Early warning of malaria epidemics in African highlands using Anopheles (Diptera : Culicidae) indoor resting density

Several highland regions of Africa recently have suffered malaria epidemics . Because malaria transmission is unstable and the population has little or no immunity, these highlands are prone to explosive outbreaks when densiti es of Anopheles exceed critical levels and conditions favor transmission. I f an incipient epidemic can be detected early enough, control efforts may r educe morbidity, mortality, and transmission. Here we present three methods (direct, minimum sample size, and sequential sampling approaches) that cou ld be used to determine whether the household indoor resting density of Ano pheles gambiae s.l, has exceeded critical levels associated with epidemic t ransmission. Data on Anopheles density before, during, and after a malaria epidemic (December 1997-July 1998) in the highlands of southwestern Uganda were evaluated to demonstrate the application of these three approaches. Du ring this epidemic, a density of 0.25 Anopheles mosquitoes per house was as sociated with epidemic transmission, whereas 0.05 mosquitoes per house was chosen as a normal level expected during nonepidemic months. The direct app roach to calculating mean Anopheles density with an allowable error of 20-5 0% of the mean would require the sampling of 102-16 houses, respectively. I n contrast, with only seven houses, the minimum sample size approach could be used to determine whether Anopheles density had exceeded the critical le vel. This method, however, would result in an overestimation of the risk of an epidemic at low Anopheles density. Finally, a sequential sampling plan could require as man) as 50 houses to conclude that risk of an epidemic exi sted, but this disadvantage is offset by the ability to preset the probabil ities of concluding that risk of an epidemic exists at both the critical an d normal Anopheles densities. Our study illustrated that it is feasible, an d probably expedient, to include monitoring of Anopheles density in highlan d malaria epidemic early warning systems.