A two-year study was conducted in three sequential phases. Initially, four matched groups of nine calves grazing similar pastures were subjected to different chemoprophylactic control programmes while a fifth group acted as unprotected controls. After being housed in the autumn, three calves from each group and two parasite-naive controls were challenged experimentally with Ostertagia ostertagi, Cooperia oncophora and Dictyocaulus viviparus. Post mortem worm counts revealed gradations of protective immunity, with the field controls showing the highest level and the naive controls the lowest. In the second grazing season the remaining animals from each group were grazed together with parasitenaive controls on two fields. No further prophylactic treatments were given. One of the naive controls developed severe parasitic bronchitis but the signs in the other groups were milder and transient. It is concluded that the calves protected by chemoprophylaxis in their first grazing season developed marked resistance to gastrointestinal and pulmonary nematodes, but that their level of immunity was influenced by the effectiveness of the prophylactic strategy used.
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