Impact of the crayfish Procambarus clarkii on Schistosoma haematobium transmission in Kenya

The Louisiana red swamp crayfish, Procambarus clarkii, which was introduced into east Africa in the 1950s or 1960s, has since widely dispersed. Previo us work by our group has shown that P. clarkii can reduce populations of th e molluscan intermediate hosts of human schistosomes through predatory and competitive interactions. Here, we investigate whether crayfish can reduce populations of Bulinus africanus and consequently, Schistosoma haematobium prevalence in school children. Children from 6 primary schools in the Macha kos and Kitui Districts of Kenya were selected for study. Schools were divi ded into 3 experimental-control pairs. At experimental schools, crayfish we re introduced into nearby aquatic habitats harboring Bulinus africanus snai ls and serving as S. haematobium transmission sites. Snail habitats near co ntrol schools did not receive crayfish. Six months after crayfish introduct ion, all infected children were treated with praziquantel. Children were th en monitored quarterly for 2 years, at which time infection and reinfection rates were compared statistically between the paired schools. In one such pair, crayfish failed to establish, resulting in neither snail control nor a reduction in transmission. At the second pair of schools, the numbers of snails were decreased by the presence of crayfish, but a clear difference i n infection rates in children could not be detected, primarily because drou ght conditions kept overall transmission rates low. At the third school pai r, crayfish established well in experimental habitats, snail numbers decrea sed precipitously, and children at the experimental school were significant ly less likely to acquire S. haematobium infections than children at the co ntrol school. Our results indicate that under certain environmental circums tances, P. clarkii exerts a significant impact on the transmission of human schistosomiasis in Kenya. Important questions remain regarding the impact of P. clarkii on Kenyan freshwater ecosystems, not the least of which is it s potential to significantly influence the epidemiology of schistosomiasis in east Africa.