The emerging of the fifth malaria parasite (Plasmodium knowlesi): a public health concern?

Abstract

After examining the most recent scientific evidences, which assessed therole of some malaria plasmodia that have monkeys as natural reservoirs,the authors focus their attention on Plasmodium knowlesi. The infectivefoci attributable to this last Plasmodium species have been identifiedduring the last decade in Malaysia, in particular in the states ofSarawak and Sabah (Malaysian Borneo), and in the Pahang region(peninsular Malaysia). The significant relevance of molecular biologyassays (polymerase chain reaction, or PCR, performed with specificprimers for P. knowlesi), is underlined, since the traditionalmicroscopic examination does not offer distinguishing features,especially when the differential diagnosis with Plasmodium malariae isof concern. Furthermore, Plasmodium knowlesi disease may be responsibleof fatal cases, since its clinical presentation and course is moresevere compared with those caused by P. malariae, paralleling a moreelevated parasitemia. The most effective mosquito vector is representedby Anopheles latens; this mosquito is a parasite of both humans andmonkeys. Among primates, the natural hosts are Macaca fascicularis, M.nemestina, M. inus, and Saimiri scirea. When remarking the possiblesevere evolution of P. knowlesi malaria, we underline the importance ofan early recognition and a timely management, especially in patients whohave their first onset in Western Hospitals, after journeys in SoutheastAsian countries, and eventually participated in trekking excursions inthe tropical forest. When malaria-like signs and symptoms are present, atimely diagnosis and treatment become crucial. In the light of itsemerging epidemiological features, P. knowlesi may be added to thereknown human malaria parasites, whith includes P. vivax, P. ovale, P.malariae, and P. falciparum, as the fifth potential ethiologic agent ofhuman malaria. Over the next few years, it will be mandatory to supportan adequate surveillance and epidemiological network. In parallel withepidemiological and health care policy studies, also an accurateappraisal of the clinical features of P. knowlesi-affected patients willbe strongly needed, since some preliminary experiences seem to show anincreased disease severity, associated with increased parasitemia, inparallel with the progressive increase of inter-human infectiouspassages of this emerging Plasmodium.