PROPHYLACTIC EFFICACY OF PERSISTENT ANTHELMINTICS AGAINST CHALLENGE WITH DRUG-RESISTANT AND SUSCEPTIBLE OSTERTAGIA-CIRCUMCINCTA

Three groups of newly-weaned Romney lambs were given either a standard oral dose of albendazole, a controlled-release capsule containing alb endazole, or a standard oral dose of moxidectin. At 10, 20, 30 and 40 days after treatment, sub-groups of lambs were Siren 10,000 infective- stage larvae of either a drug-resistant or a drug-susceptible strain o f Ostertagia circumcincta. The recommended oral dose of albendazole re moved 32 per cent of the resistant strain and over 99.9 per cent of th e susceptible O circumcincta. The recommended oral dose of moxidectin removed 91 per cent of the resistant strain and over 99.9 per cent of the susceptible parasites, None of the lambs treated with controlled-r elease capsules was challenged at 20 or 30 days after treatment, Twent y-one days after challenge, samples of faeces were taken to determine the presence of nematode eggs and cultured to establish the proportion of eggs developing to infective-stage larvae (L3). Abomasa were recov ered after slaughter and worm burdens determined, in the lambs given c ontrolled-release capsules only the resistant parasites were able to e stablish, and there were significantly fewer than in the lambs treated orally with albendazole, The proportion of the eggs from resistant pa rasites which developed to L3,vas not reduced by the presence of the c apsules, Oral moxidectin provided no protection against the establishm ent of the resistant strain and viable L3 were recovered after challen ge with resistant parasites 10 days after treatment; however, the esta blishment of susceptible O circumcincta was reduced by more than 99 pe r cent, The establishment of the susceptible parasites in the lambs tr eated with moxidectin increased with time and was not significantly lo wer than in the other groups by 30 days after treatment.