“The more we deforest, the more we lose biodiversity, the more epidemics we have… It’s the factory of pandemics”, explains Serge Morand, parasitologist at CNRS-CIRAD (Center for International Cooperation in Agronomic Research for Development), one of the French pioneers in ecology of health. A few seconds earlier, in the middle of a Thai forest, the biologist had just shown this correlation on three maps, each tracing the evolution of the three indicators mentioned above, all of which streaked Southeast Asia in an alarming dark red.
All under the watchful eye of actress Juliette Binoche and the camera of director Marie-Monique Robin. Known among other things for her investigation of the multinational Monsanto – The world according to Monsanto (ed. La Découverte), the journalist this time focused on the link between the emergence of new infectious diseases and the loss of biodiversity. A subject far from being limited to Covid-19. The WHO counted a new infectious disease every fifteen years until 1970, when the rate today is between one and five emergencies a year. In 70%, they are zoonoses, diseases present in animals before being transmitted and developing in humans. This is the case with coronavirus, but also with AIDS, Ebola virus disease, dengue, Lyme disease, chikungunya…
Being able to predict where epidemics will arise?
From her investigation, Marie-Monique Robin made a documentary, broadcast on Sunday, May 22, on TV Ushaïa. She takes Juliette Binoche aboard through eight countries, to meet a dozen health ecologists in their fields of study.
All point to this correlation between the human impact on ecosystems – deforestation, urbanization…-, the associated loss of biodiversity and the emergence of new epidemics. To the point of being able to predict where they will appear? Interviewed in the documentary, Rodolphe Gozlan, research director at the Institut de recherche pour le développement (IRD), and Mathieu Nacher, an epidemiologist, identified and compared these emergence factors in a study published in September 2019. China (including Wuhan, where the first cases of Covid-19 will be detected a few months later).
Serge Morand cites other hotspots: “West Africa, especially in the coastal strip, the Rift Valley, target regions in Brazil, but also in Europe, where there are high concentrations of livestock, such as the Netherlands.” But the most obvious hot spot, for the parasitologist, remains Southeast Asia. “Point your compass at Bangkok and draw a circle with a radius that goes all the way to Pakistan, he invites. Within that perimeter, which encompasses India, southern China and Japan and all of Southeast Asia, you have half the world’s population living. »
Meetings that shouldn’t have happened
This density is accompanied by “an impressive and increasing concentration of domestic animals, but also very significant environmental changes”, continues Serge Morand. Destruction of forests to make way for commercial plantations, such as oil palm, but also, more broadly, the conversion of traditional agricultural systems, smallholdings and varied crops, to more intensive, export-oriented production crops. Like corn in Thailand, which the population does not consume. »
The whole thing causes encounters that shouldn’t have happened and that can lead to new epidemics. Inside The pandemic factory, Serge Morand takes as an example the bat of the genus Pteraupus, reservoir of the Nipah virus, whose mortality rate is at least 40% and which spread in southwest Malaysia in 1999. This frugivore initially lived on the island of Borneo, where he saw his habitat melt under the effect of deforestation, says Serge Morand. Enough to force her to look for other territories, where there are fruit plantations. Like the mango plantations in Malaysia, at the foot of which the pigs, raised for export in this Muslim country, are placed in the shade. “Bats eat mangoes, defecate on pigs and infect them,” continues the parasitologist. We are starting to have mortality in pigs, then in humans in contact with them (breeder, slaughterhouse workers in Singapore, etc.)”
The dilution effect
The pandemic factory draws many other examples. But the documentary also takes the opposite view, showing how restoring biodiversity can help nip epidemics in the bud. This is the dilution effect, a mechanism brought to light by the Americans Richard Ostfeld and Félicia Keesing. The research duo is working on Lyme disease, transmitted by a bacterium of which white-footed mice are the reservoir in the United States and ticks are the vectors. In other words, ticks that come on this rodent to gorge themselves on blood have a 90% chance of being infected and, in turn, will transmit it, during their next meals, to other animals or walkers.
Richard Ostfeld and Félicia Keesing place the white-footed mouse in the category of “competent hosts”. But most animals that live in the forest – deer, raccoons, bobcats… – are incompetent. Understand: they can carry the bacteria, but they transmit it much less easily to ticks. Some even participate in its regulation “Like the skunk that, when washed, kills 90% of the ticks present in it”, specifies Richard Ostfeld. “The presence of a great diversity of species in a territory dilutes the impact of those most at risk of infecting ticks with the Lyme disease bacteria”, explains Félicia Keesing. Ticks will simply be spoiled by choice of food and will be less so for white-footed mice.
Strengthening biodiversity in its broadest definition
This is the dilution effect. “It doesn’t work for all zoonoses, some of which are transmitted without vectors”, specifies Serge Morand. But the scientist emphasizes the great interest in maintaining a rich biodiversity. “Biodiversity in a broad sense”, he specifies, to remind us that the challenge is not just to guarantee the presence of a large number of species in a territory. “You also need a diversity of interactions, that is, with predators – the first to disappear, often when biodiversity starts to decline, it continues. They play a crucial role in regulating populations, including those of competent hosts. This is the case of the fox, an excellent hunter of mice, including those with white paws, but which she herself hunted bitterly for man.
To the diversity of interactions, Serge Morand adds that of habitats, which is equally essential. “By simplifying or fragmenting habitats, we weaken specialized species, adapted to live in very specific environments. [comme les grands prédateurs], continues the parasitologist. On the other hand, we favor generalists, who have the ability to live almost anywhere, including environments that have been greatly modified by humans. ” We agree with you: in generalists there are a large number of rodents, including the white-footed mouse.