Isolation and Maintenance of Schistosoma Parasites

Upon exposure to light, 50–100 snails that are patent with S. mansoni infection, are induced to shed cercariae into the surrounding water. Cercariae are cleaned and concentrated over a series of sieves using distilled water and allowed to stand on ice in a 50 mL polystyrene tube for 1 h. During this time, cercariae clump, settle to the bottom and stick to the inside surface of the tube. The water is poured off and replaced with 9 mL ice-cold ‘Incomplete’ Medium 169 ([35] (link); custom made at the UCSF Cell Culture Facility) that contains 1× penicillin-streptomycin solution. Cercariae are mechanically transformed into schistosomula by passing back and forth between two 10 mL syringes attached via a 22-gauge double-headed needle (adapted from [36] (link)). After deposition into a 9 cm diameter Petri dish, cercarial heads are separated from tails by swirling in Incomplete Medium 169 and the lighter tails aspirated leaving the heads (schistosomula) settled in the center of the dish. Under sterile conditions, schistosomula are washed 3 times in Incomplete Medium 169 and allowed to settle over ice in a 1.5 mL microfuge tube. Parasites are kept on ice for up to 2 h prior to screening with compounds. As a note, Medium 169 is preferred over RPMI as a culture medium for schistosomula – worms survive with <10% mortality for up to 4 weeks whereas in RPMI, approximately 40–60% of the parasites die within 3 days with continued mortality out to two weeks (Ruelas and Caffrey, unpublished).
Adult worms, perfused from hamsters, are washed 5 times in RPMI 1640 containing 1× penicillin-streptomycin solution and 10 µg/mL amphotericin B (both supplied by the UCSF cell culture facility). After 3 further washes in Incomplete Medium 169, parasites are maintained in ‘Complete’ Medium 169 (with the addition of 10% fetal bovine serum (FBS; HyClone, Logan, Utah) at 37°C and 5% CO₂ for up to 24 h prior to screening with compounds.