Acquired immunity against Cooperia spp. and Ostertagia spp. in calves: effect of level of exposure and timing of the midsummer increase.

In two experiments, groups of calves were exposed to different levels and patterns of infection with Ostertagia spp. and Cooperia spp. The experimental design simulated the stereotypic pattern of herbage infestation, including both as normal and a delayed midsummer increase, under conditions of set-stocking. After this simulated 'first grazing season', calves were followed during the subsequent winter housing. At the end of that housing period some calves were challenged with 100,000 L3 Cooperia spp. and 40,000 L3 Ostertagia spp. and slaughtered 23 days later. All previously infected calves were protected against the establishment of the challenge infection with Cooperia spp., but not against Ostertagia spp. For the latter a significant negative correlation was found between worm count and previous level of exposure to infection. During the simulated first grazing season, changes in the ratio of Cooperia to Ostertagia eggs in the faecal egg output and the genus-specific egg count were influenced by both the level of exposure and the timing of the midsummer increase. It is concluded that acquired immunity against both parasite genera develops depending on the level of exposure to infection during a first grazing season, and that delaying the midsummer increase results in a delay of the acquisition of an effective immunity as measured by faecal egg counts and the ratio of Cooperia to Ostertagia egg output.