Study design

Areas for sampling the snails living in the river were also defined. These measured 30 m along the bank and 3 m into the main body of the water. The corners of these rectangular sampling areas were marked by pegs so that successive samplings could be performed across the same area. Each site was sampled weekly from 0800 − 0900 hours over a period of six months.

All snails found floating or attached to vegetation were collected using a scooping net with a long handle and placed on white plastic trays in order to be able to rapidly identify the different species. Snails of the Biomphalaria species, responsible for the transmission of bilharzia, were transferred into large plastic buckets filled with water from the snail sampling areas and containing appropriate aquatic plants to simulate natural environmental conditions. These were taken to the field centre to screen for the microscopic presence of Schistosoma mansoni cercariae. Snails that were not shedding cercariae were kept for a week to allow subsequent development of cercariae arising from early stages of infection.

Snails that did not shed cercariae in either screening were considered to be uninfected. These and the other snail species were taken 1 km downstream and returned to the river. This was to maintain similar proportions of snails within the sampling areas and to avoid sampling the same snails on different sampling occasions.

Before sampling of snails began environmental variables thought to be responsible for influencing the distribution of these snails along the river were also recorded. These included water flow velocity, water pH, water temperature and concentration of total dissolved solids (TDS) in the water. Water flow velocity was obtained by sprinkling methyl orange dye from the upstream mark of the sampling area and recording the time taken for the dye colour to cover the 30 m distance to the downstream mark. Values for pH and temperature were obtained by using a pH meter integrated with a temperature probe. TDS concentration was determined using a conductivity meter.