title: Mutual resistance to avermectins and milbemycins: oral activity of ivermectin and moxidectin against ivermectin-resistant and susceptible nematodes.
abstract: To determine whether there is mutual resistance to avermectin and milbemycin anthelmintics, ivermectin and moxidectin sheep drenches were tested against ivermectin-resistant and susceptible isolates of Ostertagia circumcincta and Trichostrongylus colubriformis in sheep. None of the isolates had been exposed to moxidectin previously. The dosage of ivermectin required to remove 95 per cent of the ivermectin-resistant O circumcincta and T colubriformis were 23 times and six times larger, respectively, than the dosages required to remove the same percentage of susceptible isolates. The dosages of moxidectin required to remove 95 per cent of the ivermectin-resistant O circumcincta and T colubriformis were 31 times and nine times larger, respectively, than the dosages required to remove the same percentage of susceptible isolates. It is concluded that the worms resistant to ivermectin were also resistant to moxidectin.
authors.affiliation: Merck Research Laboratories, Merck and Co Inc, Rahway, New Jersey 07065.
authors.forename: W L
authors.initials: WL
authors.lastname: Shoop
authors.forename: H W
authors.initials: HW
authors.lastname: Haines
authors.forename: B F
authors.initials: BF
authors.lastname: Michael
authors.forename: C H
authors.initials: CH
authors.lastname: Eary
mesh_terms: D000284:Administration, Oral; D000818:Animals; D000871:Anthelmintics / Q000627:therapeutic use*; D000900:Anti-Bacterial Agents / Q000008:administration & dosage / Q000627:therapeutic use; D004351:Drug Resistance; D005260:Female; D007559:Ivermectin / Q000008:administration & dosage / Q000627:therapeutic use; D018942:Macrolides; D008297:Male; D010028:Ostertagia / Q000187:drug effects / Q000302:isolation & purification; D010029:Ostertagiasis / Q000188:drug therapy / Q000469:parasitology / Q000662:veterinary*; D011897:Random Allocation; D012756:Sheep; D012757:Sheep Diseases / Q000188:drug therapy* / Q000469:parasitology; D014253:Trichostrongylosis / Q000188:drug therapy / Q000469:parasitology / Q000662:veterinary*; D014254:Trichostrongylus / Q000187:drug effects / Q000302:isolation & purification
chemical_list: D000871:Anthelmintics; D000900:Anti-Bacterial Agents; D018942:Macrolides; D007559:Ivermectin; C027837:moxidectin