January 25-31 2021
The risks of transmission by animals
The coronavirus epidemics derive from animals, but human to animal transmission is poorly understood. A recent study carried out on mink on a Dutch farm (see the News-COVID-19.info 16-22 November newsletter) showed that these animals had been infected by men with SARS-CoV-2, and that other humans had later been reinfected by the mink (a process known as re-emergence). To date, this is the only study clearly showing transmission of SARS-CoV-2 from animal to human. Certain variants in animals could therefore infect us, calling into question our anti- SARS-CoV-2 immunity, whether natural or vaccine-induced. These variants could also compromise the effectiveness of certain therapies. It is therefore necessary to study the immunological, anthropogenic and ecological factors that may lead to such re-emergence.
Recent studies have shown that certain animals are susceptible to SARS-CoV-2 (cats, ferrets, hamsters, monkeys, mink, tree shrews, racoons, bats and rabbits). But numerous other species have not been tested. The authors therefore propose to study them using three criteria:
- the susceptibility of the species (or closely related species) to the virus
- human/animal ACE2 receptor homology
- the frequency of contact with humans
For example, in amphibians, ACE2 homology is weak and no cases of infection have been noted. However, in monkeys the level of homology is high but contact with humans is rare (except in zoos and pet stores). And while homology is moderate in cats, the frequency of contact with humans is very high. Dairy cattle, which have moderate homology, are frequently in contact with humans, but their susceptibility to the virus is low. Amongst these species, domestic cats should therefore be monitored as a priority. And the variants that develop within the species need to be identified.
Why is this important? Because the risk of human-to-animal transmission is that alternative, unknown “reservoirs” are formed, that is, animals carrying the virus and likely to infect humans. They give the virus the opportunity to evolve through selection of modified strains and to change its pathogenetic or transmission potential. As within humans, this evolution occurs through the accumulation of mutations but also by recombining (different viruses infecting the same host can exchange genetic material). This is how viruses acquire the capacity of changing hosts, causing viral emergence. Specifically, a spike protein adapted to animal receptors can interact with the human ACE2 receptor and infect humans, without being recognized by neutralizing antibodies.
This can be represented schematically as follows:
As with humans, animals showing clinical symptoms can be isolated, but animal reservoirs are often asymptomatic. What should we do? The authors propose increasing monitoring of animals, practicing distancing in high-risk areas (farms, pet stores), restricting animal movement, wearing protective equipment, disinfecting, monitoring personnel in contact with animals, and limiting access. The most difficult issue is controlling wild animals and ensuring that they don’t have contact with domestic pets. Bats require particular attention since they are endemic carriers of many coronaviruses and are frequently found in mink and ferret farms. In areas exposed to wildlife, protection must be reinforced. The authors underline that in the future, such viral emergences may become more frequent due to changes in our relationship with nature caused by deforestation, climate change and urbanization.