The effects of single and repeated inoculations of various larval doses on Strongyloides ratti burden and distribution in rats

The changes in worm burden, distribution, length, and fecundity after and during single and repeated inoculations of 10, 50, or 500 larvae of Strongyloides ratti were examined in rats. Worm burden after a single inoculation of a higher larval dose reduced rapidly. Repeated inoculations of lower larval doses at weekly intervals led to a delayed peak and slower reduction of worm burden; the repeated inoculations of 10 larvae did not induce worm expulsion for at least 7 wk. In repeated inoculations at 3-wk intervals, a primary inoculation of 500 larvae induced strong resistance to reinfection at week 3, whereas no resistance was induced until week 6 in rats receiving repeated inoculations of 10 or 50 larvae. Similar dose-dependent reductions in worm length and fecundity were observed in single and repeated inoculations, and the reductions began earlier than worm expulsion. Intestinal migration of worms from the upper small intestine to the large intestine was observed during the course of single and repeated inoculations. Earlier and clearer migration was observed in rats receiving higher doses. These findings indicate that in S. ratti infection, the changes of worm burden, distribution, length, and fecundity are dependent on the inoculated larval dose.