Calculation of Anopheline Man-Biting Densities from Concurrent Indoor and Outdoor Resting Samples

Abstract

A continuing problem in malaria epidemiology is the accurate measurement of the manbiting-rate (ma of Macdonald's 1957 formula, h= mabs). Both direct catches of female anophelines as they bite and all indirect sampling methods are subject to biases of different kinds (eg Garrett-Jones and Shidrawi, 1969; Garrett-Jones, 1970). Although there are arguably more accurate methods of sampling, the indirect, pyrethrum'spray-catch'of houses is unquestionably the most productive of anophelines per man-hour. Its weakness is that some females of even highly endophilic species such as Anopheles gambiae invariably leave a house on the night that they feed, and the technique thus only gives an accurate measure of ma when the exophilic proportion of the population is very small. The present paper suggests a means for adjusting this estimate of ma to allow for the proportion of the vector population that is resting outside. When the gonotrophic cycle lasts 2 days, the morning female anopheline population comprises fed and gravid individuals in approximately equal numbers (excluding unfed and pre-gravid females). Sampled during the morning, the outside resting population should therefore be found to consist of a minority of fed mosquitoes-those that left the houses soon after feeding during the previous night-and a larger number of gravid mosquitoes that fed the night before and left either then or as half-gravids 24 hours later. Where densities are high, and natural resting sites not too abundant, this situation may commonly be observed in unsprayed areas (eg Muirhead Thomson, 1951; Brady, unpublished observations in Ghana). When it is, the data can be made use of as follows: let