Abstract
There are no specific clinical signs or symptoms of malaria. Fever attacks, anemia, or signs of severity like coma or respiratory distress cannot easily be attributed to malaria in people who are infected most of the time. Ascribing clinical manifestations to malaria is problematic in populations that are regularly exposed to the transmission of human plasmodia. The more transmission is intense and regular, the higher the prevalence of asymptomatic infections. In areas of intense and perennial malaria transmission, more than 90% of the population may be infected and the simple detection of a plasmodial infection is not enough to attribute clinical manifestations to malaria. Naturally acquired anti-malaria immunity permitting asymptomatic infections is incomplete and temporary. It is an obstacle to the estimation of the malaria burden in endemic areas. The positive association between parasite density and fever allows the attribution of clinical attacks to malaria. The relationship between parasitaemia and the risk of fever is not continuous. An age- and endemicity-dependent threshold effect of parasite density has been demonstrated and can be used to distinguish clinical attacks due to malaria from others. Clinical diagnosis and evaluation of malaria are problematic in three situations: in public health to estimate the malaria burden for health services, in clinical research to evaluate treatments or prophylactic measures (drug, vaccine, anti-vectorial devices), and in basic research on pathophysiology, immunology or genetic susceptibility to clinical malaria. No one diagnostic definition nor procedure for detecting cases is adequate for all three purposes. Case detection may be passive (in health structures for example) or active (in population). The choice of methods for diagnosis and recruitment depends on the objectives and whether a "pragmatic" or "explicative" approach is used. The radical differences between these approaches are often unsuspected or ignored.
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