Sunday, 24 July 2011

MALARIA: Co-infections of Plasmodium knowlesi, P. falciparum, and P. vivax

EID Journal Home > Volume 17, Number 7–July 2011
Co-infections of Plasmodium knowlesi, P. falciparum, and P. vivax among Humans and Anopheles dirus Mosquitoes, Southern Vietnam
Ron P. Marchand, Richard Culleton, Yoshimasa Maeno, Nguyen Tuyen Quang, and Shusuke Nakazawa

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.

Concerted control measures have considerably reduced the prevalence of malaria in Vietnam, and the parasites that cause it are now mostly restricted to forested rural areas (1). Forest malaria poses a special challenge for control because the exophilic and early biting habits of the mosquito vector Anopheles dirus render conventional vector control methods such as indoor residual spraying and insecticide treated mosquito nets difficult to apply as well as ineffective (2–4). The possibility of zoonotic malaria in Southeast Asian forests, because of the transmission of Plasmodium knowlesi from monkeys to humans (5–10), may form an additional complication.

Since surveys began in 2002, the forest populations of An. dirus mosquitoes in Khanh Phu, south-central Vietnam, have shown consistently high sporozoite infection rates (1%–2% of the thousands of specimens collected and dissected annually for microscopic examination of salivary glands), raising the question of whether all the sporozoites detected belong to species capable of infecting humans. 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). Here we report the results of the PCR analysis of 72 additional sporozoite–positive salivary glands of An. dirus mosquitoes from the forest in Khanh Phu and of 211 blood samples from the local human population.

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