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Studies on the effects of cercarial concentration and length of exposure on the infection of mice by St Lucian Schistosoma mansoni cercariae in a natural running-water habitat

The effects of cercarial concentration and length of exposure on the infection of mice by St Lucian Schistosoma mansoni cercariae were investigated in a running-water habitat. For all exposure times, mice exposed to cercarial concentrations from 2·5 to 40 cercariae per mouse acquired no infections. At higher concentrations, infection rates and worm burdens of mice increased in direct proportion to cercarial concentrations and exposure times. The highest infection rate (57·9 %) and worm burden (46 worms) were obtained in mice exposed for 256 mm to 1280 cercariae per mouse. Cercarial concentrations are low in St Lucian running waters, a fact suggesting that the risk of persons becoming infected while fording, collecting domestic water, washing clothes and swimming in habitats with reasonable flows is very low. It appears that most infected individuals acquire a low worm burden over a rather long period of time.