Malaria has recently made a comeback in the Peruvian Amazon, and deforestation may be the underlying cause. New research shows that the risk of being bitten by the primary malaria-carrying mosquito is nearly 300 times higher in cleared areas than in those that are largely undisturbed—adding to the growing evidence that conservation is critical to human health.
“This is the first conclusive study showing the association between human biting and the degree of deforestation,” says University of Miami epidemiologist John Beier. This work is reported in the American Journal of Tropical Medicine and Hygiene by Amy Yomiko Vittor of Stanford University in California and seven co-authors.
Malaria has risen steeply in the Loreto district of northeastern Peru, with the number of reported cases jumping from about 600 in 1992 to more than 120,000 in 1997. Likewise, Loreto’s human population is growing fast, facilitated by the construction of a 95-kilometer road through the forest. The increasing numbers of people are clearing ever-widening swaths of forest along the road for plantain, cassava, and other subsistence crops.
Clearing forest for cropland also has the side effect of creating more breeding sites for the Amazon Basin’s most efficient malaria-carrying mosquito, Anopheles darlingi, which prefers to lay its eggs in deep water surrounded by short vegetation. This mosquito also prefers to feed on people and went from being extremely rare in Loreto in 1991 to being the most abundant Anopheles species collected in the area’s human settlements in 1995.
To see whether malaria is linked to deforestation in Loreto, the researchers determined A. darlingi human-biting rates and vegetation composition in and around 12 villages along the road through the forest. They collected mosquitos at four sites per village: the village center, a deforested area about one kilometer away, a secondary vegetation area, and a forested area. At each site, collectors sampled mosquitos every three weeks for a year, aspirating them off their own legs for 50 minutes per hour from 6:00 p.m. to midnight. The researchers used satellite images to characterize the landscape in a one-square kilometer area around each site, including forest and grass/crop land cover.
To ensure that malaria isn’t rising simply because there are more people in Loreto for the mosquitos to feed on, the researchers also compared A. darlingi biting rates along sparsely and densely populated stretches of the road. They found that increases in human population density had no impact on A. darlingi biting rates.
In contrast, deforestation had an enormous impact on A. darlingi biting rates. The rate was a whopping 278 times higher in sites with the least forest and the most grass/crop land than in sites that were relatively undisturbed (the former had less than 20 percent forest and more than 30 percent grass/crop land, whereas the latter had more than 70 percent forest and less than 10 percent grass/crop land). In addition, A. darlingi biting got worse with progressive deforestation. Specifically, biting rates were roughly four times higher in sites with less than 20 percent forest than in sites with 20-60 percent forest.
“These results may be generalizable to other areas of the Amazon where slash-and-burn agriculture predominates,” say the researchers, adding that preventing widespread deforestation after road construction could both reduce malaria epidemics and increase biodiversity conservation.
By Robin Meadows
Vittor, A.Y. et al. 2006. The effect of deforestation on the human-biting rate of Anopheles darlingi, the primary vector of falciparum malaria in the Peruvian Amazon. American Journal of Tropical Medicine and Hygiene 74:3-11.
Anopheles darlingi. Photo by Dr. Phil Lounibos UF/FMEL