Text: Irina Yakutenko, molecular biologist, science journalist
The text was released in late February, when the world was still ‘open’, but the Russian government suspended all dog and cat shows. By now, the world has changed, the number of corona-infected is over a million people, but the scientific argumentation why pets are not dangerous and humans should not avoid contact with them still makes sense. (remark by the editor).
In December 2019 China was the first country to face the new coronavirus SARS-Cov-2
The disease has turned out to be surprisingly ‘sticky’, spreading throughout the whole territory of PRC and beyond. As of February 20th, there had been almost 76 thousand infected people around the world.
Given the unprecedented scale of the outbreak, governments began to implement all sorts of containment measures – from mandatory quarantine to restrictions on flights to China. The measures that limit contact of humans with animals, the potential virus transmitters, are also under consideration. Thus, the FSVPS (Federal Service for Veterinary and Phytosanitary Surveillance of Russia) has offered to cancel all mass gatherings with demonstration of animals, including domestic ones. Apparently, cat and dog shows, which are largely popular in big cities, fall under the prohibition. Is this reasonable? Is transmission of the virus from these animals to humans even possible?
Coronavirus SARS-Cov-2 is a member of the large Coronaviridae family. Its representatives do not only infect Homo sapiens, but also a wide variety of animals – from ferrets, bats and pangolins to dogs and cats. Typically, coronaviruses (of various types) cause mild respiratory or intestinal symptoms. But they can sometimes cause serious diseases with high mortality, such as atypical pneumonia (SARS) and Middle East Respiratory Syndrome (MERS) in humans, or infectious peritonitis in cats.
The widespread prevalence of coronaviruses among representatives of those species that are very distant from each other is a consequence of viruses’ increased ability to mutate and even ‘steal’ genes from other viruses. Viruses are prone to do this in general, but the Coronaviridae genome changes much more easily due to the fact that it isn’t coded with the DNA molecule, as it is in human bodies, for example. It is encoded within a much less stable RNA. This is one of the key reasons why they have an increased ability to penetrate interspecific barriers. Thus, in all three cases of human coronavirus epidemics (SARS, MERS and the current Wuhan coronavirus SARS-CoV-2), the diseases were zoonotic, which means that they have leaped over to us from animals.
If so, could it be that prohibiting cat and dog shows is a justified measure that can prevent a big disaster like the one we are watching live in China? Of course not. And here is why.
The first argument against such restrictions was formulated back in 1950 by the great physicist Enrico Fermi – even though for an absolutely different reason. During his stroll with colleagues, their conversation turned to aliens. Having heard the arguments in favor of their existence, Fermi, after long silence, suddenly asked: “But if everything is like this – where are they?” And although the good story is most likely anecdotal, the essence of the argument doesn’t change. Cats, dogs and all of the most important farm livestock, such as pigs, cows, chickens and turkeys, have lived next to humans for thousands of years and carry many different coronaviruses. Nevertheless there was not a single case of a dangerous or at least in any way infectious coronavirus leaping over from them onto people and beginning to actively reproduce among Homo sapiens during all that time.
The absence of such examples is not just a mere accident. Animals differ a lot in their ability to ‘create’ new viruses that can overcome interspecific barriers. There are many reasons for this inequality – starting with the excessively strong processes in the animal immune system, which do not let viruses multiply enough to start mutating. So they end up with significantly different mechanics of entering host body cells. In the case of Coronaviridae (and not only this family), bats are the main ‘suppliers’ of viruses that are dangerous for humans and other species.
There are several reasons for this: their ability to fly, for example. Flying requires so many resources that part of their cell’s nuclei literally breaks down and ‘cracks’, releasing DNA. Free-floating DNA fragments in the cell are a typical sign of viral infection, this is why after finding such a deformity, the immune system of most animals initiates an inflammatory process, which is often dangerous for the host. But that is not the case when it comes to the immune system of bats. Since DNA fragments are constantly present outside the nucleus, the immune system has learned to ignore them and soften the protective response. And when a virus appears in the bat’s body, it also may not be threatened by the immune system’s ‘anger’. On the other hand, recent work has shown that when the virus is detected in these animals, the interferon response is triggered rapidly. However, it does not take the path of developing dangerous inflammation. The peculiar reaction of the immune system prevents viruses from infecting new cells, but often does not kill them all. As a result, pathogens have a unique opportunity to persist in the body for a long time and mutate.
Cats, dogs or cows don’t have such talents – although we can’t be 100% sure that a dog or a cat virus will never leap over onto humans. However, this statement also wouldn’t have been eligible 5, 10, and 50 years ago. The fact that the world is suddenly afraid of a new infection did not change the biological mechanisms. The existing barriers that prevent the transfer of feline or canine coronaviruses to humans have not broken. Moreover, the vast majority of recent zoonotic infections are associated with the fact that either large populations animals, the potential carriers of viruses, or people who deal with those animals, suddenly find themselves in new conditions, for example, due to large-scale deforestation and mass migrations of their inhabitants to new territories. But our interaction with cats or dogs has not changed over the past 5–10–50 years, so there are no risk factors here either.
Moreover, there is no confirmed evidence that the new coronavirus is widely transmitted from animals to humans. Instead, humans infect one another. The vast majority of the current infected patients didn’t get the virus from bats or any other intermediate hosts – they were infected by neighbours and random bypassers.
And finally, it isn’t clear why it’s necessary to forbid the exhibitions. If we are to apply the ‘gun-shy’ strategy, we should terminate all the domestic animals that have coronaviruses and switch to vegetarianism. Otherwise, anything else seems to be a halfway measure.
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