WASHINGTON Clearing forests in the Amazon helps mosquitoes thrive and can send malaria rates soaring, U.S. researchers reported on Wednesday.
They found a 48 percent increase in malaria cases in one county in Brazil after 4.2 percent of its tree cover was cleared.
Their findings, published in the journal Emerging Infectious Diseases, shows links between cutting down trees, a rise in the number of mosquitoes and infections of humans.
"It appears that deforestation is one of the initial ecological factors that can trigger a malaria epidemic," said Sarah Olson of the University of Wisconsin, who worked on the study.
Experts are already worried that the destruction of Brazil's Amazon forests can help drive climate change. Big fires, set by farmers to clear land for agriculture, are the main cause of deforestation.
One team estimated earlier this month that 19,000 square km (7,300 square miles) of forest had been lost every year in Brazil from 1998 to 2007.
The new study shows the immediate health consequences, the researchers said.
"Conservation policy and public health policy are one and the same," Jonathan Patz, the professor who oversaw the work, said in a telephone interview. "How we manage our landscapes and, in this case, tropical rain forest has implications for public health."
Malaria, caused by a parasite transmitted by mosquitoes, kills about 860,000 people a year globally, according to the World Health Organization. Brazil has about 500,000 cases a year of malaria, most carried by Anopheles darling mosquito.
Patz's team has been tracking mosquito populations and how they change as forests are cut down in Brazil and Peru. They took satellite data showing changes in tree cover in one county of Brazil's Amazon region and linked it with health records showing diagnosed cases of malaria.
The malaria data was exceptionally detailed -- some of the teams used Global Positioning Satellite data to show precisely where patients lived. They documented more than 15,000 malaria cases in 2006
The conclusions were clear.
"We show that a 4.2 percent change in deforestation from August 1997 through August 2001 is associated with a 48 percent increase of malaria incidence," the researchers wrote.
Forests in Brazil are cleared by large-scale loggers and subsistence farmers alike.
"Human-altered landscapes provide a milieu of suitable larval habitats for Anopheles darling mosquitoes, including road ditches, dams, mining pits, culverts, vehicle ruts, and areas of poor clearing," the researchers wrote.
Another possible factor is that many of the farmers have started fish farms in the region. Patz said it was not possible to see those in the satellite images, but they could be providing breeding areas for mosquitoes.
"Our findings are likely generalizable to many parts of Amazonia, and build on our past entomological studies in the Peruvian Amazon," Patz added.
"This environmental epidemiology study further shows that rain forest conservation policy should be a key component to any malaria control effort in the region."
(Editing by Peter Cooney)