A review of nearly 1,000 studies, published this Wednesday, May 8, shows that human activities increase the risk of infection affecting plants, animals and individuals. Nature loss is the leading factor, followed by invasive species, climate change and chemical pollution.
Ecological upheavals caused by human activities, and primarily the loss of biodiversity, increase the risk of transmission of infectious diseases. This is the conclusion of a scientific article published this Wednesday, May 8 in the journal Nature by American researchers.
Until now, other work had certainly already shown that the risk of infection was modified by global changes such as the loss of biodiversity, global warming, chemical pollution, the transformation of landscapes or the introduction of invasive exotic species. . But the relative responsibility of these causes in the increase in diseases remained uncertain. The researchers therefore examined data from 972 studies and identifying nearly 3,000 observations of “answers” global changes in infectious diseases across 1,497 combinations “host-parasites”, THE “hosts” which can be plants, animals and humans.
Lyme disease
Result of this meta-analysis: the collapse of biodiversity is the factor which most promotes the risk of infectious diseases. Next come, in descending order, the role of invasive species, climate change and chemical pollution (in particular due to synthetic fertilizers and pesticides). The effects of this pollution on infectious risks are significantly less significant (393% less) than those linked to the loss of biodiversity.
How can we explain that biodiversity, or rather its disappearance due to homo sapiens, is this predominant among the causes of increased infectious risk? “Anthropogenic factors generally first cause the loss of rare species before that of abundant species, which therefore increases the proportion of abundant species in a given place, but these are the ones that transmit the most “effectively” the infections, because parasites infect them as a priority,” answer to Release the main author of the study, biologist Jason Rohr, professor at the University of Notre Dame, in Indiana (United States).
And to cite the example of Lyme disease in the United States. “White-footed mice are the most abundant and ‘competent’ host species at transmitting the disease-causing bacteria, carried by ticks, while larger mammals tend to be less abundant and ‘competent’ than mice, he explains. As humans shrink their natural habitat, larger mammals are disappearing because they need more space than mice. This further increases the relative proportion of mice in the landscape, which ticks love: the loss of biodiversity therefore increases the risk of transmission of Lyme disease.
Urbanization
On the other hand, notes the study published this Wednesday, urbanization is associated with a drop in the transmission of diseases. Which seems surprising, since it also contributes to the erosion of biodiversity, in particular by eating away at soils and ecosystems.. “It’s true, urbanization causes a loss of biodiversity, but to such an extent that all the wild “host” species of many parasites disappear, answers Jason Rohr. In fact, the relationship between biodiversity loss and infectious diseases is not linear: a certain level of loss can increase the risk, while a very large loss can reduce it, because all parasites need hosts to survive. So a complete loss of habitat and biodiversity can eliminate some diseases, but that said, the loss of species caused by human activities is generally associated with an increased risk of disease.
According to researchers, the reason why urbanization often leads to a reduction in disease is mainly because it is associated with better access to water, hygiene and medical infrastructure for human populations. “What is interesting, notes Jason Rohr, is that most pathogens decline with urban development, except viruses, because many of them are transmitted directly from human to human, such as that of Covid-19, which can favor their transmission in urban environments densely populated.”
Better prevention
When it comes to climate change, researchers estimate that the increase in disease in response to it will be “consistent and widespread, emphasizing the need to reduce greenhouse gas emissions in order to mitigate these negative impacts”.
Overall, they conclude that the results of their meta-analysis “should help focus disease management and surveillance efforts” in order to better prevent the occurrence of epidemics affecting plants, animals and humans. For Jason Rohr, the most urgent thing is to fight climate change “because it increases the risk of disease and causes a loss of biodiversity, which also increases this risk.”
