Chapter 210

Humanity has but three great enemies: Fever, famine and war; of these by far the greatest, by far the most terrible, is fever.

William Osler

Malaria is a protozoan disease transmitted by the bite of infected Anopheles mosquitoes. The most important of the parasitic diseases of humans, it is transmitted in 108 countries containing 3 billion people and causes nearly 1 million deaths each year. Malaria has been eliminated from the United States, Canada, Europe, and Russia; in the late twentieth and early twenty-first centuries, however, its prevalence rose in many parts of the tropics. Despite enormous control efforts, increases in the drug resistance of the parasite, the insecticide resistance of its vectors, and human travel and migration have contributed to this resurgence. Occasional local transmission after importation of malaria has occurred in several southern and eastern areas of the United States and in Europe, indicating the continual danger to nonmalarious countries. Although there are many promising new control and research initiatives, malaria remains today, as it has been for centuries, a heavy burden on tropical communities, a threat to nonendemic countries, and a danger to travelers.

Five species of the genus Plasmodium cause nearly all malarial infections in humans. These are P. falciparum, P. vivax, P. ovale, P. malariae, and—in Southeast Asia—the monkey malaria parasite P. knowlesi, which can be reliably identified only by molecular methods (Table 210-1). Almost all deaths are caused by falciparum malaria. Human infection begins when a female anopheline mosquito inoculates plasmodial sporozoites from its salivary gland during a blood meal (Fig. 210-1). These microscopic motile forms of the malarial parasite are carried rapidly via the bloodstream to the liver, where they invade hepatic parenchymal cells and begin a period of asexual reproduction. By this amplification process (known as intrahepatic or preerythrocytic schizogony or merogony), a single sporozoite eventually may produce from 10,000 to >30,000 daughter merozoites. The swollen infected liver cell eventually bursts, discharging motile merozoites into the bloodstream. These merozoites then invade the red blood cells (RBCs) and multiply six- to twentyfold every 48–72 h. When the parasites reach densities of ∼50/μL of blood (∼100 million parasites in the blood of an adult), the symptomatic stage of the infection begins. In P. vivax and P. ovale infections, a proportion of the intrahepatic forms do not divide immediately but remain dormant for a period ranging from 3 weeks to a year or longer before reproduction begins. These dormant forms, or hypnozoites, are the cause of the relapses that characterize infection with these two species.

Table 210-1 Characteristics of Plasmodium Species Infecting Humans

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