Knowlesi malaria: newly emergent and of public health importance?

Abstract

Several questions on public health impact have arisen from the discovery of a large focus of the simian malaria parasite, Plasmodium knowlesi, in the human population. P. knowlesi malaria is not newly emergent and was overlooked until molecular tools to distinguish between P. knowlesi and the morphologically similar Plasmodium malariae became available. Knowlesi malaria is a zoonosis that is widely distributed in Southeast Asia and can be fatal. Information on knowlesi malaria should be included in medical and public health guidelines to encourage the accurate diagnosis and treatment of patients, and monitor the incidence and distribution of cases. A complete emergence of P. knowlesi into the human population could be overwhelming and, although challenging, the prevention of this situation deserves serious consideration.

Figure 1

Thin blood films that show morphological similarities between blood stages of P. knowlesi, P. malariae and P. falciparum. Giemsa-stained thin blood films show that the early trophozoites of both P. falciparum and P. knowlesi appear as discrete ring forms, at times with double chromatin dots, and individual erythrocytes can be infected by more than one parasite. The late trophozoites, schizonts and gametocytes of P. knowlesi and P. malariae are similar, including the appearance of some trophozoites as band forms. The figure is modified from Ref. [2], with permission from Elsevier.

Figure 2

The number and distribution of reported human infections with P. knowlesi, the limits of natural distribution of two species of macaques and the limits of natural distribution of mosquitoes of the An. leucosphyrus group. All of the human cases of P. knowlesi shown were confirmed by sequencing and/or PCR [2,37-41] with the exception of two single cases in Peninsular Malaysia [10,13]. The map and known distribution of the An. leucosphyrus group were modified from Ref. [25], and the approximate distribution of long-tailed (M. fascicularis) and pig-tailed (M. nemestrina) macaques were adapted from Refs [27,28].