As the destruction of the world's largest rainforest continues, researchers have found that deforestation in the Brazilian Amazon is significantly increasing malaria transmission. The analysis shows that a 10% increase in Amazon deforestation is associated with a 3.3% increase in cases of malaria.
According to the research team, the key implication is that forest clearing has a direct impact on human health, in addition to the loss of other ecosystem services such as species diversity, water quality, and carbon storage.
The largest remaining tropical rainforest, the Amazon of South America, is under substantial pressure from mining, timber harvest, livestock and agricultural production, forest fires, urbanization, and infrastructure development.
The researchers explain that as land conversion for human use continues at a rapid pace to meet the demands of a growing human population for food, fiber, and resources, "unintended consequences" for ecosystem function, biodiversity, climate, and health are frequently emerging. And one such consequence could be the emergence, resurgence, and transmission of vector-borne disease, they add.
"Brazil's government-sponsored settlement of the Amazon region and development of the transAmazonian highway has led to substantial deforestation and a simultaneous resurgence in malaria, partially reversing the prior success of malaria control programs. Despite the recent focus on emerging mosquito-transmitted viruses like Zika and chikungunya, malaria is resurging in a number of regions of South America more broadly, particularly in regions undergoing rapid land conversion and political and economic turmoil. Our work provides clear large-scale evidence that deforestation increases malaria," say the researchers in their findings published in the Proceedings of the National Academy of Sciences (PNAS).
To determine whether malaria transmission in Brazil is influenced by deforestation in the Amazon, the research team — from the Department of Biology, Stanford University, Stanford; and Earth Research Institute, University of California, Santa Barbara — compared a 13-year dataset of malaria cases reported in 795 municipalities of the Brazilian Amazon between 2003 and 2015 with annual forest loss and total forest cover from the Hansen Global Forest Change dataset.
"Our results suggest a 10% increase in deforestation leads to a 3.3% increase in malaria incidence — approximately 9,980 additional cases associated with 1,567 additional square kilometer lost in 2008, the study midpoint, Amazon-wide," says the team.
The researchers found that the effect is more significant in the Amazon’s interior region — where forest cover remains high, which is particularly evident for P. falciparum - than the outer Amazonian states, where comparatively little forest remains. "The effects of deforestation on malaria are largest in the early stages of deforestation in the interior of the Amazon as forest edge habitat increases, promoting mosquito vector breeding habitat, survival, and human biting rate. But the effects attenuate as forest loss progresses, forest edge area declines, and human settlements become larger and further removed from the forest," state the findings.
At the same time, the researchers also found that increased malaria burden reduces forest clearing. The assessment shows that an approximate 1% increase in malaria cases is expected to reduce deforestation by 1.4%.
"We estimate a 1% increase in malaria incidence results in a 1.4% decrease in forest area cleared (approximately 219 fewer square kilometer cleared associated with 3,024 additional cases in 2008). This bidirectional socio-ecological feedback between deforestation and malaria, which attenuates as land use intensifies, illustrates the intimate ties between environmental change and human health," says the study.
It further says, "Malaria clearly and consistently reduces economic activity and influences human settlement, including in the Brazilian Amazon. These effects on economic activity, migration, and settlement could, in turn, influence rates of land clearing, potentially resulting in the negative feedback of malaria incidence on deforestation that we observed."
An increase in malaria cases was also correlated with "precipitation and optimal temperature" for transmission in the dry season, when malaria is most frequently transmitted.
The findings of the current study, according to the researchers, could inform forest conservation, land use policy, and public health decisions in Amazon.