US: Malaria increases when rain forest cut

Researchers from the University of Wisconsin-Madison have presented new evidence linking the incidence of malaria with the felling of tropical rainforest.

The study compared malaria cases in 54 Brazilian health districts with the extent of logging in the Amazon forest. It showed that clearing tropical forest boosted the incidence of malaria by nearly 50%.

"It appears that deforestation is one of the initial ecological factors that can trigger a malaria epidemic," says Sarah Olson, the lead author of the new report and a postdoctoral fellow at the Nelson Institute, Center for Sustainability and the Global Environment.

Olson and senior author Jonathan Patz say the clearing of tropical forests creates conditions that favour malaria's primary carrier in the Amazon, the mosquito Anopheles darlingi.

"The deforested landscape, with more open spaces and partially sunlit pools of water, appears to provide ideal habitat for this mosquito," Olson says.

She says Anopheles darlingi displaces other types of mosquitoes that prefer forest and that are far less prone to transmit malaria.

"This study of human malaria cases complements our previous work that focused more on the abundance of the malaria-carrying mosquito," Patz says.

"In those studies from the Peruvian Amazon, we showed a correlation between this mosquito's larvae and aquatic breeding sites in disturbed habitats following land clearance."

The new Wisconsin study focused on a corner of the Brazilian Amazon near Peru where detailed health and population data were collected in 2006 by Brazilian researchers. Combined with high-resolution satellite data of changes in land cover, the health data reveals the large human-health impact of relatively small changes to the forest landscape.

"A 4% change in forest cover was associated with a 48% increase in malaria incidence in these 54 health districts," says Olson.

"Unlike previous studies, our data allowed us to zoom in on areas where people are being exposed to malaria and to exclude areas where they are not being exposed."

Patz says the area was in the top five of all Brazilian counties with malaria.

"Even after we adjusted for human populations, access to healthcare and other factors, malaria hotspots paralleled locations with the most destruction of rainforests."

The message from the study, say Olson and Patz, is that tropical forest conservation may benefit human health more than we realised.

"Land-management practices show promise as useful interventions to reduce malaria risk factors," says Olson.

The research is published in the online issue of the CDC journal Emerging Infectious Diseases