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).

Anyway, I wanted to get back to some COVID 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 it will be longer than a typical blog post).

Are cats susceptible to this virus?

This one is easy…..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 and we don’t have a good answer because surveillance has been limited. One of the earliest studies was from Wuhan, China, and this study raised concern because they found anti-SARS-CoV-2 antibodies in 14.7% of cats from Wuhan in a study that 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. https://link.springer.com/article/10.1007/s12250-020-00284-5

The most relevant are studies have looked at cats living in households where people had COVID, and the rates of infection have been pretty high. A study from Hong Kong identified the virus by PCR in 12% of cats from positive households.

Studies looking for the virus by PCR will under-estimate infections because there’s a short window of time that cats are shedding the virus. The figure below from a small experimental study shows that, indicating 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 (since those stick around for at least a while after infection).

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

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

https://www.nejm.org/doi/full/10.1056/nejmc2013400

Do cats get sick?

They can. Most often, they don’t. Experimentally, it’s 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 a fatal infection. More work needs to be done in this area. I get lots of anecdotal reports about sick cats, and I suspect many of them are real. 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 was infected, it’s pretty suggestive since there aren’t many other common causes.

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 will die. The relative size of those different groups is completely unknown.

Can cats infect other animals?

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-cat transmission was more likely than all cats being infected by people). How often this occurs in households will be hard to discern because if multiple pets are infected in a household, it’s pretty much impossible to say whether the pets were spread it between each other or whether people infected them all.

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

The answer is “we don’t know”. Since cats can infect other cats, we have to assume there’s some risk of them infecting people. However, sorting that out is a challenge.

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

If a pet cat gets infected, 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 and visitors would hopefully not be present. If I get COVID and infect my cat, 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 it to someone outside the household, it would have to leave the household during the short window when it’s infectious. That can happen (e.g. vet visit, indoor-outdoor cat), but fewer vet visits would occur when the owner is sick. 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 a healthy carrier. If the cat was sick, it might be considered as a potential source, but with rampant human-human transmission, that’s not enough proof. What we’d need is for the cat and person to both best tested and have whole genome sequencing performed on the virus from both, to show it’s the same (and even with that, it’s not a 100% answer, since that same strain could be what’s circulating. However, with identical virus, it would be a pretty solid conclusion). Since there’s limited testing of cats and little likelihood that samples from both would be sequenced, odds of identifying a cat as the source are low.

Could cats be an important reservoir of the virus once it’s controlled in people?

Probably not. They are pretty susceptible but they don’t shed it for long. To maintain the virus in circulation in the cat population, an infected cat has 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 became infected, assuming there’s some degree of immunity to re-infection. For a virus that’s carried for a short period of time to be maintained in a population, you need a lot of animals and animal contact. That’s more of a concern with some wildlife species…that’s a story for another day, though.

So, we shouldn’t worry about COVID in cats?

  • Worry, no.
  • But, we should pay attention.
  • There’s a cat health risk, and we want to avoid that by reducing contact of infected people with cats. That’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 the household is limited but can’t be ignored. One issue is when you have someone isolating from the rest of the household (e.g. living in the basement). We want to make sure the cat is considered, so it’s 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 others, 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 taking the virus out of the household, exposing other cats or wildlife. Odds of a big problem or creating a 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 COVID.
  • If you are infected, try to stay away from animals…all animals…human and otherwise.
  • If your cat has been exposed, keep it inside and away from others.

Ultimately, cats are part of the family so if your family is being isolated, the cat’s part of that.