Source: Emerging Health Threats
Clearing just 4% of forested land in parts of the Amazon can significantly raise the risk of contracting malaria in local communities, according to a study published today in Emerging Infectious Diseases.“Adjusting for population, access to care, and district size, we found that malaria risk increased ≈50% in health districts when 4% of the area underwent deforestation in 1997–2000,” write Sarah Olson of the University of Wisconsin, USA, and colleagues.
The research builds on earlier work by the team, which found a correlation between deforestation levels in north-eastern Peru and the abundance of Anopheles darlingi mosquitoes — the main vector for malaria in the Amazon.
In an email to EHTF News Jonathan Patz, one of the study’s authors, says the analysis shows that “rainforest conservation policy should be a key component to any malaria control effort in the region.”
The findings probably apply to other parts of Amazonia as well, suggests Patz. Commenting on the research Andrew Githeko, Chief Research Officer at the Centre for Global Health Research at the Kenya Medical Research Institute, says the same link has been observed in Cameroon, “where deforestation has been increasing the breeding of An. gambiae [mosquitoes] with subsequent increase in malaria transmission.”
In Brazil, about half a million cases of malaria on average were confirmed each year between 1997 and 2006. Most of these cases were recorded in the Amazon Basin, according to the authors, where an area the size of Denmark was cleared of forest between 1999 and 2002. Although deforestation rates have decreased in recent years, estimates suggest the Amazon rainforest could be reduced by an additional 40% by 2030.
Olson and colleagues examined the link between deforestation and malaria in Mâncio Lima, a county in western Brazil near the border with Peru. Surveillance data on the number of people diagnosed with malaria in 2006 were available from 54 health districts covering nearly a third of the county. For the same districts the authors estimated the extent of deforestation using satellite imagery. With a statistical model, they estimated the rise in malaria risk that corresponded to a percentage change in forested land in different periods from 1997 to 2006, alongside changes in other factors such as the percentage of patients who had access to health care.
A significant increase in the chances of contracting malaria in 2006, measured as a relative risk of 1.48, was associated with changes in forest cover between 1997 and 2001.
“We found that human malaria risk is specifically associated with deforestation 5–10 years previously,” write Olson et al. “We did not find an association with deforestation before or after that time frame.” They say that this can be explained by previous research showing that shrubs begin to grow back about five years after land is cleared; these shrubs are favoured by the An. darlingi mosquitoes that carry malaria parasites in the region.
Githeko points out that the mosquitoes like to breed in clean, semi-shaded areas with slow-moving water that has a neutral pH. “Deforestation would naturally increase such habitats tremendously,” and their suitability for mosquito breeding may increase in time, he says.
The authors used changes in deforestation as a proxy variable for changes in the number of suitable breeding sites, notes Githeko, so their study design isn’t ideal. The potential to transmit malaria would also depend on the genetic variety of the mosquitoes, he explains, as some prefer to feed on animals. “However there is sufficient published data on larval ecology of An. darlingi to allow reasonable inferences.”
Olson and colleagues say that studies on mosquito characteristics and malaria risk suggest that deforestation has a bearing on patterns of malaria on the "frontier" of cleared land, where settlers begin to move into the rainforest. But they caution that because of its design, their study does not show that changes in deforested land caused a rise in the incidence of malaria in Mâncio Lima. They also point to limitations, including the lack of data on fish farming — a growing industry in the region which can also create suitable habitats for An. darlingi mosquitoes.