Field studies of some of the basic factors concerned in the transmission of malaria

Abstract

Summary Observations made on the human population and the vector mosquitoes in a coastal area of tropical Africa where malaria is holo-endemic are described. The actual malaria inoculation rate in infants as derived by analysis of parasite indices—about once in every 100 days—was much less than the theoretical rate derived from entomological data (vector densities, feeding habits and sporozoite rates)—from once every 10 days to once a day. Possible reasons for this discrepancy are discussed; the chief factor is considered to be suppression by the infants of the majority of sporozoite inoculations before parasitaemia occurs. It is not thought that selective biting of different age-groups occurs to any great extent. Studies of the vector species, A. gambiae and A. funestus, confirmed that in this area they are strongly anthropophilic, will feed within 24 hours of emergence, probably require two blood feeds to mature the first batch of eggs, have a 48-hour gonotrophic cycle, and that a significant proportion of half-gravid A. gambiae leave human habitations to mature their eggs elsewhere. The survival rate of the vector species has been measured by a new technique involving the comparison of the immediate sporozoite rate on capture with that found after keeping wild-caught mosquitoes alive for the duration of the extrinsic cycle. We estimate that in this area A. gambiae showed a natural daily mortality of the order of 7 per cent. and A. funestus of the order of 8 per cent. Mortalities among wild-caught mosquitoes kept in cages, while generally lower than these, confirm that A. funestus shows a higher mortality than A. gambiae. Such values of naturally occurring mortalities are perfectly consistent with the sporozoite rates found. The implications of these findings in relation to control by residual insecticides are discussed.