A recent study on Plasmodium knowlesi malaria in Vietnam failed to discover symptomatic P. knowlesi in the human population 1. In their study, the authors used P. knowlesi PCR primers to re-examine blood samples collected during two large cross-sectional malaria surveys in the Ninh Thuan province during 2004 and 2005. The population screened was mainly of Ra-glai ethnicity, living in hilly, densely forested areas with frequent overnight stays in the jungle. There are many parallels with the population in Sarawak Malaysian Borneo, where a large number of human cases of zoonotic knowlesi malaria occur annually 23. The authors report that vectors of P. knowlesi are in the area, but it is not known if there are macaques or indeed if they carry P. knowlesi parasites 4. Nonetheless, all of the indicators suggest that human cases of P. knowlesi could occur in this population, which is, therefore, worthy of screening.

The authors reported that during their surveys 13.6% of 4,000 individuals screened were PCR positive for the human-adapted malaria parasites, PCR for P. knowlesi was not included. Of the positives, 210 were Plasmodium malariae. Ninety-five of the 210 were selected for re-screening with P. knowlesi PCR primers. Three samples, two from children, two and three years of age respectively, and one from a young adult were confirmed P. knowlesi PCR-positive. One of the children remained positive with time. The authors conclude that the prevalence of P. knowlesi is low and asymptomatic in this population. This finding is not surprising given that the authors re-screened samples PCR-positive for P. malariae. Plasmodium malariae and P. knowlesi are phylogenetically distinct, the one notable similarity is at the morphological level. Indistinguishable morphology between these parasites gave rise to misdiagnosis and masking of symptomatic human cases of P. knowlesi malaria in Sarawak, often putting patients lives at risk 3. Therefore, to screen properly this important population in Vietnam for P. knowlesi infection, PCR negative samples found during the primary screen, those that were not amplified by falciparum, vivax, malariae or ovale specific PCR primers, would have been more likely to yield positive results for P. knowlesi. Of the three P. knowlesi positives reported, one was negative by microscopy and the others were either P. vivax or P. vivax with P. falciparum by microscopy. For this study, the authors used P. knowlesi primers Pmk8 and Pmk9 published by the Sarawak research group 3. These primers have recently been reported to cross-hybridize with a small number of Plasmodium vivax isolates in a stochastic (random) manner 5. Two of the three positive cases had microscopy confirmed P. vivax. It is possible that the persistent infection reported was due to cross-hybridization with P. vivax, an explanation that would better fit the biology of the two parasites. Taken together with sampling and other problems involving non-specific human DNA amplification reported by Van den Eede et al, this account of P. knowlesi in Vietnam may not accurately represent the situation there.

Persistent P. knowlesi infections, particularly in very young children would indeed add interest to what is currently known of P. knowlesi epidemiology. Cryptic knowlesi infections were reported recently in Thailand 6, but mostly as mixed infections with P. falciparum and P. vivax and in areas where these human malaria parasites predominate. The cases of P. falciparum and P. vivax in Sarawak, Malaysian Borneo, are in decline, particularly in areas where symptomatic P. knowlesi is reported. Plasmodium knowlesi remains zoonotic and by definition less well-adapted to the human host. In areas where Plasmodium falciparum and P. vivax are prevalent, they would perhaps be expected to out-compete P. knowlesi in the human host. It will be interesting to follow the incidence of symptomatic knowlesi malaria in neighbouring countries, particularly when the human-adapted parasites are in decline and where humans continue to utilize forested areas with known knowlesi transmission.

The authors declares no competing interests.