint referenced in the study demonstrating antibody responses in cats in Wuhan1 demonstrates that animals, most of which were strays or rescue animals developed an immune response to infection. Three of them were owned by a COVID19 patient. The lack of history of precludes any assessment of how the cats became exposed, but it is possible that each one was owned by a COVID19 patient before being abandoned.

“The data overall suggest that cats may become infected by their owners if their owners have COVID19, but there is no suggestion that they may transmit it to owners. This reflects the advice today from the British Veterinary Association that if possible, when infected, owners should keep their cats inside.

“It is possible to conceive of many routes that an infection can get into a household, but indirect means like through deliveries or on any animal are far less likely to transmit infection than direct transmission from someone who is infected. It is known that virus can live for a number of hours on inanimate surfaces like wood. However, despite over a million of cases of COVD19 globally, there is no single public report that suggests patients may have become infected by their animals.

“Animals may become infected by the high doses of virus transmitted by their infected owners in some settings. The relative size of a cat compared to a human means that there is far less exhaled breath from one cat in a house, compared to the exhaled breath volumes from a human patient. Further, the grooming behaviour of cats means that they are more likely to catch infection from an owner than vice versa. Owners exercising normal hygienic measures around their animals, including hand washing, should avoid most risks of infection from contaminated surfaces.”

Q. Zhang et al., bioRxiv (2020). doi:10.1101/2020.04.01.021196


More information: Jianzhong Shi et al. Susceptibility of ferrets, cats, dogs, and other domesticated animals to SARS–coronavirus 2, Science (2020). DOI: 10.1126/science.abb7015

Expert reactions to a study looking at susceptibility of pets to the COVID-19 virus (SARS-CoV-2): https://www.sciencemediacentre.org/expert-reaction-to-a-study-looking-at-susceptibility-of-pets-to-the-covid-19-virus-sars-cov-2/

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