The effects of chemoprophylaxis on acquired immunity against gastrointestinal nematodes in cattle.

The level and duration of exposure to gastrointestinal nematode infections are of crucial importance for the development of acquired immunity in first season grazing calves. An excessive reduction of host-parasite contact by chemoprophylaxis, pasture management, or both, causes a diminished level of acquired immunity. Moreover, the level of acquired resistance is negatively related to the degree of suppression of host-parasite contact. Whether or not a reduced resistance against establishment and development of gastrointestinal nematode infections has a negative effect on weight gain in the second grazing season, depends both on the intensity of the prophylaxis used and on the level of the challenge infection. Cross-sectional serological surveys and questionnaire surveys in The Netherlands, Germany and Belgium indicated that parasitic nematode control in first season grazing calves tends to be overprotective. Possible consequences of over-treatment, besides higher treatment costs and a reduced level of acquired immunity, are more drug residues in animal products and in the environment and increased selection for anthelmintic resistance.