Population build up of gastrointestinal nematode infections in ewes and lambs on pasture grazed by calves in the previous year.

The development of trichostrongylid infections was studied in ewes, lambs and tracer lambs grazing a pasture which has been used by calves in the previous year. Treatment with fenbendazole of the ewes two days before they were turned out to pasture virtually prevented infections with trichostrongylid species characteristic for sheep. Significant burdens of Ostertagia leptospicularis, Cooperia oncophora and Nematodirus helvetianus were found. An increased pasture infectivity level compared to spring was observed in C oncophora and O leptospicularis in August and September. The possibility that O leptospicularis may build up to pathogenic levels in sheep and cattle as a result of alternate grazing is discussed. The two other species are probably of little significance as pathogens in sheep.