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Titration of efficacy of ivermectin and moxidectin against an ivermectin-resistant Haemonchus contortus derived from goats in the field

W. E. Pomroy1,2, S. P. Hart1, and B. R. Min1

1E (Kika) de la Garza Institute for Goat Research, Langston University, Langston, OK
2Institute of Veterinary, Animal and Biomedical Sciences, Massey University, New Zealand

The objective of this study was to titrate the efficacy of ivermectin (IVM) and moxidectin (MOX) dewormers using variable dose rates of both anthelmintics against an IVM-resistant isolate of Haemonchus contortus. Yearling wether goats (45 kg BW, mean fecal strongylid egg count >1000 eggs/g) were randomly allocated to groups of 6 animals. Each group was treated as follows: I1 (IVM 0.1 mg/kg), I2 (IVM 0.2 mg/kg), I3 (IVM 0.4 mg/kg), I4 (IVM 0.8 mg/kg), M1 (MOX 0.05 mg/kg), M2 (MOX 0.1 mg/kg), M3 (MOX 0.2 mg/kg), M4 (MOX 0.5 mg/kg) and an untreated control group. The injectable cattle formulation of IVM and the pour on cattle formulation of MOX were both administered orally. Fecal egg counts (FEC) were estimated by a modified McMaster technique on the day of treatment (d 0) and d 7 and d 15 after treatment. Feces from each treatment group were cultured to determine the genera of larvae. Data were analyzed non-parametrically using a Kruskal-Wallis test. Haemonchus was the dominant genus found in pretreatment and control larval cultures but both Trichostrongylus and Ostertagia were also present in low numbers. Control group mean FEC varied by < 2% over the 3 sampling days. The percent reduction in FEC from pretreatment numbers was <80% for all dose rates of IVM, whereas the efficacy for M2, M3 and M4 were > 98% but the reduction for M1 was 93% on d 7 and 82% on d 15. On d 7 and 15 only the FEC of the 3 higher dose rates of MOX were significantly (P<.05) lower than the control group. Trichostrongylus were found to comprise 19% of control cultures, and 10%, 16%, 4%, and 22% of I1, I2, I3, and M1 cultures, respectively on d 7. Results indicate that IVM-resistance is present in H. contortus at IVM doses as high as 0.8 mg/kg but a dose as low as 0.1 mg/kg of the formulation of MOX used was effective. The survival of Trichostrongylus at 0.4 mg IVM/kg suggests the emergence of IVM resistance in this parasite as well.


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