If the pandemic continues, the risk that other animal species may be infected by Sars-Cov-2 will increase, thereby multiplying the possibilities of virus mutation and the likelihood of animals ultimately helping to spread the virus. It is therefore necessary to adopt an approach incorporating the animal dimension in the fight against coronavirus.
“In an epidemic context, cats and dogs need to be considered as human beings,” says Eric Leroy, an IRD researcher specialising in emerging viral diseases within the MIVEGEC (Infectious Diseases and Vectors: Ecology, Genetics, Evolution and Control) joint research unit. The expression is in no way intended as a provocation, but refers to the need to apply a “One Health” strategy in the fight against the Covid-19 epidemic. The “One Health” approach stipulates that human health, animal health and ecosystem health are all closely interlinked and should be tackled as a whole.
To support his statement, Eric Leroy, a member of the French National Academy of Medicine and the French Veterinary Academy, draws on various elements. First of all, an analysis of four cases of pet contamination (two cats, two dogs), whose owners had Covid-19, as well as the contamination of four tigers and three lions in a zoo. “Although not all the animals developed symptoms, a PCR analysisA method used in molecular biology to amplify a segment of DNA or RNA and measure its quantity demonstrated the presence of the virus in all of these animals, at low doses in all but one of them”, highlights the researcher. The fact that the owners of the domestic animals were infected with Covid-19 suggests human-to-animal contamination.
Genetically similar coronaviruses
To this can be added what is known about the routes of contamination of Sars-Cov-2, the coronavirus responsible for Covid-19: it is transmitted mainly through respiratory droplets – or sputum – but also when hands come into contact with the mouth after touching soiled surfaces. “In addition to living together, licking or kissing represent additional risk factors facilitating the transmission of the virus between infected people and their pets,” concludes Eric Leroy.
While transmission from humans to animals appears to be an acknowledged fact, other factors also combine to support a One Health approach. One of the main ones is the close genetic proximity between the coronaviruses that infect humans and those that infect carnivorous animals. Sars-CoV-2 actually belongs to a broad group of viruses – the Coronaviridae family – that can be subdivided into two subfamilies. One of these – the Orthocoronavirinae subfamily – is divided into four separate generaSubdivision of subfamilies in the classification of living organisms: Alphacoronavirus (alpha-CoV), Betacoronavirus (beta-CoV), Gammacoronavirus (gamma-CoV) and Deltacoronavirus (delta-CoV). “At present”, specifies the virus specialist, “seven coronaviruses are known to infect humans: these belong to the alpha-CoV genus or the beta-CoV genus. Just like the coronaviruses known to specifically infect cats and dogs.”
A high potential to evolve
Another characteristic of Sars-CoV-2 makes it a virus capable of crossing the species barrier: the low specificity of its Spike protein, which attaches onto a receptor on the outside of human cells and enables the virus to penetrate inside these same cells. Therefore it may potentially bind to the equivalent receptor in another species, much like a master key capable of opening doors with similar locks. This low specificity of the S protein, and hence its high breaking and entering capacity, can be linked with the high zoonotic potential of the coronaviruses found in several species: Sars-Cov, which caused the SARS (severe acute respiratory syndrome) outbreaks in humans in 2003, was transmitted via the masked palm civet (Paguma larvata), the Rhinolophus bat and the raccoon dog (Nyctereutes procyonoides), and Sars-CoV-2 probably via the Rhinolophus bat, the Sunda pangolin (Manis javanica), and hence dogs and cats, as described above.
In fact, other parameters also facilitate this cross-species transmission, such as the size of the viral genome. “In the field of evolution, mutations are actually errors that occur during the replication of genetic material”, goes on Eric Leroy. “And the bigger the genome, the greater the likelihood that errors will occur.” In addition, coronaviruses demonstrate a high capacity to swap genome fragments, a phenomenon known as recombination. “It’s bit like swapping over our right arms when we meet another person”, explains the researcher. If an animal is infected with two different coronaviruses, they will therefore be able to swap a fragment of their genome and form new versions of the virus.
To cap it all, the molecule responsible for multiplying the virus, RNA polymerase, does not have very high fidelityCapacity of the enzyme to insert the right nucleotide in the right place during RNA replication, increasing the probability of generating mutations.
Physical distancing for all
It is thus the accumulation of all these characteristics that gives Sars-Cov-2 a high evolutionary potential and hence the possibility of crossing the species barrier more easily than other viruses. “If the epidemic continues, contacts between infected people and animals will automatically be multiplied,” warns Eric Leroy. “New cases of domestic animal infection will inevitably occur, with a not insignificant risk of seeing the virus evolve following transmission to animals. However, a risk of pets contributing to the spread of the epidemic and of these animals themselves becoming infectious has not been demonstrated so far.” As a precautionary measure, we should therefore apply a One Health strategy, incorporating the animal dimension. What does this actually mean? “If someone in the household is infected with the virus, pets should be considered in the same way as humans, and the same health and physical distancing measures should be applied”, concludes the specialist.