Back in October/November 2020, I wrote a series of posts about what we know about SARS-CoV-2 and different animal species. It’s a dynamic field, so it’s about time I got around to updating them. We’ll start back at the beginning with one of the most susceptible domestic species: cats. A lot of research about SARS-CoV-2 in cats in the last year has largely supported our initial observations and have helped refined what we know.
Are cats susceptible to the SARS-CoV-2 virus?
Yes, cats are clearly susceptible. No change here. We’ve known that for a while and more research has just solidified that. More on that below.
How often do cats get infected?
There are a lot of papers now about SARS-CoV-2 in cats. Some are very good. Some are interesting but low-impact single case reports, and some are rushed studies (“I want to be first, not the best”) that use small sample sizes, indistinct populations or cherry-pick interesting results from what should have been more comprehensive, bigger studies.
Overall, it’s apparent that human-to-cat transmission is common in households where people have COVID-19. A small number of studies have looked at active infection using PCR testing +/- virus isolation, which is tough to do logistically. It’s a lot of work to identify infected people, arrange to sample their pets and (typically) go to the household to do that. A study from Texas (Hamers et al. 2021) identified the virus in 3/17 (18%) of cats in infected households. The results of our Canadian study (which have been presented but not yet published) were fairly similar.
Testing for virus only tells us part of the story, because of difficulty with sampling infected cats soon enough to catch them during their short-term active shedding period. We assume that we often miss infections because we get into the household to sample too late. That’s why more studies are based on looking for antibodies in the blood of house cats as an indicator of previous infection. It’s less definitive than detecting the virus, since the performance of antibody tests can be variable, but with good tests it can really help our understanding of the situation.
When we look at antibodies to SARS-CoV-2 in cats in infected households, the apparent infection rates go up. The Texas study reported a seroprevalence (the percentage of cats with antibodies) of 44% (7/16), and the seroprevalence in our preliminary data from Canada was even higher at 67%. Other studies have had variable results (for example, a study from Peru found a seroprevalence of 17-30% among cats from infected households, depending on how the testing was interpreted), but the take-home message is that human-to-cat transmission of SARS-CoV-2 is pretty common.
There are also many studies that have looked at antibodies against SARS-CoV-2 in the general cat population, usually without any information about whether the cats were exposed to an infected person. These studies are fairly easy to do (for example, by testing leftover blood from samples collected for other purposes, or collecting convenience samples from cats presented to veterinary clinics or shelters), but their value is variable. Typically, these studies report low seroprevalence among cats. One study reported close to 10% prevalence, but <2% is more common (e.g. Dileepan 2021, Klaus 2021, Smith 2021, Stranieri 2021, Udom 2021, van der Leij 2021). Positives could be cats that actually had infected owners, but the information wasn’t known or collected, or false positives, due to an imperfect test. In our surveillance study of cats from shelters or spay/neuter clinics, we found there was often very limited history about the cats (e.g. cats recently acquired off Kijiji), so we can’t use the history to put the results into context.
There’s always a lag between disease occurrence and publication of reports, so it would be expected that rates of infection in cats would increase over time as the human pandemic continues and more cats become exposed.
Risk factors for infection in cats haven’t been carefully investigated yet, often because of fairly small study sizes. A Brazilian study reported that cats that slept in the bed were at higher risk of being seropositive (Calvet 2021), something we also found in our Canadian study. That’s not too surprising as things that increase close contact (direct or shared airspace) presumably increase the risk of human-to-cat transmission.
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 paper describing a fatal infection in a cat in the UK.
In our surveillance, cats that had antibodies against the virus were more likely to have been reported as being sick at the same time as the COVID-19-infected owner, but most of the time any illness in the cats was mild (e.g. coughing, sneezing, quieter than normal). 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, there aren’t many other probable causes for the cat’s illness. However, at the same time, since infection of cats seems to be quite common, we’d expect to find incidental infection of cats that get sick or die from various other unrelated things. A small study by the US CDC (yet to be published) explored this, and the take-home message was that some cases of severe disease seemed to occur but much of the time, cats that died while infected didn’t die from the effects of SARS-CoV-2.
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, such as the high-profile outbreak in lions and tigers in the Bronx Zoo, where cat-to-cat transmission was more likely than all the big cats being individually infected by people. We also investigated one large group of infected cats, and it’s most likely there was cat-to-cat spread there too, rather than all human-to-cat infections.
Can cats infect people with SARS-CoV-2?
We still don’t know for sure if cat-to-human infection occurs. Since cats can infect other cats, we have to assume there’s some risk of them infecting people, but sorting out how much of a risk there is is a challenge. If someone got infected by a cat, it would be very difficult to determine that they got it from a cat vs a human contact, because the virus is still circulating widely in people, and contact with the infected cat would probably coincide with contact with that infected cat’s (probably infected) owner.
I think we have to assume that cat-to-human transmission is biologically possible and has probably happened. However, in the real world, it’s probably very rare given the dynamics of cat-to-human contact. If my cat gets infected, he got the virus from me, my wife or my kids. In that event, transmission from the cat to other people in the household is possible, but transmission between people is far more likely. Most cats don’t encounter a lot of different people, especially when their owners are sick. The biggest risk is likely when a cat leaves the house, such as to go to a veterinary clinic, or is surrendered to a shelter. We’ve detected infected cats in shelters, so it’s a plausible scenario, and it’s why we recommend asking about owner infection status prior to bringing animals into clinics, shelters or other places outside the home.
Do we have a SARS-CoV-2 vaccine for cats, and should we consider vaccinating cats?
My current answers are “kind of” and “no.” There’s a SARS-CoV-2 vaccine (of unknown safety and effectiveness) licensed for use in cats in Russia. In North America, there’s an experimental vaccine that has been used in mink and some zoo animals, and it would be the best option if we needed a vaccine. However, I don’t see a need at this point given the apparent rarity of severe disease. There’s more information on the possible utility (or not) of SARS-CoV-2 vaccines in pets in an earlier post.
Could cats be an important reservoir of SARS-CoV-2 once it’s controlled in people?
Probably not. Cats are pretty susceptible to the virus, but they don’t shed it 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 (which seems to be the case) . 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).
Could cats be a source of new SARS-CoV-2 variants?
Probably not. Variants occur because of random mutations. These occur when the virus replicates. So, the risk of variant emergence is directly proportional to how much transmission (and therefore virus replication) is going on. Since we don’t expect sustained transmission in the cat population, there’s limited risk of variants emerging in there.
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. Keep cats indoors if they’re in contact with any infected people.
What should be done with cats?
This hasn’t changed from the first post….
- 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.