Malaria

Abstract

Malaria is a parasitic infection of global importance. Although relatively uncommon in developed countries, where the disease occurs mainly in travellers who have returned from endemic regions, it remains one of the most prevalent infections of humans worldwide. In endemic regions, malaria is a significant cause of morbidity and mortality and creates enormous social and economic burdens. Current efforts to control malaria focus on reducing attributable morbidity and mortality. Targeted chemoprophylaxis and use of insecticide-treated bed nets have been successful in some endemic areas. For travellers to malaria-endemic regions, personal protective measures and appropriate chemoprophylaxis can significantly reduce the risk of infection. Prompt evaluation of the febrile traveller, a high degree of suspicion of malaria, rapid and accurate diagnosis, and appropriate antimalarial therapy are essential in order to optimize clinical outcomes of infected patients. Additional approaches to malaria control, including genetic manipulation of mosquitoes and malaria vaccines, are areas of ongoing research.

Fig. 1: Global distribution of malaria. Drug resistance is represented by the shaded areas (see legend). This map is intended as a visual aid only; online sources of country-specific malaria risk are provided in “Additional Resources.” Reproduced, with permission, from the Committee to Advise on Tropical Medicine and Travel, Health Canada. Canadian recommendations for the prevention and treatment of malaria among international travellers — 2003. Can Commun Dis Rep 2004;30(Suppl 1). In press.

Fig. 2: The plasmodia life cycle. The human (asexual) stage of the life cycle begins with the exoerythrocytic phase. When an infected mosquito bites a human, sporozoites in the mosquito's saliva enter the bloodstream (1). The sporozoites travel to the liver, where they invade hepatocytes (2); over a period of up to 4 weeks, the infected hepatocytes mature into schizonts. In Plasmodium vivax and P. ovale infections only, some schizonts may remain dormant as hypnozoites (3) for weeks to years before causing clinical relapses. With schizont rupture, merozoites are released into the bloodstream (4). In the erythrocytic phase, merozoites invade erythrocytes and either undergo an asexual cycle of reproduction (5) or develop into nonmultiplying sexual forms (gametocytes) (6). These gametocytes are crucial for perpetuating the life cycle, as they are ingested by a feeding mosquito (7) and undergo sexual reproduction within the mosquito midgut; thousands of infective sporozoites (8) are produced, which then migrate to the salivary glands, ready to initiate another life cycle. Photo: Lianne Friesen and Nicholas Woolridge

Fig. 3: Stages in the life cycle of Plasmodium falciparum. A: Ring forms (early trophozoites). B: Mature schizont, rarely seen in peripheral blood smears because of microvascular sequestration. C: Gametocyte, demonstrating the classic banana shape. Source: Division of Parasitic Diseases, US Centers for Disease Control and Prevention, Atlanta. Photo: CDC