ATTRIBUTABLE FRACTION ESTIMATES AND CASE DEFINITIONS FOR MALARIA IN ENDEMIC AREAS

Asymptomatic carriage of malaria parasites occurs frequently in endemi c areas and the detection of parasites in a blood film from a febrile individual does not necessarily indicate clinical malaria. In areas of low and moderate endemicity the parasite prevalence in fever cases ca n he compared with that in community controls to estimate the fraction of cases which are attributable to malaria. In areas of very high tra nsmission such estimates of the attributable fraction may be imprecise because very few individuals are without parasites, Furthermore, non- malarial fevers appear to suppress low levels of parasitaemia resultin g in biased estimates of the attributable fraction. Alternative estima tion techniques were therefore explored using data collected during 19 59-1991 from a highly endemic area of Tanzania, where over 80 per cent of young children are parasitaemic. Logistic regression methods which model fever risk as a continuous function of parasite density give mo re precise estimates than simple analyses of parasite prevalence and o vercome problems of bias caused by the effects of non-malarial fevers. Such models can be used to estimate the probability that any individu al episode is malaria-attributable and can be extended to allow for co variates. A case definition for symptomatic malaria that is used widel y in endemic areas requires fever together with a parasite density abo ve a specific cutoff. The choice of a cutoff value can be assisted by using the probabilities derived from the logistic model to estimate th e sensitivity and specificity of the case definition.