High Annual and Seasonal Variations in Malaria Transmission by Anophelines and Vector Species Composition in Dielmo, a Holoendemic Area in Senegal

We conducted a three-year entomologic study in Dielmo, a village of 250 inhabitants in a holoendemic area for malaria in Senegal. Anophelines were captured on human bait and by pyrethrum spray collections. The mosquitoes belonging to the Anopheles gambiae complex were identified using the polymerase chain reaction. Malaria vectors captured were An. funestus, An. arabiensis, and An. gambiae. Anopheles funestus was the most abundant mosquito captured the first year, An. arabiensis in the following years. The annual entomologic inoculation rates calculated by enzyme-linked immunosorbent assay were 238, 89, and 150 for the first, second, and third years, respectively. Each year there was a peak of transmission at the end of the rainy season, but transmission occurred year round. The heterogeneity of transmission was found at four different levels: 1) the relative vector proportion according to the place and method of capture, 2) the human biting rate and relative proportion of vectors by month and year, 3) the infection rate of each vector by year, and 4) the number of infected bites for all vectors, and for each species, for the year. Our data show that even in areas of intense and perennial transmission, there exist large longitudinal variations and strong heterogeneity in entomologic parameters of malaria transmission. It is important to take these into account for the study of the variations in clinical and biological parameters of human malaria, and to evaluate this relationship, a very thorough investigation of transmission is necessary. The interpretation of malaria parameters such as parasit- emia, morbidity, mortality, and associated immune responses depends on having precise information and close follow-up of variations in malaria transmi~sion.'~~ It is very well known that the transmission of malaria in Africa is not homoge- neo~s.~ The vector species and density, the Plasmodium spe- cies, the number of infective bites per human per year (also called the annual entomologic inoculation rate (EX)), and the monthly Em, are changeable. Many studies have com- pared transmission between villages in the same area:-8 but few have shown that significant differences may also occur within the same location over a several year follow-up study. Such variations have to be taken into account in longitudinal studies on the development of malaria immunity, but so far these have been studied very little. A longitudinal study be- gan in 1990 to evaluate malaria infections and the mecha- nisms of protective immunity in a population living in Diel- mo, a village in a holoendemic area of Senegal. During a four-month follow-up of the entire population conducted during the 1990 rainy season, the cumulative prevalence of P. falciparum, P. malariae, and P. ovale were 98.6%, 50.5%, and 40.3%, respe~tively.~ The preliminary studies showed that the transmission was continuous throughout the year and that the vectors were Anopheles funestus and mos- quitoes of the An. gambiae complex.'0*'' Three species of this complex were noted in the study area: An. gambiae, An. arabiensis, and An. melas. Malaria transmission by these different vectors was stud- ied from 1992 to 1995 using the polymerase chain reaction (PCR), which identifies the species of the An. gambiae com- plex.'* In this paper, we have investigated the relative fre- quencies of the different vector species (An. funestus, An. gambiae, An. arabiensis, andAn. melas) collected within the study area according to the time, season, place, kd method of capture, and