From Asia, the Novel (and Old) Fifth Malaria Plasmodium of Human Beings

ABSTRACT

A novel form of human malaria Plasmodium knowlesi has been identified in small epidemic foci occurred during the last decade in Malaysia. Based on a review of the available literature data, the Authors underline the diagnostic importance of molecular biology examinations performed with primers which are specific of Plasmodium knowlesi, since the standard hemoscopy may fail in distinguishing Plasmodium knowlesi from Plasmodium malariae, due to their similar appearance. P. knowlesi has been reported as a causative parasite agent of life-threatening and even lethal forms of malaria. In humans, its clinical picture is more severe a compared to that of P. malariae, since the disease is characterized by a greater parasitemia, versus that is referred in the course of P. malariae disease. The most effective carrier of P. knowlesi is represented by the mosquito Anopheles leucosphyrus, which is attracted by both humans and monkeys. Among primates, the natural hosts of P. Knowlesi are known until now and have been represented by Macaca fascicularis and Macaca nemestina, while other monkeys including Saimiri scirea and Macaca mulatta, which cannot become infected. These might be useful in eventual experimental models. When facing the potentially severe evolution of human disease by P. knowlesi, we remark the key role played by a prompt disease recognition, which is expected to be more easy and obvious in patients followed in endemic countries at elevate risk, but should be carefully implemented for subjects coming back to health care services of western countries, presenting with a number of typical signs and symptoms of malaria, after travelling in South-East Asia where they were engaged in staying or making excursions in the tropical forest. In these last cases, both diagnosis and treatment should be prompt, timely, and appropriate. According to literature data, in non-severe human cases the old and trivial chloroquine remained very effective against P. knowlesi, achieving the disappearance of signs and symptoms in 96% of cases within the first day of pharmacological therapy. On the ground of the emerging epidemiological figures, P. knowlesi was added to Plasmodium vivax, Plasmodium ovale, Plasmodium malariae, and Plasmodium falciparum, as the fifth etiological agent of human malaria. During the next years, it will become mandatory to plan an adequate surveillance programme of the epidemiological evolution of this novel form of human malaria, paying also maximum attention to the clinical presentation of patients affected by P. knowlesi malaria, which are expected to suffer from a more severe clinical course, according to the time elapsed from the appearance of the early signs and symptoms. Some preliminary clinical figures suggest that a greater severity is related to an increased parasitemia, and parallels the increased interhuman infectious passages of parasites.