Schistosoma mansoni oviposition in vitro reflects worm fecundity in vivo: individual-, parasite age- and host-dependent variations

In an attempt to determine whether in vitro oviposition of adult S. mansoni reflects the fecundity status of worms in vivo, Mongolian gerbils and ICR, BALB/c and SCID mice were infected with about 100 cercariae and examined on an individual basis, 5-12 weeks later, for worm burden, counts of eggs in liver and small intestine, and for the rate of egg deposition of ex-vivo female worms cultured in vitro, singly or in pairs, over a 3-5 day incubation period. The percentage of egg-laying female worms and the number of eggs laid/female after 3 days in culture showed, like worm fecundity in vivo, wide inter-worm variability, especially in 5-, 6- and 12-week-old worms; varied significantly with the age of the parasite with a maximum level attained by worms of approximately 8 weeks of age; and differed in worms recovered from different host species and strains. The data taken together indicate that measuring the egg-producing ability of S. mansoni in vitro reflects the fecundity status of worms in vivo and additionally provides likely explanations for hitherto poorly understood findings on schistosome fecundity.