The epidemiology of malaria

Abstract

Epidemiologists have recently paid greater attention than in the past to the epidemiology of clinical malaria as opposed to the epidemiology of malarial infection. This change of emphasis has been stimulated in part by the need for better clinical definitions of malaria in the evaluation of control measures such as insecticide-treated materials and malaria vaccines. Methods of determining mortality from malaria and of defining severe and uncomplicated malaria have been devised. The limited data available indicate that malaria-attributable mortality and the incidence of severe malaria do not increase with an increase in the entomological inoculation rate above a threshold value, an observation that has important implications for the likely long-term effects of attempts to contain malaria through vector control. Study of the epidemiology of severe malaria in Africa has shown different epidemiological patterns for the two most frequent forms of this condition: cerebral malaria and severe malarial anaemia. Severe malarial anaemia is seen most frequently in areas of very high malaria transmission and most frequently in young children. In contrast, cerebral malaria predominates in areas of moderate transmission, especially where this is seasonal, and it is seen most frequently in older children. Study of patients with uncomplicated malaria has established the relationship between fever and parasite density and has demonstrated ways of defining fever thresholds. Algorithms have been developed to help in the diagnosis of malaria in the absence of parasitological confirmation but this approach has proved difficult because of the overlap in symptoms and signs between malaria and other acute febrile illnesses such as pneumonia.

PIP: Even though the malaria life cycle was discovered a century ago, new aspects of the epidemiology of the disease have recently been described. While early malaria surveys sought mainly to determine the extent of infection in given communities, epidemiologists are now increasingly interested in the epidemiology of clinical malaria. This shift in focus was prompted partly by the need for better clinical definitions of malaria in evaluating control measures such as insecticide-treated materials and malaria vaccines. Available data indicate that malaria-attributable mortality and the incidence of severe malaria do not increase with an increase in the entomological inoculation rate above a threshold value. This observation has important implications for efforts to contain malaria through vector control. Cerebral malaria and severe malarial anemia are the 2 most frequent forms of severe malaria in Africa. Severe malarial anemia is seen most frequently in areas of very high malaria transmission and most often in young children. Cerebral malaria, however, predominates in areas of moderate transmission, especially where such transmission is seasonal, and is found mostly in older children. The study of patients with uncomplicated malaria has shed light upon the relationship between fever and parasite density, and has helped to define fever thresholds. Algorithms developed to help diagnose malaria in the absence of parasitological confirmation have been problematic.