Co-infections of Plasmodium knowlesi, P. falciparum, and P. vivax among Humans and Anopheles dirus Mosquitoes, Southern Vietnam

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.

Figure 1

Results and interpretation of the PCR analyses of sporozoite-positive salivary glands of Anopheles dirus mosquitoes in Khanh Phu forest, Vietnam. CSP, circumsporozoite protein; ssu, small subunit; shaded cells, PCR products present; F, Plasmodium falciparum; V, P. vivax; K, P. knowlesi; M, P. malariae; ?, unknown.

Figure 2

Dynamics of biting density and sporozoite positive salivary glands (including Plasmodium knowlesi) of Anopheles dirus mosquitoes in Khanh Phu forest, Vietnam. The solid red line connects the points of mean monthly An. dirus density (bites/person-night). Bars indicate the monthly number of sporozoite-positive salivary glands: white, P. knowlesi; gray, other species. The first mosquito found with P. knowlesi in August 2008 was described in Nakazawa et al. (11).

Figure 3

Age analysis of persons tested for Plasmodium knowlesi infection, Khanh Phu, Vietnam. A) Age groups of P. knowlesi–positive persons (n = 32; mean age 15.8 y) compared with P. knowlesi–negative persons (n = 179; mean age 26.2 y); p = 0.0004 (significant) by 2-tailed t test with unequal variance. B) Age groups of P. knowlesi–positive persons compared with ages of those positive for other parasites (n = 93; mean age 24.5 y); p = 0.004 (significant) by 2 tailed t test with unequal variance. C) Age groups of P. falciparum–positive persons (n = 95; mean age 23.4 y) compared with P. falciparum–negative persons (n = 116; mean age 25.6 y); p = 036 (not significant) by 2-tailed t test with unequal variance.

Figure 4

Frequency of single and co-infections among 124 human blood samples (A) and 73 mosquito salivary glands (B) positive for Plasmodium spp. infection by PCR. P. malariae was discarded. Gray bars indicate P. knowlesi infection or co-infection. F, P. falciparum; V, P. vivax; K, P. knowlesi.