Recovery of infective Schistosoma mansoni schistosomula from liquid nitrogen: a step towards storage of a live schistosomiasis vaccine.

Successful recovery of infective schistosomula of Schistosoma mansoni following storage at -196 degrees C is reported. The technique involves a two-step cooling procedure--slow cooling (0.65 degrees C min-1) to an intermediate temperature of -28 degrees C, followed by rapid cooling into liquid nitrogen (10,000 degrees C min-1). Rewarming (10,000 degrees C min-1) and rapid dilution to remove the cryoprotectant (17.5% methanol) yielded motile organisms some of which developed to adult worms in mice after intramuscular injection. The percentage of schistosomula developing to adult worms was small (0.44%), but is a significant step towards storage of trematode larvae and of a live attenuated vaccine for schistosomiasis.