Trichinella spiralis: role of different life cycle phases in induction, maintenance, and expression of rapid expulsion in rats.

The capacity of different phases of the life cycle of Trichinella spiralis to induce rapid expulsion was examined. The phases examined included enteral preadults, enteral adults, and parenteral larvae. All had the ability to induce rapid expulsion although there were significant quantitative differences in their inductive capacity and in the kinetics of expression. Immunization with preadults required only a 48-hr enteral exposure to 2000 worms to induce strong rapid expulsion. In contrast rats required a 14-day exposure to adult worms to elicit a comparable response. After immunization with adults the reaction was demonstrable for only 2 weeks. Parenteral larvae produced only a weak rapid expulsion reaction by themselves and this response did not develop until some 8 weeks after challenge. When immunization with the enteral phases (preadult and adult) was combined with exposure to parenteral larvae a strong and enduring rapid expulsion reaction was observed. Phase specificity was also observed in the susceptibility of worms to the rapid expulsion response. The preadult phases, from infectious larvae to worms of up to 2 days of age were highly susceptible. Older worms, from 3 to 4 days old were not susceptible to rapid expulsion and could invade and establish themselves in the primed intestine for at least a 48-hr period without apparent adverse effects.