Abstract
BackgroundIn tropical Africa, where malaria is highly endemic, low grade infections are asymptomatic and the diagnosis of clinical malaria is usually based on parasite density. Here we investigate how changes in malaria control and endemicity modify diagnostic criteria of Plasmodium falciparum attacks.Methods and FindingsParasitological and clinical data from the population of Dielmo, Senegal, monitored during 20 years, are analyzed in a random-effect logistic regression model to investigate the relationship between the level of parasitemia and risk of fever. Between 1990 and 2010, P. falciparum prevalence in asymptomatic persons declined from 85% to 1% in children 0–3 years and from 34% to 2% in adults ≥50 years. Thresholds levels of parasitemia for attributing fever episodes to malaria decreased by steps in relation to control policies. Using baseline threshold during following periods underestimated P. falciparum attacks by 9.8–20.2% in children and 18.9–40.2% in adults. Considering all fever episodes associated with malaria parasites as clinical attacks overestimated P. falciparum attacks by 42.2–68.5% in children and 45.9–211.7% in adults.ConclusionsMalaria control modifies in all age-groups the threshold levels of parasitemia to be used for the assessment of malaria morbidity and to guide therapeutic decisions. Even under declining levels of malaria endemicity, the parasite density method must remain the reference method for distinguishing malaria from other causes of fever and assessing trends in the burden of malaria.
Highlights
In tropical Africa, where malaria is highly endemic, most individuals are semi-immune and asymptomatic malaria infections are highly prevalent
Malaria control modifies in all age-groups the threshold levels of parasitemia to be used for the assessment of malaria morbidity and to guide therapeutic decisions
Following early studies in the 1980s that have shown that parasitemia was much higher during malaria attacks than during asymptomatic chronic infections and that it was possible to use parasite density thresholds to confirm or to discard the diagnosis of clinical malaria with a low risk of error [1,2], the parasite density method has become the reference method for measuring malaria morbidity in research and clinical trials in highly endemic areas of tropical Africa [3,4,5,6,7]
Summary
In tropical Africa, where malaria is highly endemic, most individuals are semi-immune and asymptomatic malaria infections are highly prevalent. Numerous studies have been conducted during the two past decades using various approaches and models to investigate the relationship between the dynamics of parasitemia and the occurrence of fever, and to determine levels of Plasmodium falciparum parasitemia which constitute the best threshold values for attributing to clinical malaria a fever episode in a given area and population [3,4,5,6,7,8,9,10,11,12,13,14] These studies have shown that age and endemicity level are the two most important factors that determine these levels. We investigate how changes in malaria control and endemicity modify diagnostic criteria of Plasmodium falciparum attacks
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