The incubation period between the bite of the mosquito and the onset of disease is ap-proximately 2 weeks. With P. malariae and with strains of P. vivax in temperate climates, however, this period is often more prolonged. Individuals who contract malaria while tak-ing antimalarial suppressants may not experience illness for many months. In the United States, the interval between entry into the country and onset of disease exceeds 1 month in 25% of P. falciparum infections and 6 months in a similar proportion of P. vivax cases.
The clinical manifestations vary with the species of plasmodia but typically include chills, fever, splenomegaly, and anemia. The hallmark of disease is the malarial parox-ysm. This manifestation begins with a cold stage, which persists for 20 to 60 minutes. During this time, the patient experiences continuous rigors and feels cold. With the conse-quent increase in body temperature, the rigors cease and vasodilatation commences, ushering in a hot stage. The temperature continues to rise for 3 to 8 hours, reaching a maximum of 40 to 41.7°C before it begins to fall. The wet stage consists of a decrease in fever and profuse sweating. It leaves the patient exhausted but otherwise well until the onset of the next paroxysm.
Typical paroxysms first appear in the second or third week of fever, when parasite sporulation becomes synchronized. In falciparum malaria, synchronization may never take place, and the fever may remain hectic and unpredictable. The first attack is often severe and may persist for weeks in the untreated patient. Eventually the paroxysms be-come less regular, less frequent, and less severe. Symptoms finally cease with the disap-pearance of the parasites from the blood.
In falciparum malaria, capillary blockage can lead to several serious complications. When the central nervous system is involved (cerebral malaria), the patient may develop delirium, convulsions, paralysis, coma, and rapid death. Acute pulmonary insufficiency fre-quently accompanies cerebral malaria, killing about 80% of those involved. When splanchnic capillaries are involved, the patient may experience vomiting, abdominal pain, and diarrhea with or without bloody stools. Jaundice and acute renal failure are also common in severe ill-ness. These pernicious syndromes generally appear when the intensity of parasitemia ex-ceeds 100,000 organisms per cubic millimeter of blood. Most deaths occur within 3 days.