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AbstractAIM: To confirm the presence of multiple anthelmintic resistance on a sheep farm in New Zealand.METHODS: Three groups of 10 weaned Romney-cross lambs were treated either with an oral dose of ivermectin (0.2 mg/kg), or a benzimidazole/levamisole (BZ/LEV) combination (4.75 albendazole and 7.5 mg/kg levamisole), or were left untreated. Ten days later, animals were necropsied, and adult worms recovered and identified from the abomasa and small intestines. Pre- and post-treatment faecal nematode egg counts (FEC) were recorded, and larval cultures were performed.RESULTS: In a faecal egg count reduction test (FECRT), adjusted to reflect pre- and post-treatment larval culture results, ivermectin resistance was detected in Teladorsagia (Ostertagia), Trichostrongylus and Haemonchus spp, while BZ/LEV combination-resistant Teladorsagia and Trichostrongylus spp were also present. Adult worm counts confirmed these results, and identified the species involved as Teladorsagia circumcincta, Trichostrongylus colubriformis and H. contortus.CONCLUSION: Multiple, multi-generic anthelmintic resistance was confirmed on a sheep property in New Zealand. This included the first confirmed case of ivermectin resistance in T. colubriformis from sheep in New Zealand.

AbstractAIMS: 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 colubriformisparasites, 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 FECRTs. There was no significant difference between Treatments 3 and 4, although this approached significance in Ostertagia spp, as measured by LDA.CONCLUSIONS: Anthelmintic treatments to adult ewes around lambing time are likely to be more selective for resistance than additional treatments administered to lambs in the autumn. Farmers wishing to slow the emergence of anthelmintic resistance on their farms should look to minimise the administration of peri-parturient treatment of ewes. A trend to slower development of resistance where a proportion of lambs were left untreated at each drench suggests further work on this aspect of management of resistance is warranted.

AIMS: To determine whether: a) using a controlled-release anthelmintic capsule (CRC) instead of a programme of 5 oral drenches administered at 3-4 week intervals, would delay the development of anti-parasite immunity in lambs; b) the use of ivermectin instead of albendazole, administered either as a CRC or as a programme of 5 oral drenches, would delay the development of anti-parasite immunity in lambs; c) lambs treated with CRCs would have higher liveweight gains than lambs drenched orally 5 times at 3-4 week intervals, and; d) delayed onset of anti-parasite immunity is associated with reduced liveweight gains in the period following anthelmintic treatment.METHODS: Three field trials were conducted, 1 on a research farm and 2 on commercial sheep farms, in which groups of 30 lambs were treated with either a CRC containing albendazole, a CRC containing ivermectin, 5 oral drenches with albendazole, or 5 oral drenches with ivermectin, administered at 3-4 week intervals. Liveweights and faecal nematode egg counts (FECs) were recorded in all trials. Immunoglobulin-G (IgG) antibody levels to Ostertagia circumcincta and Trichostrongylus colubri-formis adult and larval antigens were measured in Trials 1 and 3, and fleece weights and resistance of animals to nematode challenge infection were measured in Trial 1.RESULTS: CRC-treated lambs had higher levels of antibodies to O. circumcincta infective-stage larvae (L3) than orally drenched lambs in Trial 3, but no other immunological differences due to mechanism of delivery were detected. Antibody levels were lower in lambs treated with ivermectin than albendazole, as a CRC or oral drench in Trial 1, but this was not associated with any measurable effects on FEC or productivity. No significant differences (p>0.05) were detected between drench types (al-bendazole vs ivermectin) or delivery mechanisms (CRC vs oral drenching) in any of the production parameters measured, in any of the trials. Albendazole-CRCs failed to control FECs in all 3 trials.CONCLUSIONS: Although some differences between treatments in antibody levels were detected these were not associated with measurable differences in level of parasitism or productivity of lambs. CRC use did not appear to offer substantial gains in productivity over a structured programme of 5 oral drenches administered at 3-4 week intervals.

ExtractAllan Murphy (this issue; Murphy, 2001) presents an argument, based on our recent article in this journal (Leathwick et al, 2000), designed to convince the reader that moxidectin is superior to abamectin at controlling ivermectin-resistant parasites and should therefore be the macrocyclic lactone anthelmintic of choice in this country. His letter, however, fails to adequately address several important issues.