Resistance of Trichostrongylus colubriformes to oxfendazole

first communication in Uruguay

Abstract

The anthelmintic susceptibility of gastrointestinal nematodes isolated from a herd 01 1146 Merino lambs is analyzed. A first assay was designed to establish, under field controlled conditions, the anthelmintic involved in the failure of efficacy. A total of 80 lambs was randomly divided into four groups (n=20) and treated with the dose commercially recommended in Uruguay for oxfendazole (2,5 mg/kg), albendazole (3,8 mg/kg) and levamisole (5,O mg/kg). A group was kept as control, without treatment. Results expressed in geometric means for days "O" and "10" post treatment, showed a reduction in egg counts (REC) of 0%, 59,9% and 99,2% respectively, for oxfendazole, albendazole and levamisole. The purpose of the second assay was to experimentally infect lambs to carry out under controlled conditions, an efficacy trial with the anthelmintic of lower REC values. A total of 12 sheep, 7 to 12 months old, was experimentally infected with 23.000 larvae per animal and was exposed 10 different treatments (n=4), 34 days post infection. Two groups received 5 mg/kg and 10 mg/kg of oxfendazole respectively. A third group was kept untreated, as control. The necropsy of all animals was performed six days after infection. With 5 mg/kg dose, oxfendazole had no control on Thricostrongylus colubriformis, (0.0%), while with a 10 mg/kg dose, it had 11.0% control. The involvement of the resistance fenomenon at farm level is discussed, as well as the risks of future use of drugs from the group bencimidazole.