Schistosomiasis in rice fields and implications for biocontrol: case studies from Senegal and Madagascar show sharp contrasts in detection of intermediate snail hosts

Abstract Approximately 230 million people worldwide are infected with schistosomes, and more than 90% of those requiring treatment live in sub-Saharan Africa. Senegal and Madagascar both have high prevalences of schistosomiasis, which has a significant impact on human health and on each country’s economy. In Africa, schistosomiasis is caused by Schistosoma trematodes with a life cycle that depends on a definitive human host and intermediate Bulinus or Biomphalaria snail hosts. Drug treatment kills adult stages of the parasite but does not prevent reinfection, and elimination strategies therefore require prevention or targeted control of the intermediate host snail. In Senegal, irrigation canals and rice fields often harbour schistosome host snails and are well-described potential sites of infection. In Madagascar, it remains unknown whether similar habitats harbour host snails despite schistosomiasis and rice cultivation being highly prevalent. This study presents results of a snail survey in Senegal in 2024 and repeated surveys in 2022 and 2023 in Madagascar as case studies to compare schistosomiasis as an aspect of human health in the context of rice farming in the 2 countries. To account for differences in sampling effort, total snail counts per site were normalized by the number of sweeps deployed. Detection of intermediate host snails for the parasite was significantly associated with country. While snails were found in fields and canals in Senegal, competent hosts were not detected in Madagascar, indicating possible differences in habitat and population dynamics and that geographic differences may require different research and intervention approaches.