I’ve let the blog slip over the past week so it’s catch-up time. (I’ve been busier on Twitter – @weese_scott if anyone wants to follow that).

I want to get back to some COVID-19 discussion, and rather than a multi-species update, I figured I’d back up and focus on an overview of one species at a time. We’ll start with cats (so this will be longer than a typical blog post).

Are cats susceptible to the SARS-CoV-2 virus?

Yes, cats are clearly susceptible. This has been shown in multiple experimental studies and infected cats have been found in the “real world,” infected by their owners.

How often do cats get infected?

That’s a good question, but we don’t have a good answer because surveillance has been limited. One of the earliest studies from Wuhan, China, raised concern about this because they found anti-SARS-CoV-2 antibodies in 14.7% of cats from that city, even though they did not target cats with known exposure to infected people. Finding antibodies indicates that the cats were previously infected. In contrast, another study of cats in Wuhan didn’t find any cats with antibodies.

The most relevant studies are those looking at cats living in households with people who had COVID-19, in which the rates of infection appear to be pretty high. A study from Hong Kong identified SARS-CoV-2 by PCR in 12% of cats from COVID-19-positive households.

Studies looking for the virus by PCR will under-estimate the number of infected cats, because there appears to be only a short window of time that cats will shed the virus. This is illustrated in the figure below from a small experimental study, which shows the shedding time for experimentally infected cats and cats infected by those cats.

The logistics of sampling cats right around the time their owners are infected are challenging, so looking for antibodies against the virus can tell us more, because antibodies stick around for longer after infection.

Our (small, so far) study found antibodies in ~50% of cats living in households with infected people. A pre-print of a study from France had somewhat similar results, finding antibodies in 24-59% of cats from positive households (depending on how the tests were interpreted).

So, my assumption is that cats living with people with COVID-19 are quite commonly infected. Whether it’s 5%, 15% or 50% we don’t know yet, but I think human-to-cat transmission in households is likely pretty common.

Figure from Halfmann et al. N Engl J Med 2020 (https://www.nejm.org/doi/full/10.1056/nejmc2013400).

Do cats get sick from SARS-CoV-2?

They can, but most often if appears they don’t. Experimentally, clinical signs in cats have been pretty unremarkable. Most infected cats have been reported to be healthy, but it’s not always the case. There are reports of sick cats, including a pre-print describing what appeared to be a fatal infection with SARS-CoV-2 in a cat from the UK. More work needs to be done in this area. I get lots of anecdotal reports about sick cats that have been exposed to the virus, and I suspect many of them really are due to to SARS-CoV-2. When an otherwise healthy adult indoor cat with no contact with other cats develops signs of upper respiratory tract infection around the time its owner had COVID-19, it’s pretty suggestive since there aren’t many other probable causes for the cat’s illness.

Similar to people, most exposed cats probably don’t get sick or get mild disease. A subset get more serious disease, and a smaller subset may even die from the infection. The relative size of those different groups is completely unknown.

Can cats infect other animals with SARS-CoV-2?

Yes. Experimentally, cats have been shown to infect other cats. That’s also been seen outside the lab, with the outbreak in lions and tigers in the Bronx Zoo (where cat-to-cat transmission was more likely than all the big cats being infected by people). How often this occurs in households will be hard to figure out, because if multiple pets are infected in a household, it’s pretty much impossible to say whether the pets spread it between each other or whether people infected them all.

Can cats infect people with SARS-CoV-2? (Yes, people are animals too, but I assume you know what I mean.)

We don’t know. Since cats can infect other cats, we have to assume there’s some risk of them infecting people. However, sorting out how much of a risk is a challenge.

Why haven’t we figured out cat-to-human transmission yet?

If a pet cat gets infected with SARS-CoV-2, it almost certainly got it from its owner(s). Your average pet cat mainly or only has contact with its owners, especially when an owner has COVID-19 and visitors hopefully are not around. If I get COVID-19 and infect my cat, and then the rest of my family gets sick, did I infect them or did the cat? Most likely, it was me, and it would be essentially impossible to differentiate.

For a cat to spread SARS-CoV-2 to someone outside the household, it would have to leave the household during the short window when it’s actually shedding the virus. That can happen (e.g. veterinary visit, indoor-outdoor cat), but fewer veterinary visits would occur when the owner is sick due to the human-to-human transmission concerns. Even then, if the cat infected someone at the vet clinic, a link to the cat would be hard to find, especially if the cat was not showing any signs of illness. If the cat was sick, it might be considered as a potential source, but with rampant human-to-human transmission, that’s not enough proof. What we’d need is for the cat and person to both be tested and have whole genome sequencing performed on the virus from both, to show it’s the exact same virus (even then we can’t be 100% certain, since cat and person could have been infected by the same source (e.g. another person), but with identical virus in both, it would be a pretty solid conclusion). Since there’s limited testing of cats and little likelihood that samples from both owner and cat would be sequenced, the odds of identifying a cat as the source of a human infection are low.

Could cats be an important reservoir of SARS-CoV-2 once it’s controlled in people?

Probably not. Cats are pretty susceptible but they don’t shed the virus for long. To maintain the virus in circulation in the cat population, an infected cat would have to interact with another susceptible cat within a few days (and on and on…). Most cats don’t do that. In community cat colonies, I could see it spreading through the group, but it would likely burn out quickly as most of the cats became infected and recovered, assuming there’s some degree of immunity to re-infection. In order to maintain a virus in a population when it’s only carried for a short period of time, you need a lot of animals and a lot of animal-to-animal contact. That’s more of a concern with some wildlife species (but that’s a story for another day).

So, should we worry about SARS-CoV-2 in cats?

• Worry, no.  But we should pay attention to it.
• There’s a cat health risk, and we want to avoid that by reducing contact of infected people with cats. It’s probably most important with older cats and cats with underlying diseases that may make them more susceptible to severe disease.
• The risk of cats spreading the virus in a household is limited, but can’t be ignored. When you have someone isolating from the rest of the household (e.g. living in the basement), we want to make sure pets like cats are considered, so they’re not tracking the virus from the infected person to the rest of the family. It’s easy to see how someone might do a great job staying away from other people, but not think about the cat that runs back and forth between them and the rest of the family.
• We also don’t want cats tracking the virus out of the household and exposing other cats or wildlife. The odds of this causing a big problem or creating a wildlife reservoir are very low, but not zero. A little prudence makes sense.

What should be done with cats?

• Cats are people too, when it comes to SARS-CoV-2.
• If you are infected, try to stay away from animals – all animals, human and otherwise.
• If your cat has been exposed to SARS-CoV-2, keep it inside and away from others.

Ultimately, cats are part of the family – so if your family is being isolated, the cat needs to be a part of that.