Before coronavirus disease 2019 (Covid-19), there was SARS, Zika, HIV, Ebola ... For half a century, epidemics of infectious diseases have multiplied around the world and experts agree on the origin of this acceleration: the profound transformation of natural environments caused by human activity. These researchers point to a chain of responsibilities, with which we are all associated as consumers or travelers: deforestation, hunting and trade in wild animals, globalized trade, etc.
"We are approaching these viruses, they are not the ones that jump on us," summarizes the biologist Barbara Rhétoré. The current pandemic linked to the coronavirus does not tell another story: the hypothesis favored for the moment is that of an agent transmitted by a bat, via a specimen of pangolin, the most poached species in the world, sold on a Wuhan market.
At the source of epidemics, the destruction of ecosystems. And it is hardly necessary to raze a forest to change its balance, says entomologist Philippe Grandcolas. “It doesn't have to be devastation. It can be a lot more insidious. If you exploit environments, if you deplete them, by selective exploitation in a forest, by cutting large trunks, by making paths or by hunting, new equilibria settle in the groups of organisms which are interacting with each other, ”explains the CNRS research director.
Vulnerable animals
"The rarefaction of species is something a little paradoxical," notes biologist Serge Morand. “By losing species, you lose pathogens. But in an environment, viruses and bacteria are also regulators, just like predatory animals. If there is a loss of diversity, there are fewer interactions, resilience and stability, ”continues the research director at CIRAD (Center for International Cooperation in Agronomic Research for Development).
Philippe Grandcolas observes that "in simplified ecosystems", the reservoir species, that is to say the one in which the virus reproduces, can become "overabundant": "It is no longer controlled by other species which are its natural antagonists and it is more likely to transmit infectious agents. Before the disappearance of the species, it is the collapse of their populations that worries Barbara Rhétoré. “From a genetics point of view, the immune defenses are weakened when the specimens become scarce. Populations are less resistant to changing effects and the emergence of diseases. "
Imbalances can thus occur within the very species carrying a virus, develops Philippe Grandcolas: “We all have within us a lot of microbes that interact with each other. In the same way as in humans, taking antibiotics can promote the appearance of yeast infections, if you oversimplify the reservoir species, there are dominances that are established and that can become harmful. It can be billiard effects with several bands: if we influence several species, it will have actions on yet another species which will cause us problems. "
Too close to humans
For a pandemic to occur, the virus must meet humans. Large-scale destruction of natural habitats, for example to produce palm oil or soybeans, encourages infected species to migrate while humans encroach on wilderness areas to make a living from farming and hunting. "The installation of populations on the edge of ever more fragmented forests exposes them to microbiological hazards", explains Jean-François Guégan, director of research at the National Institute for Agronomic Research (INRAE). "99.99% of microbes are positive germs," he recalls. It is only because of the encounter with other species that they do their evolutionary work, adapt and cause disease. But, according to this specialist in the transmission of infectious diseases, "nasty beasts out of the woods, there are larger and more virulent than the Covid-19".
The Nipah virus, which appeared in 1998, is a dramatic illustration of this. Threatened by deforestation and fires in Borneo, bats fled to the milder areas of Singapore and Malaysia to find food. They have taken up residence in orchards and on pig farms. Reveling in rambutan fruit, they also defecated on these trees, contaminating the animals. “Pigs have been amplifiers of the virus. Farmers have also contracted it, with mortality rates of 70%, ”continues Jean-François Guégan. Recalling that Singapore and Malaysia are Muslim countries, Serge Morand notes that the production of pork was not dedicated to local trade. “Neither the pigs nor the bats should have been there. This little story, it repeats itself more and more. "
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The Covid-19 is a glaring example: the bushmeat trade also has deleterious effects. All of the animals that have never had to experience such a level of promiscuity are housed together: pangolins, snakes, etc. A culture broth favorable to the passage of the "species barrier" by certain viruses. "To be able to spread optimally, a virus must not be too dangerous for the host, or else it must kill it very slowly, otherwise it eliminates itself," observes Gilles Kleitz, director of the ecological transition and resources department. natural to AFD. In a situation of high confinement between animals, even if there is a high morbidity, the virus will constantly find hosts. This is how aggressive strains are selected despite everything, whereas in nature they would rather be eliminated.
Strengthen controls
What to do ? “There are solutions, reassures Gilles Kleitz. They require courage, political will and to accept that there are things that change radically. This agronomist by training insists on the sanitary supervision of breeding. “Over the past 40 years, veterinary services have been dismantled and liberalized in emerging countries, such as China, India and Brazil. The latter have seized economic opportunities to intensify their breeding in very poorly controlled conditions. Among the first solutions, there are controls on the markets, border and import tests and precise monitoring of the veterinary status of farms. "
Experts also believe that combating poaching and the uncontrolled bushmeat trade is essential. But according to Gilles Kleitz, who talks about the emergence in recent years of "pathogen freeways", it requires much more resources than currently: "Policies for biodiversity conservation and species protection in southern countries is less than 1 / 10,000, or even 1 / 100,000, of public budgets. Poor gamekeepers are usually absolutely helpless to deal with this kind of thing. "
The sale of live animals on the markets is a specificity of Southeast Asia. If wildlife and its habitat are particularly abused, should we conclude that only this part of the world should change its practices? Pointing to the "very worrying problem" of Lyme disease, Philippe Grandcolas evokes a local drift: "Currently in France, foxes are considered harmful. We take orders and kill a million foxes a year. This is based on old ideas that foxes are transmitters of rabies. By killing so many foxes, we give free rein to populations of rodents, which are nevertheless a reservoir of Lyme disease. What if the chicken thief wanted us good?