Cats may be able to contract SARS-CoV-2, but evidence is limited

A preliminary study suggests that cats can contract the new coronavirus, yet this happened in an artificial context where scientists gave the felines high doses of SARS-CoV-2. The research did not prove convincingly that cats were able to transmit the virus further, between themselves or to humans.

One key question that scientists have not been able to answer satisfactorily during the COVID-19 pandemic is: Can domestic animals contract SARS-CoV-2, the virus that can lead to this disease?

Moreover, if the answer is “yes,” which domestic animals are at risk, and are they able to shed the virus further, transmitting it to other animals and humans?

One report from Hong Kong indicated that a pet Pomeranian tested “weak positive” for infection with SARS-CoV-2. The results suggested that this was a case of human-to-animal transmission, and the experts who tested it kept the animal in quarantine for a few days, even though it did not show any symptoms.

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In the meantime, the dog — which was 17 years old — has died, though, whether the virus was in any way responsible for its passing remains uncertain.

Since then, another dog in Hong Kong — this time, a German Shepherd — tested positive for infection with the virus, though once more, the canine does not show any symptoms.

And most recently, a cat in Belgium that showed respiratory and digestive symptoms also tested positive for a coronavirus infection. In this case, however, some scientists have raised doubts about whether this was a SARS-CoV-2 infection, and whether the cat’s symptoms were related to the infection.

Cats and ferrets, yes, dogs and pigs, no?

Because of the high volume of contradictory information around the possibility of SARS-CoV-2 transmission in domestic animals, researchers from China decided to take further steps to look into the matter.

The researchers hail from the Harbin Veterinary Research Institute at the Chinese Academy of Agricultural Sciences, and the National High Containment Laboratory for Animal Diseases Control and Prevention, both in Harbin, China.

While their study has not yet undergone peer review, and they have not published it in any scientific journal, the authors are making their findings available online in preprint form.

In their research, they looked at whether the new coronavirus could infect a range of domestic or companion animals, including cats, dogs, ferrets, chickens, ducks, and pigs.

Their most salient finding was that cats and ferrets appear to be the most susceptible to infection with SARS-CoV-2, while in dogs, pigs, chickens, and ducks, the virus “replicates poorly,” according to the study authors.

Yet, it is unclear how likely cats and ferrets are to contract the virus, given the completely artificial method of infection that the scientists used.

Cats inoculated with high load of virus

The researchers inoculated five 8-month-old domestic cats with a high dose of the virus, which they delivered directly to the cats’ noses.

After 6 days, they euthanized two of the cats to collect samples from different parts of their bodies and check for viral RNA. From the rest of the cats, the researchers collected and analyzed fecal samples.

The researchers found that post mortem, the two euthanized cats presented viral RNA in their noses, mouths, and tonsils, while one of them also had traces of the virus in its trachea, and another in its small intestine. They found no viral RNA in the cats’ lungs.

In terms of infectious viral RNA — traces of virus that could actually replicate further — the researchers only found those in the two cats’ noses, mouths, tonsils, and trachea.

As for the other three cats, “[i]n the transmission study, viral RNA was detected in the feces of two virus-inoculated subadult cats on day 3 [following infection], and in all three virus-inoculated subadult cats on day 5,” study author Jianzhong Shi and colleagues write. The researchers had isolated all of these cats in separate cages.

The study team exposed a sixth young cat to the three that it had already inoculated with the virus, and this cat became infected too. The investigators found traces of viral RNA in its feces on the third day after exposure.

When they did corresponding experiments using kittens aged 70–100 days, the researchers obtained similar results. Thus, they write that:

“These results indicate that SARS-CoV-2 can replicate efficiently in cats, with younger cats being more permissive and, perhaps more importantly, the virus can transmit between cats via respiratory droplets.”

Insufficient evidence

Nevertheless, all of these cats, as well as other animals, would not usually come into direct contact with such high quantities of the virus.

Moreover, the team performed its experiment on a small number of animals and only tested infection through exposure in one cat. All of this means that its findings are unlikely to be representative of real-life situations.

Speaking to Nature, distinguished virologist Prof. Linda Saif, from Ohio State University in Wooster, said that “[t]his suggests the virus may not be highly transmissible in cats.”

Previous research on SARS-CoV, the virus that causes severe respiratory syndrome (SARS), has suggested that cats are able to contract the coronavirus and pass it on to other cats and that the same applies to ferrets.

However, Prof. Saif told Nature, “there was no indication during the SARS pandemic that SARS-CoV became widespread in house cats or was transmitted from cats to humans.”

Given the continuing lack of strong evidence that domestic cats are likely to transmit SARS-CoV-2 to other cats, and the current lack of evidence that they can pass it on to humans, Prof. Saif and other specialists have urged further studies on this topic.

Yet there are already reports of panic-stricken owners looking to rehome their feline companions amid fears that their cats might cause them to contract the new coronavirus.

This is despite bodies, such as the World Small Animal Veterinary Association (WSAVA), continuing to emphasize the lack of evidence in this respect.

“To date, we confirm that there is still no information proving that SARS-CoV-2 infection in a dog or cat can be passed on to other animals or to humans,” WSAVA officials write in their latest update from March 26, 2020.

“The focus in the control of COVID-19 […] undoubtedly needs to remain firmly on reducing the risk of human-to-human transmission,” Prof. Dirk Pfeiffer, an epidemiologist affiliated with the City University of Hong Kong, also stressed in a comment for Nature.

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