Attenuation ofSchistosoma mansonicercariae with a molluscicide derived fromMillettia thonningii

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Abstract
A chloroform extract of molluscicidal plant secondary compounds from the seeds of a West African legumeMillettia thonningiiwas used to attenuate cercariae ofSchistosoma mansoniprior to infection of NMRI female mice. Exposure of cercariae to 0·3, 0·6, 1·3, 2·5, 10 or 20 mg/l concentrations of extract for 30 mm, immediately before standardized infection, was associated with a concentration-dependent decline in worm establishment at 55 days post-infection. The mean numbers of adult worms established declined from about 17 worms/mouse with control cercariae and those exposed to 0·3 mg/lMillettiato 0·1 worms/mouse after 10 mg/l pre-treatment. Mice exposed to cercariae after 20 mg/l pre-treatment had no adult worms at 55 days post-infection. The activities of cercariae 30 mm after exposure toMillettiaextract at concentrations up to 2·5 mg/l were similar to those of control larvae: none was immobile. Exposure to higher concentrations ofMillettiaprogressively reduced swimming activity and increased the proportion of immobile cercariae. After pretreatment with 20 mg/lMillettiathe majority of larvae were immobile. Levels of anti-parasite antibodies (estimated by ELISA assay using an adult worm-based antigen preparation) rose between 21 and 55 days p.i. with control cercariae and those pre-treated with 2·5 mg/lMillettia. After 5 mg/l pre-treatment, cercariae induced a reduced antibody rise, while mice exposed to cercariae after 10 mg/l pre-treatment showed no rise in anti-parasite antibody levels. These results are discussed in the context of protocols which could possibly useMillettia-attenuated cercariae to induce useful levels of protection in mice towards further cercarial challenge.