The consequences of reducing transmission of Plasmodium falciparum in Africa.
Snow RW., Marsh K.
Malaria transmission intensity in Africa varies over several log orders, from less than one infected bite per year to more than one thousand. In this review we examine the consequences in terms of age pattern, clinical spectrum and overall burden of disease and discuss the possible implications for interventions that reduce exposure to infected bites. With very low transmission intensity, all age groups are susceptible to severe malaria. With increasing transmission intensities, older children and adults suffer less severe disease and with high transmission rates the majority of severe cases occur in infants under one year of age. This pattern reflects the increasingly rapid acquisition of immune responses that limit the life-threatening effects of malaria with increasing exposure to the parasite. The clinical spectrum of severe malaria varies with transmission: with high transmission, severe malarial anaemia dominates and cerebral malaria is rare. As one moves towards lower transmission rates, cerebral malaria accounts for an increasingly large proportion of cases. Although the population risk of severe disease falls with age, the risk of death at an individual level may rise with age after an initial fall from very high case fatality rates in children aged under 6 months. Of central interest to malaria control is how the overall amount of disease in childhood varies with transmission. Data from a number of sources suggest that, with low transmission, the amount of malarial disease rises with increasing exposure but that this saturates relatively early. A key issue is whether the same pattern obtains for deaths, both those directly due to malaria and those from all causes. The methodological limitations of ecological comparisons between different areas are discussed before presenting a review of attempts to use this approach in Africa. This suggests that children living in areas of low malarial endemicity have all-cause mortality rates about half of those of children living in areas of moderate to high transmission. Deaths in the first year of life rise linearly with increasing exposure to malaria over a wide range of transmission intensities; by contrast all-cause mortality in children aged 0-4 years appears to saturate at relatively low transmission intensities. These data suggest that interventions that reduce exposure to malaria parasites, such as insecticide-treated bed nets (ITNs), will have the greatest chance of a sustained effect when used in areas where disease burdens are high but the frequency of parasite exposure is low-to-moderate. In conditions of high transmission, initial reductions in mortality may prove difficult to sustain as the reduced level of transmission may still lie on the part of the curve where mortality has saturated. However, at all levels of transmission the overall balance of benefits, including reduced load on families and health services from non-life-threatening malaria, favours the widespread introduction of ITNs in endemic areas of Africa.