INCIDENCE OF PLASMODIUM-FALCIPARUM INFECTION IN INFANTS IN RELATION TO EXPOSURE TO SPOROZOITE-INFECTED ANOPHELINES

The relationship of the incidence of Plasmodium falciparum infection t o entomologic inoculation rates (EIRs) was studied in 163 children les s than one year of age in a Tanzanian village to determine likely effe cts of transmission-reducing interventions on infection incidence. A t otal of 66,727 Anopheles gambiae s.l. and 17,620 An. funestus mosquito es were caught in 1,056 light trap collections from 139 houses over a period of more than two years. Time period-specific human biting rates were estimated for 11 village neighborhoods. Sporozoites were detecte d by ELISA in 4.4% of the An. funestus and 2.5% of the An. gambiae s.l . Eight hundred seventeen pairs of blood slides with approximately two -week intervals between slides were used to estimate incidence of para sitemia by fitting reversible catalytic models to parasite positivity data. Estimated EIRs during the four weeks preceding each intersurvey interval averaged 1.6 (SD = 2.1) per adult per night. Parasites were p resent at the end of 31% of the 443 intervals that commenced with a pa rasite-negative slide. Attack rates were comparable with those in west ern Kenya, and the proportion of bites resulting in human infections w as strongly dependent on mosquito density. Incidence of infection incr eased with the EIR up to approximately one bite from a sporozoite-carr ying mosquito per adult per night. However, higher levels of transmiss ion observed locally in the wet season did not result in a correspondi ngly higher incidence. These data suggest that transmission-reducing m easures cannot be expected to reduce incidence of infection at the hig hest levels of EIR.