Abstract
Resistance to the establishment of Ostertagia ostertagi was tested in calves which had been infected daily with 1000 larvae for 30, 80, 150, and 250 days. The proportion of a challenge dose of 30,000 larvae which became established decreased steadily and by Day 250 was only 1 20 of the proportion that became established in susceptible control animals. Neither the age of the host nor the age of the larvae used affected the proportion that became established. It is shown that the increasing resistance to the establishment of worms, and an average life span of 28 days, of worms which developed beyond the early fourth stage, adequately account for the course of infections in calves infected daily. Evidence is presented which suggests that the host's previous experience of infection increases the proportion of the worms that are inhibited in their development.
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