[Interference between anthelmintics and immunity in gastrointestinal strongylosis of sheep]

Groups of very similar sheep (breed, sex, age, hemoglobin type, family) were immunised against H. contortus by repeated infections. Self-cure and immune reactions expelled parasites but some of them still remain in the gut of animals; they have been eliminated by dosage of anthelmintics. Animal were challenged; a discrepancy exists between numbers of eggs passed in each group (immune and dosed animals, immune and non-dosed animals and non-immune animals); the best fitting estimated equation on experimental values (or on moving average points) is used for statistical tests. Immunity vanishes partly (or totally) when animals have been dosed; a residual population of worms is thought to be responsible for immunity because of a permanent booster.