Effects of cercarial concentration and length of exposure on the infection of mice by Schistosoma japonicum and S. mekongi cercariae.

The effects of cercarial concentration and length of exposure on the infection of mice by Schistosoma japonicum and S. mekongi were studied under laboratory conditions. For all cercarial concentrations of 10, 20 and 30, mice exposed for 1 min acquired little or no infection. Total worm recovery tended to rise with an increase either in cercarial concentration or exposure time. The highest infection rate and worm burden were obtained in mice exposed for 64 min to 30 cercariae per mouse. The present investigation suggested that exposures of 1 min or less carried little risk of infection at low cercarial concentration. There was a greater risk of infection in prolonged exposure to low cercarial concentration. Prolonged exposures to high cercarial concentration carried the greatest risk of high infection rate and worm burden.