Malaria: even more chronic in nature than previously thought; evidence for subpatent parasitaemia detectable by the polymerase chain reaction.

In high endemicity areas, malaria is a chronic disease: examination of blood films reveals that up to half of the population, particularly children, harbour parasites at any one given time. The parasitological status of the remainder was addressed using the polymerase chain reaction, a technique 100 to 1000 times more sensitive than microscopy, on a series of samples from Dielmo, a holoendemic area of Senegal. Two-thirds of the microscopically negative individuals were found to harbour subpatent levels of Plasmodium falciparum, suggesting that more than 90% of the exposed population at any one time, i.e. in a cross-sectional survey, are chronically infected. This also means that the range of parasite loads harboured by humans with various degrees of exposure is remarkably large, probably reflecting a large range of effectiveness of the defence mechanisms against malaria parasites, none of which is fully efficient.