Short report: Entomologic inoculation rates and Plasmodium falciparum malaria prevalence in Africa

Epidemiologic patterns of malaria infection are governed by environmental p arameters that regulate vector populations of Anopheles mosquitoes. The int ensity of malaria parasite transmission is normally expressed as the entomo logic inoculation rate (EIR), the product of the vector biting rate times t he proportion of mosquitoes infected with sporozoite-stage malaria parasite s. Malaria transmission intensity in Africa is highly variable with annual EIRs ranging from < 1 to > 1,000 infective bites per person per year. Malar ia control programs often seek to reduce morbidity and mortality due to mal aria by reducing or eliminating malaria parasite transmission by mosquitoes . This report evaluates data from 31 sites throughout Africa to establish f undamental relationships between annual EIRs and the prevalence of Plasmodi um falciparum malaria infection. The majority of sites fitted a linear rela tionship (r(2) = 0.71) between malaria prevalence and the logarithm of the annual EIR. Some sites with EIRs < 5 infective bites per year had levels of P. falciparum prevalence exceeding 40%. When transmission exceeded 15 infe ctive bites per year, there were no sites with prevalence rates < 50%. Annu al EIRs of 200 or greater were consistently associated with prevalence rate s > 80%. The basic relationship between EIR and P. falciparum prevalence, w hich likely holds in east and west Africa, and across different ecologic zo nes, shows convincingly that substantial reductions in malaria prevalence a re likely to be achieved only when EIRs are reduced to levels less than 1 i nfective bite per person per year. The analysis also highlights that the EI R is a more direct measure of transmission intensity than traditional measu res of malaria prevalence or hospital-based measures of infection or diseas e incidence. As such, malaria field programs need to consider both entomolo gic and clinical assessments of the efficacy of transmission control measur es.