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This paper presents the results of microgeographical studies of human water contact behavior and Schistosoma mansoni transmission levels and intensity of infection in four rural areas in Machakos District, Kenya. The relationship between intensity of infection (geometric mean egg counts) in 3502 persons aggregated in 120 household clusters and eight independent variables was investigated using straight and stepwise linear regression and mapping techniques. Results indicate that the two water contact variables, mean frequency per person and mean duration per person, as well as mean number of sites used per person, a transmission index and mean distance to the most frequently used site were the strongest predictors of geometric mean egg counts. All three distance variables were usually negatively associated with infection although intensity of infection and water contact declined relatively slowly with distance from the streams. This pattern appears to be owing to a combination of the relatively short distances, a general lack of safe alternative water sources and the use of more distant water contact sites both inside and outside the study area during periods of drought. The study of snail-to-man transmission identified number of infected snails as the major transmission variable and number of contacts as the major predictor variable. Mapping of total egg counts at the household cluster level and total number of infected snails revealed spatial association with transmission sites. All results varied considerably between study areas, owing to differences in exposure levels, transmission patterns and environmental factors. Findings are discussed in relation to the epidemiology and control of schistosomiasis and suggestions are made for further spatial studies.

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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.

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In an operational Schistosoma mansoni field-study in an area about 20 km 2 (population approximately 8000), transmission detection by simple snail sampling was compared with cercariometry. Between 1985 and 1987, 62 field sites were sampled at fortnightly intervals. Of a total of 2758 field observations, 89.8% gave full snail data; 64.4% full cercarial data; and 61.7% complete data for both methods. The complete data sets showed significant but not strong correlations between Biomphalaria pfeifferi (total and infected with S. mansoni or other trematodes) and cercarial (S. mansoni and non-human) recoveries. Non-human (but not S. mansoni) cercarial recovery decreased with deteriorating cercariometry filter quality. Both snail and cercarial recoveries diminished significantly with increasing water flows at the time of collection. Many samples yielded infected snails or cercariae, but not both, and neither method detected significantly more transmission sites. The method of choice for detecting transmission in a large-scale field-study depends on logistical and financial considerations. Relatively simple snail sampling allows quick, cheap and widespread data collection adequate for most purposes but more complicated cercariometry is still valuable for specific, small-scale studies.

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Integrated sampling for human prevalence, intensity, and incidence of Schistosoma haematobium, as well as for human water contact and snail distribution and density was carried out in the Volta lake farming village of Agbenoxoe at various times between 1978 and 1980. Nuclepore filters were used for determining egg output. Snail sampling was by the man-time method. A new system of recording human water contact was introduced for the peculiar condition at Agbenoxoe. Results indicated significant focality of infection and transmission in the compact village, concentration in the 5- to 19-year-old age group, and distinct seasonality of transmission. Water contact was frequent but of short duration. Only a few children under the age of 5 entered the water, and age-specific curves for duration of water contact paralleled the curve for geometric mean of egg counts (log10 of eggs + 1) for males and the prevalence curve for females. Water contact for males was of longer duration than for females and included more time playing and wading. These activities probably accounted for the much higher incidence and prevalence rates recorded for males over females in the village. The concentration of infection, transmission, and water contact in the 5- to 19-year-old age groups at Agbenoxoe and villages like it supports a control strategy of treating only this age span with drugs.

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After three years of cercarial transmission control using focal application of niclosamide and weed removal in water contact sites (WCSs) in the project area of the Volta Lake, the numbers of WCSs infested with cercariae and infected snails were reduced by over 90% in areas of both high and low endemicity. This, combined with selective population chemotherapy, reduced the prevalence of Schistosoma haematobium infection by 72% in the area of low endemicity and 40% in the area of high endemicity. The intensity of infection in the villages was reduced by 78% in both areas. The overall annual cost of the cercarial transmission control programme was US $1.09 per capita.

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Schistosoma haematobium miracidia were detected in sentinel snails placed in 16 human water contact sites in the Volta Lake, each month from March 1973 to November 1977. Results showed that rates of infection were seasonal, and that infected snails were more often found in water contact sites sheltered by emergent plant growth than in exposed open beach sites with no emergent vegetation. Sentinel snail infection rates were correlated with natural snail infection rates and with epidemiological levels of schistosomiasis in village inhabitants. After two years of chemotherapy and mollusciciding, levels of disease and sentinel snail infection rates dropped in two-thirds of the villages. In the remaining villages, however, the sentinel snail infection rates were not correlated with the fall in epidemiological level, because of ecological changes in the water contact sites.It is concluded that, unless control measures are continued, the constant changes in the lake shore environment will lead to a rapid re-establishment of previous levels of disease transmission.

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Results of 5 years of sampling for Bulinus rohlfsi in human-water contact sites of villages along the Volta Lake, Ghana, have confirmed that the aquatic macrophyte, Ceratophyllum, is the most important ecological factor for sustaining high levels of cercarial transmission of Schistosoma haematobium. Data available so far indicate that growth of this weed largely determines the size of the snail populations. Increasing density of Ceratophyllum correlates with increasing levels of cercarial transmission potential in the water contact sites and of S. haematobium infection in the village populations.

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In Ghana, Schistosoma haematobium exists as two strains, one transmitted by Bulinus rohlfsi and the other by B. globosus. In Anyaboni, a resettlement town, where the field station of the UNDP/WHO Schistosomiasis Research and Control Project is located, the residents contract the "rohlfsi" strain of the parasite from the Volta Lake and the "globosus" strain from a stream near the town. The present studies indicate that there is mixing of the two parasite strains on a community and an individual basis. In Anyaboni, the parasite developed well in both B. rohlfsi and B. globosus. In another village 25 km from Accra, where B. globosus was the only vector, the parasite developed well in B. globosus but was refractory in B. rohlfsi. In a village near the Volta Lake where B. rohlfsi was the sole vector, the parasite developed well in B. rohlfsi but was refractory in B. globosus. However, complete separation of the two strains is uncommon in Ghana because extensive mixing has already occurred owing to migration of people and snails.

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In the present ecological study of cercarial transmission of Schistosoma haematobium in the Volta Lake, Ghana, habitat observations and sampling of Bulinus truncatus rohlfsi were conducted within a 60-km stretch of shoreline. Observations revealed that human water contact sites in each village undergo constant changes in shape and vegetation. Snail sampling surveys in water contact sites were carried out monthly (for 27 months) in 8 villages using newly designed palm-leaf traps, and in 8 additional villages (for 16 months) using a modification of Olivier & Sneidermann's man-time method. Results to date confirm the finding by Chu & Vanderburg that cercarial transmission in the lake takes place almost exclusively within water contact sites. Additional results indicate that even within individual water contact sites this transmission is focal, most infected snails being found very close to the shoreline. Transmission also varies significantly according to shape, vegetation, and geographical location of the water contact sites, and is distinctly seasonal in most villages. These findings lead us to conclude that control of cercarial transmission in the Volta Lake is both attainable and feasible with existing methods.

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