The response of sheep given six sensitizing infections each of 3000 infective larvae of H. contortus at fortnightly intervals to a challenge infection of 3000 infective larvae has been studied. While a large proportion of the challenge infection (ca. 3000 parasites) persisted to termination of the experiment on day 141 in sheep (group 3) in which the challenge was superimposed upon the sensitizing infections, only a mean of 437 worms, including both 4th-stage larvae and adults, was recorded at this time in another group (group 5) in which the sensitizing infections were terminated by anthelmintic treatment on day 90, i.e. 8 days before challenge. The proportion of the total worm burdens of the challenge infections arrested at the 4th larval stage was increased for both groups (25% for group 3 and 18% for group 5) by comparison with unsensitized control groups (4% for group 6 and 2% for group 7).

As the effect of immunological control was expressed more vigorously in the group (group 5) in which the sensitizing infections were terminated before challenge it seems likely that recovery of immunological competence occurred in these animals following removal of the sensitizing infections. This conclusion suggests that, owing to the persistence of the sensitizing infections and therefore exposure of the immune mechanism to continuous antigenic insult, the animals which received the superimposed challenge infection had entered a phase of immunological exhaustion during the challenge period.