Drenching adult ewes: Implications of anthelmintic treatments pre- and post-lambing on the development of anthelmintic resistance

• Published in: New Zealand veterinary journal 2006-12, Vol.54 (6), p.297-304
• Main Authors: Leathwick, DM, Miller, CM, Atkinson, DS, Haack, NA, Alexander, RA, Oliver, A-M, Waghorn, TS, Potter, JF, Sutherland, IA
• Format: Article
• Language: English
• Subjects: Albendazole - pharmacology; Albendazole - therapeutic use; Animal Husbandry - methods; Animals; anthelmintic resistance; Anthelmintics - pharmacology; Antinematodal Agents - pharmacology; Antinematodal Agents - therapeutic use; capsule; Drug Resistance; Feces - parasitology; Female; Nematoda - drug effects; Nematoda - genetics; Nematode Infections - prevention & control; Nematode Infections - transmission; Nematode Infections - veterinary; New Zealand; Ostertagia; Parasite Egg Count - veterinary; parasites; Parasitic Sensitivity Tests - veterinary; Pregnancy; Random Allocation; Selection, Genetic; Sheep; Sheep Diseases - prevention & control; Sheep Diseases - transmission; Teladorsagia; Trichostrongylus
• ISSN: 0048-0169; 1176-0710
• DOI: 10.1080/00480169.2006.36714

AIMS: To test the hypothesis that peri-parturient anthelmintic treatment of adult ewes, either pre-lambing with a control- led-release capsule (CRC) or at tail-docking with a short-acting oral formulation, would increase the rate of development of anthelmintic resistance, as compared to not drenching ewes and giving an additional drench to lambs in the autumn. Also, to evaluate the potential of routinely leaving 15% of the heaviest lambs untreated when drenching, as a means of slowing the development of anthelmintic resistance. METHODS: A replicated farmlet trial was run from 1999- 2004. Eleven farmlets, each consisting of five paddocks, were initially seeded with Ostertagia (Teladorsagia) circumcincta and Trichostrongylus colubriformis parasites, these being a mixture of albendazole-susceptible and -resistant isolates to yield a 96% reduction in faecal nematode egg count (FEC) on drenching. Four prescriptive drenching regimes were applied; Treatments 1-3 were replicated three times and Treatment 4 twice. Treatments were as follows. Treatment 1: Ewes were given an albendazole CRC pre-lambing, and any ewes exceeding 65 kg liveweight were given two capsules simultaneously; lambs were given a five-drench preventive programme of treatments, orally, of albendazole on Days 0, 21, 42, 70 and 98 after weaning. Treatment 2: Ewes were given a single oral treatment of albenda- zole at docking (2-3 weeks after lambing), and lambs were given the same five-drench preventive programme as in Treatment 1. Treatment 3: Ewes remained untreated, while lambs were given a six-drench preventive programme of treatments, orally, of al- bendazole on Days 0, 21, 42, 70, 98 and 126 after weaning. Treatment 4: Ewes remained untreated, while lambs were given the same six-drench preventive programme as in Treatment 3, but the heaviest 15% of lambs were left untreated each time. Albendazole-resistance status was measured at least twice-yearly, using faecal egg count reduction tests (FECRTs) and larval development assays (LDA). In addition, controlled slaughter of drenched and undrenched tracer lambs was undertaken in the last 3 years. RESULTS: Resistance to albendazole increased most rapidly in Treatment 1, as measured by FECRT and LDA results, and worm burdens in tracer lambs. In Treatment 2, resistance developed slower than in Treatment 1 but faster than in Treat- ments 3 and 4, as measured by LDA; resistance in Treatment 2 developed more quickly than in Treatment 4, as measured by FEC