AsianScientist (Apr. 9, 2011) – An emerging malarial strain primarily infecting monkeys in Southeast Asia has been discovered in a Wellcome Trust-funded study.
The study, published in PLoS Pathogens, confirms that the species has not yet adapted to humans and that monkeys are the main source of infection.
Malaria is a deadly infectious disease carried by infected mosquitoes. When the mosquitoes feed on their hosts, they inject the parasites into the bloodstream of their hosts. Over a million people are killed each year this way.
Five human-infecting strains of the malaria-causing plasmodium are known to exist in humans, one of which is the recently identified Plasmodium knowlesi. Thought previously to infect only monkeys, human P. knowlesi infections have been found to be widely distributed in Southeast Asia and to be a major cause of malaria in Malaysian Borneo. The study now clarifies the question of whether the infection is transmitted from person-to-person or from infected monkeys.
Led by Professor Balbir Singh at the Malaria Research Center, Universiti Malaysia Sarawak, researchers from the Sarawak State Health Department, St George’s University of London and the London School of Hygiene and Tropical Medicine focused on examining blood samples from 108 wild macaques from different locations around Sarawak. The results reveal that 78 percent of the macaques were infected with P. knowlesi, while many were infected with one or more of four other species of monkey-borne malaria parasites not yet found in humans.
Through comparisons of the molecular identity of parasites from monkeys and parasites isolated from patients, the team constructed a diagram of the knowlesi species and its preferred host. Their analysis revealed that the spread of the knowlesi species is more frequent among wild monkeys than from monkeys to humans, and monkeys remain the dominant host.
“Our findings strongly indicate that P. knowlesi is a zoonosis in this area, that is to say it is passed by mosquitoes from infected monkeys to humans, with monkeys acting as a reservoir host,” explains Professor Singh.
“However, with deforestation threatening the monkeys’ habitat and increases in the human population, it’s easy to see how this species of malaria could switch to humans as the preferred host. This would also hamper current efforts aimed at eliminating malaria.”
Using molecular data, they estimate that the knowlesi species evolved from its ancestral species between 98,000-478,000 years ago, long before human settlement of the area. This means that monkeys are most likely to have been the initial host for the parasite when the species first emerged. The estimate also indicates that the species is as old as, or older than, the two most common human malaria parasites, P. falciparum and P. vivax.
The article can be found at: Lee KS et al. (2011) Plasmodium knowlesi: Reservoir Hosts and Tracking the Emergence in Humans and Macaques.
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Source: Wellcome Trust.
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