Malaria transmission in relation to rice cultivation in the irrigated Sahel of Mali

Seven cross-sectional entomological surveys were carried out from September 1995 to February 1998 in three irrigated rice growing villages and three villages without irrigated agriculture in the area surrounding Niono, located 350 km north-east of Bamako, Mali. The transmission pattern differed markedly between the two zones. In the irrigated zone, the transmission of malaria was fairly constant over the seasons at a low level. In the non-irrigated zone, transmission was mostly below detection level during the dry season, whereas it was high toward the end of the rainy season. In the irrigated zone, high densities of mosquitoes were correlated with low anthropophily, low sporozoite indices and probably low survival rates. In the non-irrigated zone, mosquito densities were lower and these relationships were less pronounced. Differential use of mosquito nets in the two zones may have been an important factor in the observed differences in transmission. The presence of cattle may also have played an important role. Two mosquito-catching methods (human landing catch and spray catch) were compared.