Resistance of Cooperia to ivermectin treatments in grazing cattle of the Humid Pampa, Argentina.

A field experiment and a controlled efficacy trial were conducted to demonstrate resistance of cattle trichostrongylid nematodes to endectocidal compounds in grazing cattle on the Humid Pampa, Argentina. Male Polled Hereford calves, aged 9-11 months old, with a history of frequent treatments with compounds of the avermectin/milbemycin class, were used for the trial. The field experiment involved six groups of 10 animals each, which were subcutaneously treated with either ivermectin (IVM), long-acting ivermectin (LA-IVM), moxidectin (MXD) or doramectin (DRM) at a dosage of 0.2mg/kg BW. A fifth group was treated orally with fenbendazole at a dosage of 5mg/kg BW; the sixth group of calves served as non-treated control. Faecal samples were collected from each animal on the day of treatment and at 14 days after treatment. Nematode egg counts were performed to estimate the faecal egg count reduction test (FECRT). The FECRT showed that reductions were 65% (IVM), -20% (LA-IVM) and 85% (DRM). Egg counts from calves treated with MXD or fenbendazole were reduced by 95 and 100%, respectively. For the controlled efficacy trial, six animals with the highest egg counts from the IVM, LA-IVM and non-treated control groups, were necropsied on day 18 after treatment to determine numbers and nematode species in the abomasum and small intestine. The results indicated that efficacy against Cooperia oncophora was 62.7 and 48% for IVM and LA-IVM, respectively. Neither of the IVM formulations showed efficacy against Trichostrongylus spp. in the small intestine. However, the absence of Trichostrongylus spp. in some animals of both treated and control groups precluded a valid assessment of efficacy or resistance. It was concluded that resistance of trichostrongylid nematodes to the avermectin/milbemycin class of compounds in grazing cattle of the Humid Pampa, Argentina, may be strongly established on farms where cattle are treated at frequent intervals throughout the year.