Climate change, destruction of habitats, proliferation of farm animals… Ongoing changes support the emergence of new infectious diseases. ANSES takes stock.
The Covid pandemic has recently underlined this once more: animal health and human health are closely linked. But what is the responsibility of wild species and farm animals in emerging infectious diseases? On the occasion of the International Agricultural Show, ANSES takes stock.
Gilles Salvat, Deputy Managing Director of the Research and Reference Division, recalls that humans share nearly 60% of infectious diseases with animals. While we have observed almost continuous emergence over the past few decades, 75% of emerging infectious diseases have an animal origin, whether they come from wildlife or domestic animals.
Closer contact with wild animals
The reference study on the role of wildlife in emerging diseases is that of Jones et al., published in Nature in 2008. Élodie Monchâtre-Leroy, director of the ANSES rabies and wildlife laboratory in Nancy, sums it up: “By studying all the emergences from 1940 to 2004, he shows that 43% of them have an origin in wildlife”.
And for good reason: humans are particularly sensitive to pathogens that they are not used to encountering. “However, wildlife carries a large number of micro-organisms which are foreign to man and once morest which he is not used to defending himself”recalls the veterinary doctor.
As soon as humans enter preserved environments of great biodiversity, such as primary forests, to exploit wood, metals or install large crops, it leads to the movement of wild species towards anthropized environments. “This disruption of ecosystems and this rapprochement favor the risk of virus transfer from animals to humans”summarizes Gilles Salvat.
Exchanges between wild and domestic animals
The intensification of animal husbandry also plays an amplifying role. Indeed, the concentration of susceptible animals facilitates the exchange of pathogens with wildlife, in the case of extensive farming, and the transmission of diseases within intensive farming. The H5N1 virus currently circulating between wild and domestic birds in Europe, and worldwide, is a good example. “As soon as there are more and more contacts between wildlife and humans or wildlife and domestic animals, we increase the probability that pathogens can pass to humans or animals. domestic and that there is an emergence”explains Élodie Monchâtre-Leroy.
Some species of wildlife also take advantage of the disruption of ecosystems to proliferate. This is the case, for example, of wild boar in Europe, or invasive alien species, such as the raccoon in France. These species develop in new territories, bringing with them the micro-organisms they harbor.
Finally, establishments of domestic species on the edge of primary forests can encourage contact with the wild. Gilles Servat recalls that the Nipah virus in Malaysia originated in the contamination of pig farms on the edge of primary forest in 1999. This virus, transmitted to pigs through the consumption of fruit contaminated by fruit bats, is responsible for a hemorrhagic fever. “There was probably a modification of this virus and currently in Bangladesh, we have contamination by this Nipah virus from contaminated date juice, probably by bats, and human-to-human transmission”shares the expert.
Climate change as a catalyst
Not to make things any easier, climate change also leads to a change in the distribution of disease vectors. In northern countries, it should thus lead to the emergence of diseases formerly confined to developing countries, whether in wild or domestic animals. “Climate change will have an impact on migratory species, in particular migratory birds, which might modify the distribution map of a certain number of diseases and the distribution of a certain number of vectors. [de maladies, ndlr] »explains Gilles Servat.
The tiger mosquito, now present throughout France, is a good example of this phenomenon. This pioneer is certainly not the last. “Certain species of ticks such as Hyalomma, originally from southern countries, are found around the Mediterranean, assures Gilles Servat. Hyalomma is the tick responsible for transmitting Crimean-Congo hemorrhagic fever. This tick is present in Corsica, on the French Mediterranean coast and in Spain we had the first autochthonous cases of Crimean-Congo fever. »