The clinical response to infection depends both on the species of the parasite and on the immunological status of the patient. Non-immune travellers to malarious areas risk severe attacks. Acute malaria also occurs where exposure is limited or seasonal and where the collective immunity is relatively low. In these circumstances it can occur in epidemic proportions and affect all age groups in the community.
Acute falciparum malaria is a potentially fatal disease causing prolonged, irregular high fever, intense headache and vomiting. Complications include cerebral malaria (characterized by confusion, convulsions and rapidly progressive coma), hypoglycaemia, septicaemia, pneumonia, pulmonary oedema, acute renal failure and massive haemolysis. Chronic or repeated infection often leads to splenomegaly and progressive anaemia. Splenic rupture is a dangerous complication of vivax malaria, and P.malariae infection occasionally gives rise to a fatal nephrotic syndrome.
Pregnant women, if untreated, are at particularly high risk death from falciparum malaria, especially where transmission is intermittent. In holoendemic areas they are partially protected by a measure of immunity. This reduces the risk of congenital infection, but it does not protect the placenta particularly in primigravidae, can harbour large numbers of malaria parasites. The fetus is thus inevitably exposed to the effects of placental insufficiency.
Largely as a result of passive transfer of maternal antibodies across the placenta, infants born to immune mothers living in holoendemic areas are unlikely to acquire malaria for several months after birth. Thereafter, they risk death from severe and recurrent acute attacks during infancy and early childhood. From the age of five until adulthood the severity and frequency of these attacks decrease as immunity develops.
Except among pregnant women, severe malaria is infrequent in adults who have always lived in areas of high transmission.