Forests are mosquito hotspots but deforestation can increase malaria mortality

In 1990, India reported approximately 33 million cases of malaria. By 2019, that number plummeted to 5.5 million due to a host of government interventions designed to reduce malaria incidence. Deaths from malaria saw a similar decline. New research finds that forests may have played a marginal role in avoiding additional malaria deaths, even though forests are generally considered malaria hotspots.
Forests – with their dense canopies and humid weather – are hotspots for malaria breeding and transmission in India. Malaria is transmitted when a female Anopheles mosquito carrying the Plasmodium parasite, bites a human host during feeding. Forested districts contributed to 32% of malaria cases and 42% of malaria mortality between 2000 and 2019, while harbouring just 6.6% of the country's population, government data shows.
'Mosquitos like the shade and humidity provided by forests. Forests provide ideal conditions for the entire duration of the mosquito's 12-day life cycle. Forest cover also makes it harder to spot breeding habitats, which makes it difficult to clear them,' said Ramesh Chand Dhiman, former director of the ICMR – National Institute of Malaria Research.
But forest loss may also aggravate malaria transmission and mortality. The extent to which deforestation impacts malaria incidence and mortality in India is still being understood. Researchers from Ohio State University examined the impacts of deforestation on malaria mortality in 628 districts in rural areas, and found that on average, the loss of one square kilometre of forest resulted in 0.16 additional deaths due to Plasmodium falciparum per 100,000 people. P falciparum causes the most number of malaria deaths in India.
Vexed relationship
Deforestation has been linked to a rise in malaria transmission in the Amazon in South America. A study analysing trends in deforestation and malaria in the Brazilian Amazon across 13 years found that on average, across 795 municipalities, a 10% increase in deforestation led to a 3.3% increase in malaria incidence. 'The effect is larger in the interior and absent in outer Amazonian states where little forest remains,' a 2019 study, by Stanford researchers, says.
In this study too, the main driver is P falciparum. The researchers hypothesise malaria incidence worsens in the interiors of the forest (where forest cover is high, compared to the outer states where forest cover is low) because after deforestation gives way to settlements and agriculture, 'malaria exposure leads to temporary immunity (in these parts), and housing quality and healthcare access improve.'
The study goes on to state that these effects are strongest in the interior in the early stages of deforestation. '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 forest,' the study says.
But because the mechanisms of malaria transmission are so complex and region-specific, the same trends are not seen everywhere. An analysis of malaria transmission in 17 Sub-Saharan African countries across 14 years found that forest cover and deforestation had no role to play in malaria incidence in more than 60,000 children studied.
'We speculate that our findings may differ from those of previous studies because deforestation in Sub-Saharan Africa is largely driven by the steady expansion of smallholder agriculture for domestic use by long-time residents in stable socio-economic settings, where malaria is already endemic and previous exposure is high,' the researchers say, adding, 'In much of Latin America and Asia, deforestation is driven by rapid clearing for market-driven agricultural exports by new frontier migrants without previous exposure.'
In India, the picture is still emerging. In Assam's Sonitpur, a longitudinal study found that deforestation did, in fact, lead to an increase in malaria incidence, while other studies have found deforestation changed the composition of malaria-carrying mosquito species in parts of the state.
Researchers from Ohio State University have tried to capture this relationship at scale, looking at impacts of deforestation on malaria mortality in rural parts of the country. The study went a step further by trying to assign an economic value to the services forests give in preventing additional malaria deaths.
In order to isolate the role of forests in malaria transmission, the researchers limited their analysis to three years – from 2013 to 2015 – in 628 districts. According to Daniela Miteva, lead author of the Ohio study, this is a short enough window to rule out behaviour changes that could influence malaria transmission, and is before the government introduced schemes distributing medicated nets 2016 onwards.
'We calculated the area of forest lost in a district, and then the changes in malaria within the cluster, which we defined as a 5 kilometre buffer area around a rural village. We then used methods that allow us to quantify what the causal impact is of the reducing forest within a district on malaria mortality in a cluster,' she said.
The analysis only included mortality from P falciparum from the Malaria Atlas and World Health Organisation, since no data on mortality from P vivax was available. On average, a loss of one square kilometre of forest resulted in 0.16 additional deaths per 100,000 people.
In economic terms, the ecosystem services provided by the forest in preventing these additional deaths was calculated to be between $1.26 and $85.9 per hectare, per year. But this impact is short term, and subject to change over the long term. 'The conversion of forests to urban areas may alter exposures and habitats for malaria; for this reason, the longer term impact of forest loss on morbidity and mortality is unclear,' the study says.
Land use effect
The exact mechanisms of how malaria is transmitted are determined by a number of factors, including exposure, vulnera bility to the disease, immunity, land use change, and prevailing climatic factors. India has fifty-eight species of anopheline mosquitoes, of which six are major carriers of malaria. Increasing humidity, rainfall and rising temperatures due to climate change are projected to intensify and lengthen the transmission period for malaria.
The role of deforestation is less certain. Small forest clearings could make malaria transmission worse by opening up new channels of exposure to humans, but a complete clearing of forest and removal of mosquito habitats could result in less or no malaria transmission. 'This is what researchers call a U-shaped relationship, where forest clearing can lead to an increase in exposure, but only to an extent,' said Miteva.
Dhiman is not convinced that forest cover can prevent additional malaria deaths, precisely because in his practice, high malaria endemicity and mortality are seen in areas with more forest cover. However, Dhiman did note that deforestation can change the species composition of malaria carrying mosquitos, which carries its own implications. In a paper published in 2020, Dhiman found that deforestation in the Bokajan region of Assam replaced the prevailing Anopheles minimus species with the Anopheles culicifacies species.
The forested northeast region is normally suited to the Anopheles minimus, but the clearing of forest resulted in 'availability of open area exposed to sunlight, which is suitable for breeding of An. culicifacies v ector,' which is more resistant to insecticides like DDT, the paper said. 'Land use change can definitely impact malaria incidence. In this case, we found that reduction of forest cover made it suitable for another mosquito to inhabit in the plains. In other cases, land use change that introduces mosquito habitats, like irrigation canals, can lead to more malaria incidence,' Dhiman said.

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