The clinical manifestations of malaria vary with geography, epidemiology, immunity, and age. In areas where malaria is highly endemic, groups at highest risk include young children (6 to 36 months), who can develop severe illness, and pregnant women, who are at risk for anemia and delivering low birth weight newborns. In areas where malaria is transmitted throughout the year, older children and adults develop partial immunity after repeated infections and are at relatively low risk for severe disease.
Travelers to malarious areas generally have had no previous exposure to malaria parasites or have lost their immunity if they left the endemic area; they are at very high risk for severe disease if infected with Plasmodium falciparum [1,2]. For this reason, it is important to consider malaria in all febrile patients with history of travel to malarious areas.
The clinical manifestations of malaria will be reviewed here. The epidemiology, pathogenesis, diagnosis, and treatment of malaria are discussed in detail separately. (See related topics.)
Following the bite of an infected female Anopheles mosquito, the inoculated sporozoites go to the liver within one to two hours. Individuals are asymptomatic for 12 to 35 days (depending on parasite species), until the erythrocytic stage of the parasite life cycle (figure 1). Release of merozoites from infected red cells when they rupture causes fever and the other manifestations of malaria. The relapsing species Plasmodium vivax and P. ovale can present as a new infection weeks or months after the initial illness due to activation of residual hypnozoites in the liver .
The incubation period for P. falciparum infection is about 12 to 14 days. Longer incubation periods are more likely in semi-immune individuals and individuals taking ineffective malaria prophylaxis. The incubation period for P. vivax and P. ovale is also about two weeks; most relapses due to these species occur within three months. The incubation period for P. malariae is about 18 days; however, low-grade asymptomatic infections can persist for years (table 1). P. falciparum and malariae have no dormant (hypnozoite) phase, hence do not relapse. (See "Overview of non-falciparum malaria".)