COVID-19: From humans to animals and then back to humans

Although the novel coronavirus SARS-CoV-2 is a zoonotic disease—that is, one of animal origin—it currently infects only humans, with the exception of a few rare cases. But scientists warn that a return to animal populations cannot be ruled out. The virus, which likely originated in bats, is thought to have passed through pangolins before infecting humans. But researchers are concerned about a possible anthroponosis of SARS-CoV-2—that is, the virus spreading from humans to animals—which could turn animals into a reservoir for the disease. This concern is particularly acute in light of the recent discovery of the virus spreading in mink farms in the Netherlands and Spain, from which it is believed to have been transmitted back to humans. This larger-scale transmission cycle does not bode well for the prospect of the virus's re-emergence in humans if it is not controlled. Even if the probabilities are very low, the implications are too serious to be dismissed. Scientists are calling for extensive surveillance of pets, livestock, and wildlife. To date, researchers have gathered experimental evidence of infection and, in some cases, transmission of SARS-CoV-2 in certain mammals such as monkeys, ferrets, cats (domestic and wild), dogs, hamsters, minks, squirrels, pigs, and wild boars—all of which could hypothetically become intermediate hosts. Modeling of the molecular interactions between the virus and its potential entry point in animals—the ACE2 protein—shows that infection is also possible in rabbits, sheep, goats, cattle, and horses. However, clinical manifestations and the possibility of asymptomatic cases cast doubt on the actual risks involved. For now, animal studies have been too small, limited, and sometimes even contradictory to confidently make certain claims that could have serious consequences for public health. Experimental and computational studies alone will not be sufficient to confirm that a species cannot be infected with SARS-CoV-2.

A multidimensional approach is necessary, incorporating real-world epidemiological and diagnostic studies as well as sampling a large number of animals before determining the possibility of infection.

The risk assessment must also take into account our ability to isolate, protect, or contain domestic, farm, and wild animals. Because once SARS-CoV-2 begins circulating beyond human populations, it will become very difficult to track the virus’s evolution and interrupt its transmission back to humans. Early surveillance and preventive measures are essential.