Cats: carriers of COVID-19?
Cats have become the focus of several COVID-19 related news articles in recent weeks...
First it was a cat in Belgium with respiratory disease that tested positive for the virus. Then a COVID-19 positive tiger at the Bronx Zoo in New York created a media storm. Nevertheless, now several weeks later, only a handful of reports of felines testing positive have surfaced worldwide. Is this the tip of the iceberg, or an actual reflection of the low risk to cats?
Scientists have been trying to establish just how susceptible cats are to COVID-19. The most controlled way to do this is by experimental infections. Two studies have now been published that describe cat infections with SARS-CoV-2 in laboratories. The first study was conducted in China and published in Science on 8th April 20201. The second was performed in the US and published in the New England Journal of Medicine (NEJM) on 13th May 20202.
Cats in both studies were infected with large quantities of virus directly into their noses, with the US study also infecting cats via their mouths, tracheas and eyes whilst the cats were anaesthetised. Results showed that the cats became infected with SARS-CoV-2 because the virus was subsequently readily detectable in nasal swabs collected later.
The main question asked by the NEJM study was “can infected cats pass SARS-CoV-2 onto other cats?”. To answer this, the researchers infected three cats in separate cages, then introduced a new uninfected cat into each cage. The new cats were tested daily for the presence of infectious virus. These three newcomer cats all became positive for virus within 5 days, confirming that cat-to-cat transmission can occur in a laboratory setting like this. But what does this mean for people and their pets?
The dose of virus used experimentally to infect the cats is likely to be significantly higher than the virus dose that cats naturally receive if they are living with an infected human. This means that the risk of pet cats becoming infected in the first place will be much lower. A Chinese survey in Wuhan has found that some cats living in the region have antibodies to SARS-CoV-23, suggesting natural infections can occur, but how common this is in other countries we do not know yet.
An important finding from the new experimental study was that none of the infected cats showed any symptoms of disease. This is reassuring from an owner’s perspective: there appears to be minimal chance of a pet getting sick from SARS-CoV-2.
Nonetheless, the NEJM study authors hypothesised that the lack of clinical signs could lead to silent transmission of virus between cats and their owners. At present, there is no solid evidence to support this idea. Also, we do not know the dose of virus it takes to infect a cat or a human, and we don’t know what dose of virus cats might be exposed to in an infected household.
Only one study so far has examined the risk of infection in cats naturally exposed to SARS-CoV-2. Nine pet cats living closely with vet students suffering from COVID-19 were tested evidence of infection. Encouragingly, no cat had any virus or virus-specific antibodies4. Further research that moves away from experimental infections and focuses on monitoring natural infections in cats will be essential for properly assessing the real risk to cats and their owners. Doubtless time will tell...