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Malaria surges in deforested parts of the Amazon, study finds

  • A recent study found that deforestation significantly increases the transmission of malaria, about three times more than previously thought.
  • The analysis showed that a 10 percent increase in deforestation caused a 3.3 percent rise in malaria cases.
  • The study’s authors analyzed more than a decade of data showing the occurrences of malaria in nearly 800 villages, towns and cities across the Brazilian Amazon.
  • They also controlled for the “feedback” from malaria, by which a rise in the incidence of the disease actually slows deforestation down.

Tropical deforestation may spur the transmission of malaria at levels much higher than once thought, according to a recent study.

Disease ecologist Andrew MacDonald and his Stanford University colleague Erin Mordecai analyzed more than a decade of data showing the occurrences of malaria in nearly 800 villages, towns and cities across the Brazilian Amazon. They also looked at satellite-tracked deforestation over that same time frame.

Understanding the effect each variable has on the other is tricky. That’s because the rise in malaria cases that follows deforestation appears, in turn, to diminish continued deforestation. The disease slows local economies and discourages people from settling in high-malaria areas.

A soy field abuts the forest in the Brazilian Amazon. Image by Rhett A. Butler/Mongabay.

MacDonald and Mordecai found that a 1 percent rise in the incidence of malaria corresponds to a 1.4-percent dip in deforestation. This “feedback” has clouded conclusions from earlier research about deforestation’s true effect on malaria, MacDonald, now based at the University of California, Santa Barbara, said in an email.

To cut through the confusion, the researchers controlled for malaria’s impact on deforestation in their analysis. They then found that previous estimates had been about three times lower than the actual effect of deforestation on malaria.

The team calculated that a 10 percent rise in deforestation led to a 3.3 percent average increase in malaria transmission. For 2008, that amounted to another 9,980 cases across the Brazilian Amazon. They reported their results in the journal Proceedings of the National Academy of Sciences on Oct. 14.

Plasmodium falciparum, the parasite that causes malaria. Image by Mae Melvin/CDC via Wikimedia Commons (Public domain).

Geographically, deforestation touched off more malaria in the midst of the Amazon rainforest than it did around its fringes, probably due to how deforestation happens in the forest’s interior, MacDonald said.

“Here there is small scale clearing for new settlements, which creates more forest edge habitat (and more mosquito breeding habitat), as well as increases human biting rate and human-mosquito contact rate,” he said. “In the outer Amazon, forest patches are fewer and farther between with less human contact with forest edge habitat.”

A capuchin monkey in a Brazilian forest. Image by Rhett A. Butler/Mongabay.

Still, MacDonald added, it’s unlikely that malaria alone could stop deforestation altogether. What is clear from the data is that the advantages of preventing deforestation could extend beyond keeping carbon locked away in the trees and maintaining functioning ecosystems.

“I think these results suggest that if conservation actions are focused on the large intact regions of the interior of the Amazon rainforest, it may have benefits for health in terms of reductions in malaria transmission,” he said.

Banner image of an Anopheles mosquito by Jim Gathany/CDC via Wikimedia Commons (Public domain).

John Cannon is a staff writer at Mongabay. Find him on Twitter: @johnccannon

Citation:

MacDonald, A. J., & Mordecai, E. A. (2019). Amazon deforestation drives malaria transmission, and malaria burden reduces forest clearing. Proceedings of the National Academy of Sciences, 201905315. doi:10.1073/pnas.1905315116

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