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A type of malaria transmitted from monkeys to humans via mosquitoes is caused by two different populations of parasites, increasing the risk of genetic change leading to more-virulent infections, a study has found.
The researchers analysed Plasmodium knowlesi infections in humans and wild macaques in Malaysia, where monkey malaria is rising. They found that the parasites in pig-tailed macaques and long-tailed macaques were genetically distinct from one another.
The study, led by researchers at the London School of Hygiene & Tropical Medicine, United Kingdom, and the University of Malaysia Sarawak, was published in PLOS Pathogens last month.
“It is very important to find out if there are different species of mosquitoes transmitting each of the two types of parasites.”
David Conway, London School of Hygiene & Tropical Medicine
“We didn’t expect a major subdivision in the parasite population like we found,” says senior author David Conway, a biologist at the London School of Hygiene & Tropical Medicine. “The fact that there are two different groups of P. knowlesi circulating may mean more opportunities for the parasite to change and adapt to its new environment, becoming more virulent in humans.”
Although infection by P. knowlesi is usually referred to as zoonotic, as it is mainly transmitted from wild monkeys to humans through mosquitoes, direct human-human transmission is possible, says Conway. “We suspect it has been happening. We just don’t have evidence of it yet, because there is very little data on the epidemiology of transmission.”
The findings increase understanding of the interaction between the parasite populations and their hosts, but also raise further questions about whether and how the two types of infections vary, says Timothy William, an infectious disease physician at the Jesselton Medical Centre, Malaysia, who was not involved in the study.
“It would be interesting to investigate how these different subtypes influence the clinical manifestations in humans who are infected with them, in terms of severity, morbidity and mortality, and also in the different populations by age, gender and place they were infected, William tells SciDev.Net.Scientists must next study the mosquitoes that transmit the parasites, which may help develop ways to prevent and control monkey malaria, Conway says. “It is very important to find out if there are different species of mosquitoes transmitting each of the two types of parasites,” he says.
The P. knowlesi parasite was previously thought to infect only wild macaques in South-East Asia, but human infections have increased in recent decades. While common malaria species such as P. falciparum and P. vivax have declined in the region, P. knowlesi and the microscopically near-identical species P. malariae are on the rise, reaching 35 per cent of malaria notifications in Malaysia in 2011 and 62 per cent in 2013, up from just one per cent in 1992.
Clearing trees for agriculture means humans have been moving into the macaques’ habitat, and are now increasingly exposed to the same mosquitoes that usually bite monkeys.
 Paul C. S. Divis and others Admixture in humans of two divergent Plasmodium knowlesi populations associated with different macaque host species (PLOS Pathogens, 28 May 2015)
 Timothy William and others Changing epidemiology of malaria in Sabah, Malaysia: increasing incidence of Plasmodium knowlesi (Malaria Journal, 2 October 2014)
 Timothy William and others Increasing incidence of Plasmodium knowlesi malaria following control of P. falciparum and P. vivax malaria in Sabah, Malaysia (PLOS Neglected Diseases, 24 January 2013)