collapsing frog populations lead to outbreaks of malaria

Red-eyed tree frog, 'Agalychnis callidryas', in the rainforest of Costa Rica, May 2019.

Lhe One Health concept, better known by its original name One Health, has invaded public health discourse. Adopting or promoting an integrated approach of the human, animal and environmental spheres to face the health crises to come almost takes the place of an imposed figure to run for a position or funding. All researchers know the risk: hiding a poor science behind a rich concept.

The study just published by the journal Environmental Research Letters promises, on the contrary, to make a date. For the first time, a team of researchers highlights the link between the collapse of an animal population and a human health crisis. And not just any: on the one hand the carnage caused by the fungus Batrachochytrium dendrobatidis (Bd), which makes amphibians the most endangered animal group, according to the International Union for Conservation of Nature. On the other, malaria, a scourge responsible for 627,000 deaths in 2020.

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The researchers set sail for Costa Rica and Panama. Before the arrival of Bd in Central America in the mid-1980s, these two countries were jewels of amphibian biodiversity. The fungus has wreaked havoc there: 500 species have declined, 90 others have purely and simply disappeared.

Karen Lips, of the University of Maryland, had followed the crisis at the forefront. “During a discussion, we wondered what such devastation could have caused on human healthsays Michael Springborn, environmental economist at the University of California Davis. As amphibians are known to eat mosquitoes and mosquitoes transmit malaria, we decided to go and see what was happening with malaria attacks in these two countries. »

“A natural experiment”

The two researchers and their colleagues knew they could have detailed data on the progress of the fungus: year after year, the local authorities have listed its progress, across the 136 cantons, from the northwest to the southeast of Costa Rica, between 1986 and 1993, then from west to east of Panama, from 1993 to 2010.

They collected information, on the same scale, on malaria cases. The result is spectacular: three years after the first significant declines of frogs, toads or salamanders, the cases explode and remain on a high plateau (more than one person in a thousand) for six years, before falling.

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The mechanism seems quite obvious: amphibians devour insects, especially their larvae, tadpoles gorge themselves on mosquito eggs and larvae floating in ponds. In what proportion? In the absence of data on mosquito densities in Central America, the researchers suggest rare work conducted elsewhere: a study carried out in Indiana in 2003 showed a 98% drop in mosquito larval populations in the presence of salamanders. And the conclusion to impose itself: fewer amphibians, more insects and therefore more malaria.

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