A comparison between two in vitro techniques to detect resistance to ivermectin in field populations of Cooperia spp. in cattle

Abstract

Anthelmintic resistance in beef cattle production is a well-known worldwide problem, contributing to the economic losses caused by gastrointestinal nematodes. Resistance to ivermectin (IVM) is present in 93.5% of farms in Argentina, Cooperia spp. being the most prevalent genus (100%). Diagnosing AR under field conditions is currently done using the faecal egg count reduction test, which has been long used but lacks sensitivity to detect resistance in its early stages. In trying to improve this, in vitro techniques have been developed for different compounds and different parasites, and tested mainly in sheep parasites. As part of a large study on IVM-resistant populations of Cooperia spp. in beef farms, this assay was designed to evaluate two in vitro techniques, the micro-agar larval development test (MALDT) and the larval migration inhibition test (LMIT), on proven resistant (R) and susceptible (S) field populations. Both populations had been previously characterised by controlled-efficacy tests, showing that the efficacy of ivermectin against R and Se Cooperia was 66.3% and 99.5%, respectively. For the MALDT, eggs of both Cooperia isolates were exposed to twelve anthelmintic concentrations, from 4.7x10⁻¹⁰M to 2.18x10⁻¹¹M. The obtained EC50 values were: 6.93x10-9M (95%CI: 6.37x10⁻⁹M to 7.49x10⁻⁹M) for the R population and 8.33x10-10M (95%CI: 7.86x10⁻¹⁰M to 8.8x10⁻¹⁰M) for the S one, with correlation coefficients (R²) of 0.92 y 0.93, respectively; the resistance factor (RF) was 8.31. For the LMIT, ensheathed L3 were exposed to eight concentrations, from 10⁻⁵M a 5x10⁻⁹M. The EC50 values were 6.33x10-8M (95%CI: 5.30x10-8M to 7.49x10-8M) for the R population, and 8.03x10-8M (95%CI: 5.49x10-8M ? 1,19x10-7M) for the S population, with R² of 0.87 y 0.52, respectively; and a RF of 0.79. Based on these preliminary results, the MALDT would be a useful in vitro technique to detect field populations of IVM-resistant Cooperia nematodes.