An experiment has been conducted to examine the effects of discontinuous infection with H. contortus in sheep on the development of resistance to infection. When sheep were given four doses of 3000 larvae at fortnightly intervals and these sensitizing infections were removed on day 56, there was no evidence of resistance at that time nor of any residual effect of the sensitizing infections on the response to challenge at day 140. When, however, the sensitizing infections were permitted to persist until day 132, there was strong resistance to the establishment of a challenge infection whether or not the sensitizing worm burden was removed prior to challenge, and, in the case of superimposed challenge, resistance was coupled with rejection of most of the sensitizing population. When a single infection with 3000 larvae was followed 22 days later with a further single dose of 3000 larvae, there was no evidence of resistance whether or not the first infection was removed with anthelmintic. If the interval between two doses of 3000 larvae was extended to 76 days there was resistance to the establishment of the second dose if this was superimposed on the first, but not if the first infection was removed 8 days prior to the second dose.

It is suggested that the manifestations of resistance appear in a sequence of (i) resistance only to a superimposed infection, followed by (ii) resistance to a challenge infection in the presence or absence of a previously existing worm burden, coupled in the former case with rejection of much of the existing population. This sequence could be related crudely to a product of larval dose and time. Although larval dose and time were to some extent confounded in the design of the experiment strong resistance was associated with prolonged uninterrupted infection.