Chapter 70: Malaria

DEFINITIONS AND EPIDEMIOLOGY

Malaria is a leading cause of childhood morbidity and mortality worldwide, with approximately 212 million cases occurring in 2015. The burden of this disease is borne largely by children in sub-Saharan Africa, where approximately 90% of infections and 70% of malaria-associated deaths occur.^1,2 In the United States, local malaria transmission, which was once endemic, has been extremely rare since the 1950s.³ However, American physicians continue to encounter patients with malaria, mostly immigrants, refugees, returned travelers, and military personnel, who acquired their infections in endemic areas. An average of 1700 malaria cases and 13 related deaths occur in the United States every year. Most of the cases were acquired in sub-Saharan Africa and caused by Plasmodium falciparum, although Plasmodium species infections are imported from other malaria-endemic regions, as well.⁴

Malaria is a parasitic infectious disease that has been in existence for centuries. The term malaria is of Italian origin, meaning “bad air,” reflecting the belief in medieval times that it was caused by exposure to foul air in swamps and marshes. This is true to some extent, because the mosquito vector breeds well in warm, humid environments. As a result, the disease is highly prevalent in tropical and subtropical areas, including sub-Saharan Africa, the Indian subcontinent, Southeast Asia, and South America (Figure 70-1). Malaria is caused by members of the parasitic protozoan genus Plasmodium, within which there are approximately 120 species that infect mammals. Human malaria is caused by four Plasmodium species: Plasmodium falciparum, P. vivax, P. ovale, and P. malariae. Transmission occurs throughout the tropics and subtropics, specifically in sub-Saharan Africa, the Indian subcontinent, Southeast Asia, and the western Pacific.⁵ P. falciparum is the most prevalent species worldwide. P. vivax is also common, although it is not typically found in sub-Saharan Africa. P. malariae occurs sporadically in all malaria-endemic areas, but is largely restricted to sub-Saharan Africa and the Western Pacific.⁶ P. ovale is the rarest of all four species, and is found mainly in Africa and the Western Pacific.⁶ An additional species, P. knowlesi, a species that typically infected Southeast Asian macaques, has caused disease in humans in Malaysia and other areas of Southeast Asia.⁷

PATHOGENESIS

MALARIA LIFE CYCLE

The life cycle of Plasmodium takes place in both the human host and the mosquito vector (Figure 70-2; see also Figure 70-3). As shown in Figure 70-2: (1) the female Anopheles mosquito takes a blood meal, during which it injects sporozoites (human infective stage) into the human host. Then the sporozoites pass ...