Can cats and dogs catch Covid?

Our very own Chris Smith took on the challenge...

So can cats and dogs, and other animals, catch covid? Or, more accurately, can they catch SARS-CoV-2, the coronavirus that causes Covid-19 disease in humans?

The answer is absolutely, yes. The Covid-19 coronavirus is actually not naturally an infection of humans: it's what's known as a "zoonosis" - an infection that has spread to us from animals.

And while the jury is still out on whether the virus escaped from a Chinese laboratory along the way, it nevertheless almost certainly began as an infection in a bat; we know this because the genetic codes of coronaviruses carried naturally by bats is remarkably similar to the genetic code of the Covid-19 SARS-CoV-2 virus.

Around the world billions of people have caught the infection, and many of them have exposed their pets, and their farm animals in the process.

There are reports of household cats and dogs, as well as mink, ferrets and polecats testing positive for the virus.

And in the Chinese city of Wuhan, where the pandemic began, 15% of the feral cats tested in the aftermath of the outbreak were positive for antibodies to the virus, proving they'd been exposed, possibly through scavenging through human waste.

These animals are susceptible to infection because they are mammals like we are, and share similar biochemistry making them targets.

But they also seem to fare better than humans, on average, when they catch the disease, so why is that?

The reason is that viruses evolve to become highly optimised to their natural hosts.

The best outcome for any virus is to be minimally disruptive for their host, while achieving maximum infectivity and therefore transmissibility.

Disable your host too significantly, or kill them even, and they won't go about their business infecting others quite so readily! You'll also eventually run out of victims to infect.

So in its bat host, the Covid-19 virus is relatively benign. But when it jumps the species barrier into us, the virus is ill-adapted to our physiology. The immune evasion that works well in a bat turns into a massive case of overkill in some people.

But in your dog or cat, on the other hand, the reverse is true: the virus struggles to gain much of a toehold; it cannot grow very efficiently and the infection tends to smoulder rather than turn into an inferno before the immune system smothers it.

And it's not just a one-way street. Some infections trivial for humans are lethal to wild animals. Two viruses that cause the common cold in us, called RSV and HMPV, can wipe out chimpanzees, killing up to one in five young animals for the same reason: that adaptations that make these viruses better bed fellows for us can render them lethal for even our close animal cousins.

So this is why biologists, conservationists and healthcare and animal practitioners talk about "one health": viruses can and do spread between species with highly unpredictable results; and that's why we need to try to keep tabs on them.