Malaria is caused by obligate intracellular protozoa of the genus Plasmodium, including P. falciparum, P. malariae, P. ovale, P. vivax, and P. knowlesi. Plasmodium has a complex life cycle that enables survival in different cellular environments in the human host and in the mosquito vector. There are two major phases in the life cycle, an asexual phase (schizogony) in humans and a sexual phase (sporogony) in mosquitoes. The erythrocytic phase of Plasmodium asexual development begins when the merozoites released from exoerythrocytic schizonts in the liver penetrate erythrocytes. When inside the erythrocyte, the parasite transforms into the ring form, which enlarges to become a trophozoite. These latter two forms can be identified with Giemsa stain on blood smear, which is the primary means of confirming the diagnosis of malaria.
The parasites usually are transmitted to humans by female Anopheles mosquitoes. Malaria also can be transmitted through blood transfusion via contaminated needles and transplacentally to a fetus.
Malaria is a worldwide problem with transmission in more than 100 countries with a combined population of more than 1.6 billion people. Malaria is an important cause of fever and morbidity in the tropical world. The principal areas of transmission are sub-Saharan Africa, southern Asia, Southeast Asia, Mexico, Haiti, the Dominican Republic, Central and South America, Papua New Guinea, and the Solomon Islands. Approximately 1000 to 2000 imported cases are recognized annually in the United States, with most cases occurring among infected foreign civilians from endemic areas who travel to the United States and among U.S. citizens who travel to endemic areas without appropriate chemoprophylaxis.
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Fever of Unknown Origin
The clinical manifestations of malaria range from asymptomatic infection to fulminant illness and death, depending on the virulence of the infecting malaria species and the host immune response. The incubation period ranges from 6 to 30 days, depending on the Plasmodium species (Table 123-1).The most characteristic clinical feature of malaria, which is seldom noted with other infectious diseases, is febrile paroxysms alternating with periods of fatigue but otherwise relative wellness. The classic symptoms of the febrile paroxysms of malaria include high fever, rigors, sweats, and headache. Paroxysms coincide with the rupture of schizonts that occur every 48 hours with P. vivax and P. ovale (tertian periodicity) and every 72 hours with P. malariae (quartan periodicity).
Short-term relapse describes the recurrence of symptoms after a primary attack that is due to the survival of erythrocyte forms in the bloodstream. Long-term relapse describes the renewal of symptoms long after the primary attack, usually due to the release of merozoites from an exoerythrocytic source in the liver. Long-term relapse occurs with P. vivax and
P. ovale because of persistence in the liver and with P. malariae because of persistence in the erythrocyte.