Evidence for multiple anthelmintic resistance in two nematode parasite genera on a Saanen goat dairy

Multiple resistance was demonstrated within the experimental milking goat herd at the Ruakura Agricultural Centre. The development of this resistance was probably exacerbated by the atypical animal management and parasite control programmes under which the dairy was managed. Resistance by Teladorsagia (Ostertagia) spp. to members of the three broad-spectrum families of anthelmintic is reported for the first time. Worm counts in a controlled slaughter trial demonstrated multiple anthelmintic resistance in a mixed population of Teladorsagia (T. circumcincta and T. trifurcata) and Trichostrongylus spp. The efficacies of oxfendazole, morantel citrate and ivermectin in reducing the Teladorsagia population were 43.8%, 75.6% and 93.1%, respectively. Ivermectin removed more than 99% of the small intestinal Trichostrongylus spp. but oxfendazole and morantel were less effective, removing 37.9% and 87.7%, respectively. Only treatment with ivermectin totally eliminated faecal nematode egg counts 8 days after treatment. The lack of agreement between faecal nematode egg depressions and worm counts in lambs given ivermectin suggests that the drug may have affected the fecundity of the surviving nematode population or the assay lacked the sensitivity required. These data suggest that the sole use of faecal egg depression would be inappropriate for accurate assessment of anthelmintic effectiveness.