Multigeneric resistance to oxfendazole by nematodes in cattle
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A GROWING number of reports describing anthelmintic resistance by gastrointestinal nematodes of cattle have appeared in the literature (Eagleson and Bowie 1986, Geerts and others 1987, Jackson and others 1987, Pinheiro and Echevarria 1990, Borgsteede 1991, McKenna 1991). In September 1990, a routine drench check was undertaken on an intensive bull beef rearing operation in the Waikato province of New Zealand. Faecal worm egg counts from a random selection of calves preand 10 days after treatment with oxfendazole indicated an efficacy for the anthelmintic treatment of 53 per cent. Two trials were then conducted: the first, a faecal egg count reduction test (FECRT) to define the drench resistance status of the nematode parasite population on the property; and the second, a slaughter trial to confirm the benzimidazole resistance. For the FECRT, 28 autumn-born Friesian bull calves harbouring natural worm burdens were randomised into four treatment groups each of seven animals on the basis of faecal egg counts and liveweight. The calves were dosed to individual liveweight with oral formulations of either ivermectin (0-2 mg/kg, Ivomec; Merck Sharp & Dohme), levamisole (7 5 mg/kg, Nilverm; Mallinckrodt Veterinary), oxfendazole (4 53 mg/kg, Synanthic; Mallinckrodt Veterinary) or water (1 ml/5 kg). Faecal egg counts were evaluated at seven, 10 and 14 days after treatment. Larval cultures were prepared by treatment group on day 10. For the slaughter trial, six calves were removed from pasture to an indoor feedlot where each was offered a daily ration of 2-0 kg lucerne/barley concentrate (NRM Feeds), with an unlimited supply of meadow hay and fresh water. After a 15-day conditioning period the calves were weighed and randomised, on the basis of liveweight and their 'off pasture' faecal egg counts, into two groups of three. The calves of group 1 were then treated, to individual liveweights, with an oral formulation of oxfendazole at 4-53 mg/kg. The calves in group 2 received a water placebo at