Malaria

Abstract

Malaria is a tropical parasitic infection of the red blood cells caused by the protozoal species Plasmodium falciparum, P. vivax, P. ovale, P. malariae, and P. knowlesi. It is transmitted through the bite of the female Anopheles mosquito. The average incubation period is twelve to fourteen days. Congenital and blood-borne transmissions can also occur. P. falciparum and P. vivax account for most human infections but almost all deaths are caused by P. falciparum, with children under five years of age bearing the brunt of morbidity and mortality in endemic countries. P. falciparum is dominant in sub-Saharan Africa whereas P. vivax predominates in Southeast Asia and the Western Pacific. P. ovalae and P. malaria are less common and are mainly found in sub-Saharan Africa. P. knowlesi primarily causes malaria in macaques and is geographically restricted to southeast Asia. While taking a blood meal, the female anopheline mosquito injects motile sporozoites into the bloodstream. Within half an hour, the sporozoites invade the hepatocytes and start dividing to form tissue schizonts. In P. vivax and P. ovale, some of the sporozoites that reach the liver develop into hypnozoites and stay dormant within the hepatocytes for months to years after the original infection. The schizonts eventually rupture releasing daughter merozoites into the bloodstream. The merozoites develop within the red blood cells into ring forms, trophozoites, and eventually mature schizont. This part of the life cycle takes twenty-four hours for P. knowlesi; forty-eight hours for P. falciparum, P. vivax, P. ovale; and seventy-two hours for P. malariae. In P. vivax and P. ovale, some of the sporozoites that reach the liver develop into hypnozoites and stay dormant within the hepatocytes for months to years after the original infection. The hallmark of malaria pathogenesis is parasite sequestration in major organs leading to cytoadherence, endothelial injury, coagulopathy, vascular leakage, pro-inflammatory cytokine production, and tissue inflammation. Malaria is the most frequently imported tropical disease in the UK with an annual case load of around 2000. P. falciparum is the predominant imported species, and failure to take chemoprophylaxis is the commonest risk factor.