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.