Transmission of Schistosoma mansoni was monitored by routine snail sampling for Biomphalaria pfeifferi and by supplementary cercariometric measurements in 4 neighbouring study areas in Machakos District, Kenya. After 1 year, extensive, population-based chemotherapy with a single dose of praziquantel was given in 3 areas, but only minimal treatment in the fourth. In the year preceding treatment, seasonal transmission of S. mansoni and other non-human trematodes occurred in all 4 areas, despite some ecological differences and the effects of earlier treatment campaigns in 1 of the study areas. After treatment of all infected subjects in one area in which there had been earlier chemotherapy campaigns, S. mansoni transmission remained very low. It was reduced for at least 2 years after chemotherapy targeted at either all heavily infected subjects or all infected school children, but it was unaffected in an area where treatment was restricted to those few very heavily infected cases at risk of developing, disease. Nowhere was transmission entirely eliminated by chemotherapy and that of non-human trematodes continued unabated. The snail data correspond well with the human, parasitological data. Targeting school children was as effective as more extensive campaigns, but chemotherapy alone never stopped S. mansoni transmission: reinfection was inevitable, at rates determined by ecological factors affecting snail populations.

The prevalence of Schistosoma mansoni infections of Biomphalaria pfeifferi shows seasonal variation. Field data from Zimbabwe show annual ranges from 0 to 7%. In this paper a mathematical model of B. pfeifferi population dynamics and S. mansoni epidemiology is used as a framework for analysis of these patterns. Snail fecundity is a function of snail age and of temperature, and is apparently affected by other seasonal factors. The pre-patent period is dependent on temperature. Infection affects snail fecundity and mortality. Parameter values are derived from previous field and laboratory studies. The force-of-infection is estimated from the analysis of size-prevalence data. Using observed temperatures, model output agrees well with field data on snail abundance and prevalence of infection over a 14-month period. Seasonal variation in prevalence largely reflects variation in the pre-patent period and in snail population age structure. The possible role of seasonality in the force-of-infection is discussed. Prevalence patterns are not greatly affected by year-to-year differences in temperature. Significant seasonal variation in snail–man transmission rates is expected.

Populations of Bulinus globosus and Biomphalaria pfeifferi were studied in a river habitat in Zimbabwe over a period of 12 months. Data were obtained on the prevalences of infections of Schistosoma haematobium (also S. mattheei) and S. mansoni respectively. Population parameters showed the following patterns for both snail species. (1) A patchy distribution correlated with the distributions of aquatic plants. (2) Life-expectancies of only a few weeks. (3) Recruitment rates correlated with water temperature and showing a distinct seasonal peak. (4) Spatial variation in recruitment. (5) A redistribution of snails during the rainy season. Epidemiological parameters showed the following patterns. (1) Large seasonal variations in the prevalence of patent infections. (2) Evidence from size-prevalence curves that suggests a variable force-of-infection from man to snail, correlated with water temperature. (3) Prevalences of infection that were higher in the vicinity of (±60 m from) major water contact sites. Local prevalences of infection for B. globosus sometimes exceeded 50% and may have approached 100% if pre-patent infections are included. Snail numbers may limit transmission at these locations. Attention is drawn to the need to make field observations at an appropriate spatial scale and also to the implications for the effectiveness of focal snail control as a means of reducing transmission.

Irrigation for intensive sugar cultivation started in the early 1980s at Richard Toll, some 100 km from the mouth of the Senegal River. Infections with Schistosoma mansoni were first seen in late 1988. This study records quantitative snail surveys for over 3 years from 1992 at sites representing different habitats in and around the irrigation scheme. Populations of both Biomphalaria pfeifferi (the intermediate host of S. mansoni) and Bulinus spp. (mainly B. truncatus, the local host of S. bovis) peaked in late ‘spring' or early ‘summer', depending on the habitat, and then remained low until the following ‘spring'. B. pfeifferi favoured smaller, man-made habitats with most transmission between May and August each year. The less abundant Bulinus spp. favoured larger natural and man-made habitats with most S. bovis transmission between April and July. S. mansoni infections were more, but S. bovis infections were less abundant than other trematodes in their respective snail hosts. Ecological changes in the early 1980s due to sugar irrigation pre-dated similar, more widespread changes in the late 1980s when the completion of dams across the Senegal River prevented seasonal rain fed floods and sea water intrusion. S. mansoni has since spread rapidly around Richard Toll. The incompatibility of the local S. haematobium strains with the dominant bulinid snails has so far prevented an epidemic of urinary schistosomiasis at Richard Toll, but the invasion of similar downstream habitats by susceptible B. globosus is worrying. The principal control measure, chemotherapy, given in the ‘winter' would minimise the rate of reinfection. It could be reinforced by judicious mollusciciding within the sugar irrigation scheme but not elsewhere.

The construction of the Diama dam on the Senegal River and the ensuing ecological changes have led to a massive outbreak of Schistosoma mansoni infection in Northern Senegal, associated with very high intensity of infections, due to extremely intense transmission. The vectorial capacity of Biomphalaria pfeifferi from Ndombo, near Richard-Toll was investigated in order to assess the role of the snail–parasite relationship in this particular epidemiological situation. The results revealed an unusually high compatibility between the Senegalese S. mansoni strain and its local snail intermediate host, B. pfeifferi. The snail infection rate after exposure to a single miracidium per snail was 87%. The cercarial production of infected snails was very high, with a mean total production of 50456 cercariae per snail. No significant difference was found in the total cercarial output between snails exposed to 1 miracidium and those exposed to 5 miracidia. The increase in the rate of cercarial output was significantly greater in snails exposed to 5 miracidia, but there was a higher mortality in this group. The chronobiological cercarial production pattern showed a peak around mid-day. The implications of these findings on the epidemiology of schistosomiasis in Northern Senegal are discussed.