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Deforestation increasing cases of ‘monkey malaria’ in humans, study says

  • Since 2004, an increasing number of human Plasmodium knowlesi malaria cases have been identified throughout Southeast Asia, according to study.
  • Researchers found that between 2008 and 2012, forest loss around villages in Kudat and Kota Marudu districts in Sabah, Malaysia, was strongly correlated to the number of P. knowlesi malaria cases in the villages.
  • Study concludes that deforestation is resulting in “distinct public health consequences” in the region, which need to be urgently addressed.

Deforestation could be increasing risk of “monkey malaria” in humans in Malaysia, suggests a new study published in Emerging Infectious Diseases.

Caused by the protozoan parasite Plasmodium knowlesi, and transmitted by female Anopheles mosquitoes, “monkey malaria” normally infects forest-dwelling macaques like the long-tailed macaque (Macaca fascicularis). However, in recent years, it has become the most common form of human malaria in many parts of Malaysia, researchers say. This could be due to widespread clearing of forests, and other environmental changes, the study concluded.

“The dramatic rise in the number of P. knowlesi malaria cases in humans in Malaysia in the past ten years has been most common in areas with deforestation, as well as areas that are close to patches of forest where humans, macaques and mosquitoes are coming into closer and more frequent contact,” lead author Kimberly Fornace, Research Fellow at the London School of Hygiene & Tropical Medicine, said in a statement. “This suggests that there is a higher risk of P. knowlesi transmission in areas where land use is changing, and this knowledge will help focus efforts on these areas and also predict and respond to future outbreaks.”

Scientists first detected a large cluster of human Plasmodium knowlesi malaria cases in 2004, in Malaysian Borneo. Since then, an increasing number of human Plasmodium knowlesi malaria cases have been identified throughout Southeast Asia, the authors write in the paper.

The Plasmodium knowlesi pathogen normally infects macaques like long-tailed macaques (Macaca fascicularis). However, in recent years, the number of human cases is dramatically rising. Photo by Sakurai Midori , Wikimedia Commons. CC by SA 3.0.

To find out if this recent increase in the incidence of human P. knowlesi malaria is due to increasing deforestation, researchers from Malaysia, U.K. and Australia collected data on the number of P. knowlesi malaria cases between 2008 and 2012 from villages in Kudat and Kota Marudu districts in Sabah, Malaysia. They also mapped the local forest, land use and environmental changes around 450 villages in these districts to evaluate if changes in forest cover was linked to the spread of malaria.

The team found that during the five-year period of study, 50 percent of the region’s villages lost more than 10 percent forest cover with a five kilometer (~3 miles) radius. Nearly 40 percent of the villages lost more than 10 percent of forest cover within one kilometer (~0.62 miles) radius alone.

This forest loss, the team found, had a strong link with the number of P. knowlesi malaria cases in the villages. This could be due to a number of reasons, the researchers say.

Loss of forest cover could be bringing humans in closer — and more frequent — contact with both the macaques and the mosquito vectors, they add. This is because of shifts in human movement patterns due to reduction in forest cover, and expansion of agriculture into habitats of macaques and the mosquitoes.

Deforestation could also be changing macaque or mosquito habitats, and their densities, resulting in greater disease transmission.

“Increased density of long-tailed macaques has been reported as a response to deforestation; loss of previous habitats can result in crowding within forest patches, with potential implications for disease transmission,” the authors write.

The study also confirmed that P. knowlesi was the most common cause of malaria in humans in the region.

“Given our findings, we view deforestation as having distinct public health consequences which need to be urgently addressed,” Fornace said.

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