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.