Abstract
A single Anopheles dirus mosquito carrying sporozoites of Plasmodium knowlesi, P. falciparum, and P. vivax was recently discovered in Khanh Phu, southern Vietnam. Further sampling of humans and mosquitoes in this area during 2009-2010 showed P. knowlesi infections in 32 (26%) persons with malaria (n = 125) and in 31 (43%) sporozoite-positive An. dirus mosquitoes (n = 73). Co-infections of P. knowlesi and P. vivax were predominant in mosquitoes and humans, while single P. knowlesi infections were found only in mosquitoes. P. knowlesi-co-infected patients were largely asymptomatic and were concentrated among ethnic minority families who commonly spend nights in the forest. P. knowlesi carriers were significantly younger than those infected with other malaria parasite species. These results imply that even if human malaria could be eliminated, forests that harbor An. dirus mosquitoes and macaque monkeys will remain a reservoir for the zoonotic transmission of P. knowlesi.
Highlights
A single Anopheles dirus mosquito carrying sporozoites of Plasmodium knowlesi, P. falciparum, and P. vivax was recently discovered in Khanh Phu, southern Vietnam
In 2008, evidence was found for the co-infection of P. knowlesi, P. falciparum, and P. vivax in the salivary glands of 1 mosquito among 17 that had been processed by PCR with malaria parasite species–specific primers [11]
All residences within the commune were mapped, and all persons were registered at the research station and assigned unique code numbers based on residential location and family relations. (All methods in this study that involved human participants in the field were certified as permitted standard procedures by the National Institute of Malariology, Parasitology and Entomology in Hanoi.)
Summary
Study Area Khanh Phu (12°14′N; 108°56′E) is a commune with ≈3,000 inhabitants, mainly of the Raglai ethnic minority, who live between the forested foothills on the east side of the Truong Son mountain range in southcentral Vietnam (Khanh Hoa Province), an area where malaria was previously hyper- to holo-endemic [3]. Analysis of samples from infected persons by thin-smear microscopy showed that approximately two thirds were caused by P. falciparum, one third by P. vivax, and a very small number by P. malariae during the 2003–2009 study period This malaria prevalence typically affects the poorest members of the local Raglai ethnic community, whose livelihood partly depends on excursions into the forest to collect products such as bamboo and rattan, or to cultivate their plots on the mountain slopes. Sixty-eight percent of all blood slides in both samples were from persons in the 2 southernmost villages of Khanh Phu (Nga Hai and Da Trai) This represents an area where ≈1,100 persons are living, spread over an area of 1.7 km at 12°12.5′N and 108°55.5′E. All persons found to be infected with parasites (as diagnosed by microscopy) received treatment, according to the policy of the Vietnam Ministry of Health
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