Side-resistance to macrolactones in cattle nematodes

Abstract

Cattle ranches in Rio Grande do Sul (RS), are predominantly extensive and the control of gastrointestinal nematodes relies on the continuous usage of anthelmintic drugs. Macrocyclic lactones (MLs) are the most frequently used products. There have been reports of anthelmintic resistance from gastrointestinal parasites in RS. The objective of this work was to determine the efficacy of 10 commercially available MLs in naturally infected cattle. A total of seventy Nelore and Charolais of abaut one year old and their crosses were studied. Each group (n=7) was treated with: doramectin 1% (Dectomaxâ, Pfizer), moxidectin 1% (Cydectinâ, Fort Dodge), four ivermectin 1% (Ivermectinaâ, Ouro Fino; Ivermectinaâ, Jofadel; Imectinâ, Irfa; Dectiverâ, Lapisa); ivermectin 3.1% (Ivomec Goldâ, Merial), two abamectin 1% (Abamectinaâ, Jofadel; Abactinâ, Biofarm), and the combination ivermectin 2.25% plus abamectin 1.25% (Solutionâ, Intervet) at their recommended doses. Fecal samples were collected at 0, 7, 14 and 21 days postreatment for the fecal egg count reduction test (FECRT) and larval culture, using modified RESO analysis. At 21 days after treatment overall efficacy was 0, 0, 19, 89, 30, 13, 27, 0, 0, and 67% respectively. Trichostrongylus sp. and Cooperia sp. have shown resistance to all products. Although, ivermectin 3.1% nor the combination ivermectin plus abamectin were used previously against this parasite population, the data confirm the deep development of side-resistance between MLs in the most important cattle nematodes.