Asymptomatic Plasmodium parasitemia and the ecology of malaria transmission.

Tip O’Neill, former speaker of the United States House of Representatives, famously pointed out that all politics is local. The same is true of malaria. Malaria transmission is not homogeneous through an endemic area but spotty, and depends on two primary factors: location of mosquito breeding sites, and clustering of human habitations where people serving as reservoirs of parasites for mosquito infection live. 1 Whether malaria transmission in focal areas of a malaria-endemic region is stable or unstable is an important determinant of morbidity and mortality. Stable malaria implies that prevalence of infection is sufficiently high to engender a substantial level of clinical immunity within a population, i.e. the presence of parasitemia in the absence of malaria-related symptoms. In contrast, unstable malaria, characterized by spatial and temporal variability, is associated with lower levels of clinical immunity within a population, the propensity for epidemics, and a higher rate of severe disease in adults. Clinical immunity to malaria within a population occurs in the context of premunition, defined as the presence of an immune response that produces control of but not complete elimination of parasitemia and pathological sequelae.