Studies on cryopreservation of third-stage larvae of Parastrongylus cantonensis

Attempts have been made to increase the survival rate of third-stage larvae of Parastrongylus cantonensis during cryopreservation. Parasites were obtained from experimentally infected snails, Biomphalaria glabrata. The maximum non-toxic concentrations of cryoprotectants dimethyl sulphoxide (DMSO) and glycerol were determined as 10% v/v in NCTC 109 medium. Different pre-cooling times and serum additives were assayed to determine their effects on the viability of larvae. Larvae pre-cooled at -20 degrees C for 15-20 minutes before transfer to -70 degrees C had the optimum survival rate. The presence of serum was essential for further cooling to -196 degrees C. There were no differences between native and heat-inactivated sera on the larval viability. The survival of larvae recovered after storage at -196 degrees C was higher in sera collected from the rat, man, or fetal calf than in sera collected from the mouse, dog, or horse. Viability assessed by mouse-infections showed that thawed larvae retained infectivity for susceptible hosts, but it was significantly lower than that of unfrozen control worms.