A study published recently adds evidence to the argument that deforestation aids the spread of malaria.
Researchers compared deforestation patterns to malaria rates in nine states in the Brazilian Amazon. They found that places with the highest incidences of malaria were impacted forest patches between 0.1 and 5 square kilometers in size.
The researchers write that these forest patches contain the shaded, watery, forest-edge habitat preferred by the mosquitos that transmit malaria.
To keep malaria from becoming an even bigger threat, the authors call for better monitoring of mosquito populations, land planning, and income generation schemes for forest-dwelling communities.
Scientists have long suspected a relationship between deforestation and some infectious diseases. For instance, the 2014 Ebola crisis has been linked to logging that may have put workers and their families in close contact with infected bats.
For malaria – one of the leading causes of death in tropical regions – there has been some evidence that the mosquitos that transmit it (called “vectors”) breed more readily in places where forest has been cleared. Now, a study published recently in Nature’s open-access journal Scientific Reports, adds to the hypothesis that deforestation aids the spread of malaria in the Amazon.
For their study, researchers at institutions in Brazil and the U.S. attempted to find patterns between deforestation and malaria infection in nine states in the Brazilian Amazon. They looked at patches of rainforest that had been deforested or degraded (collectively termed “impacted” in the study), breaking them down into different size categories. They then compared these deforestation patches to local rates of malaria infection recorded between 2009 and 2015.
What they discovered surprised them – the places with the highest incidences of malaria were impacted forest patches comprising between 0.1 and 5 square kilometers, which were the mid-range of the patch sizes they looked at in their study. In other words, these medium-size deforested patches seem to be the sweet spot where forest extraction activities correlate to more malaria infections.
The researchers think this may have something to do with habitat preferences of larval Nyssorhynchus darlingi mosquitoes, which are the primary malaria vectors in the regions they studied. They write that these mosquitos prefer laying their eggs in water at forest edges, but that they also need partial shade. A hallmark of deforestation is that it fragments a forest landscape, effectively creating more forest edges. And more forest edges mean more places for mosquitoes to breed.
The researchers didn’t find statistically significant correlations between small areas (less than 0.1 square kilometers) of deforestation and malaria incidence, which they say is because there wasn’t enough human presence to aid mosquito reproduction.
“Likely in this condition, vector proliferation, presence of a small human population and small number of domestic animals that could represent additional source of blood for mosquitoes, are not stable and in sufficient number to sustain a large [mosquito] population and malaria transmission,” lead author Leonardo Suveges Moreira Chaves told Mongabay. Chaves works in the Department of Epidemiology at the University of Sao Paulo.
The authors also write that forest fragmentation may help malaria-carrying mosquitoes spread to other areas after they metamorphose into adults.
“Furthermore, the new landscape delineated by the pattern of deforestation and soil occupation may favor dispersal of Ny. darlingi by creating forested areas interspaced by deforested areas, which are linked by forest corridors along [small streams] and shaded dirt roads,” the researchers write.
Another pattern the study uncovered, which affirms findings from previous studies, is that the number of malaria cases in the Amazon tended to shoot up in the dry season. The dry season also is when most logging takes places because, according to the researchers, rain makes it more difficult to access and work in forested areas.
“The driest months were associated with the largest deforested and impacted areas and the greatest numbers of malaria cases,” the researchers write.
In addition to helping create optimal nurseries for mosquito larvae, the researchers write that the act of deforestation can also introduce malaria to new places as loggers travel from one forest to another.
“It is not uncommon that the first forest invaders are carriers of [malaria] because of previous [exposure] in other areas where transmission is endemic,” Chaves said.
As malaria rates continue to rise in the Brazilian Amazon, Chaves and his colleagues write that measures must be taken to combat the disease and keep it from reaching “unsustainable levels.” They recommend enhanced monitoring and controlling of mosquito populations, improvement of land use planning, and better income generation for forest-dwelling communities so they don’t need to rely on logging to survive.
If this isn’t done, Chaves says there will continue to be a “vicious cycle of deforestation, degradation, vector proliferation, extreme poverty and malaria as well as other vector-borne infectious diseases.”