Epidemiological basis for clinical diagnosis of childhood malaria in endemic zone in West Africa

Abstract

It is difficult to distinguish childhood malaria from other common febrile disorders by parasite count alone, because of the wide variation in tolerance of parasitaemia among individuals. We postulated that the proportion of febrile episodes among young children that can be attributed to parasitaemia varies according to simple clinical criteria. We studied 1114 children aged 2-9 years, who attended a dispensary in the Republic of Niger, with a case-control approach; each of 557 febrile children was matched with a non-febrile control by sex, age, ethnic group, and day of presentation. Febrile episodes were classified according to three clinical criteria: the presence of a likely non-malarial cause; the duration (less than 3 or more than 3 days before presentation); and the intensity (below 39 degrees C or 39 degrees C and above). There was no evidence for an association between febrile episodes and parasite count during the dry, low-transmission, season. During the rainy, high-transmission, season, by contrast, there was a highly significant relation (p less than 0.0001) between the likelihood of fever and the parasite count; each clinical criterion strengthened the association. There was no association between parasitaemia and low intensity fevers, with an obvious cause, that started 3 or more days before presentation, even in the rainy season; however, the relative risk of a fever that met all three criteria developing in those with vs those without parasitaemia was 27.5. The proportion of febrile cases attributable to detectable parasitaemia (population attributable risk) ranged from 0 to 0.92. Our results suggest that simple clinical criteria may be valuable in the selection of febrile patients for antimalarial treatment. In this geographic area, high fever of short duration and with no other obvious cause that occurs during the rainy season is most likely to be malaria.

PIP: From 1987-1988, researchers randomly selected 557 febrile children aged 2-9 years who attended a health center in Galmi, Niger and matched them with 557 afebrile children. They wanted to test their hypothesis that the proportion of febrile episodes attributable to parasitemia varied according to simple diagnostic criteria. These 3 criteria included origin of fever, duration (3 or 3 days before presentation), and intensity (39 degrees Celsius or 39 degrees Celsius). 39 of the afebrile controls had parasite counts of 10,000/ul and 9 had 100,000/ul. Therefore relative immunity against malaria that resulted from exposure produced significant variation among individuals. 4% had parasite counts 10,000/ul during the dry season. Further, fever was not associated with parasitemia then (p=.46). On the other hand, fever was 14 times more likely to occur in those with a parasite count of 100,000/ul than those with a parasite count of 10,000/ul during the rainy season (p.0001). Indeed at least 90% of febrile episodes during the rainy season could be attributed to exposure. The relative risk for a fever to be caused by parasitemia when the fever was of unclear origin, lasted 3 days before presentation, and was 39 degrees Celsius stood at 27.5. By combining the relative risk with the proportion of cases that were exposed, the researchers found the population attributable risk (ARp) to be .92. The researchers proposed using an ARp.5 as part of a clinical decision strategy to help determine which febrile children should receive specific antimalarial treatment. In this study, all children with a fever of unclear origin, unless it was a low grade fever lasting 3 days, and children with fever of another obvious cause, short duration, and high degree (44% of 285 febrile children during the rainy season) would have received antimalarial treatment.