Nematode control practices and anthelmintic resistance in dairy calves in the south west of England

A postal survey of worming practices on West Country dairy farms was undert aken and farmers were requested to send faecal samples for nematode egg cou nts. The majority of the farmers who responded had a nematode control polic y which was based on a mixture of anthelmintics and pasture rotation. Sixty -five per cent turned out calves on to the same paddock each year and 57 pe r cent treated their stock with anthelmintics during the second year. Ninet y farms submitted samples for analysis but only 16 samples contained suffic ient eggs to justify repeat egg counts and only eight of these produced eno ugh eggs for in vitro tests. The small number of positive samples, even int o the latter part of the year suggests a heavy use of anthelmintics with re latively clean pasture. One Somerset farm had larvae which developed in hig h concentrations of ivermectin, and eggs were still being passed after two treatments with ivermectin at the manufacturer's recommended dose seven day s apart. Of 100 male nematodes isolated from two of the calves, 88 were Coo peria species and 12 were Nematodirus species. A controlled trial with this isolate in eight Friesian male calves showed a 44 per cent reduction in eg g counts at day 7 but no significant reduction in worm burden postmortem. T his is the first reported case of ivermectin-resistant Cooperia species in cattle in the United Kingdom.