A case of moxidectin failing to control ivermectin resistant Ostertagia species in goats
Leathwick, D.M.
Veterinary Record 136(17): 443-444
1995
ISSN/ISBN: 0042-4900 PMID: 7631480 Document Number: 441925
In mid 1993, 24 goats which had been isolated from other stock since their purchase 13 days earlier, were dosed orally with ivermectin (Ivomec liquid for sheep and goats) at 0.2 mg/kg liveweight. Examination of faecal samples collected before and 10 days after drenching showed no significant decrease in mean faecal egg count (FEC). The animals were maintained in isolation and 18 days later were randomly allocated to 3 treatment groups to receive either ivermectin or moxidectin (Vetdectin oral drench for sheep) at 0.2 mg/kg liveweight or left untreated as controls. Because of low FECs in the first test, multiple samples of faeces per animal and counts per sample were taken. Despite the high precision of the counts (sensitivity 1 egg recovered = 8 eggs per gram) there was considerable between animal variation in FEC reduction and a number of animals, including 3 who had received moxidectin, showed sizeable increases in FEC after drenching. Culture of faeces collected after drenching produced a pure population of Ostertagia species. Mean pre- and post-drench FECs were 108 and 84 epg for the ivermectin group and 130 and 274 epg for the moxidectin group. The author concludes that the failure of both ivermectin and moxidectin to reduce FECs was probably due to the resistance of the Ostertagia species and not to other factors.