In the present experiment the response of sheep to a single infection with Haemonchus contortus at graded doses, and their responses to two subsequent infections with 3000 infective larvae, were studied. The response of the sheep was investigated by estimation of faecal worm egg counts on alternate days throughout the course of the experiment and by following the development of the parasite by differential worm counts performed on animals slaughtered at crucial times.

Initial infections were established with graded doses of 500–2700 infective larvae. The proportion of infective larvae which became established at each dose, development of the worm burden and fecundity per adult worm were similar for each dose.

Following first challenge infection with 3000 infective larvae there were no effects which could be related to the previous level of infection. Whereas there was no difference in egg production per female worm, a small but consistent increase in the number of retarded larvae was observed in the previously infected animals (6% of the burden was in the 4th larval stage at day 56) compared with the previously uninfected control animals (2% in the 4th stage at day 56).

Following a second challenge with 3000 infective larvae an increase in the number of retarded larvae was again observed, and, in addition, egg production per female worm was reduced compared with egg production in previously uninfected sheep given a similar challenge infection.

The results of the initial infections suggest that control of the growth and development of H. contortus infection by factors independent of an active host response does not occur in worm burdens established from doses as high as 2700 infective larvae.