A survey of anthelmintic resistance and parasite management practices on sheep farms in New Zealand : a dissertation presented in partial fulfilment of the requirements for the degree of Master of Veterinary Studies (Veterinary Parasitology) at Massey University
The occurrence of anthelmintic resistance in sheep nematodes in New Zealand was conducted by using DrenchRite and Inhouse larval development assays (LDA) on 25 randomly selected farms. Samples from 6 farms were put onto both DrenchRite and Inhouse LDA plates and were available for comparison. Both showed a similar LC50 and LC50 well for benzimidazoles but not for levamisole. Including results from both assay systems it is concluded that anthelmintic resistance in Trichostrongylus (either suspected or high level) was demonstrated in 60% of the farms (9/15) to benzimidazole (BZ), resistance to levamisole (LEV) in 66% of farms (10/15), combination drench (BZ+LEV) on 43% of farms (3/7) and avermectin on 1 of 8 farms. For those farms where Trichostrongylus was the predominant genus there was resistance to at least one anthelmintic on all tested farms (n=12). A survey of parasite control procedures over 2002/2003 was conducted on 38 farms. The principle findings were: about 58% farmers (n=37) performed quarantine drenching of brought-in sheep for which a majority of farmers (52%) used macrocyclic lactones alone or in combination with other anthelmintics; about 78% (n=37) of farmers followed a 5-6 drench programme to lambs/hoggets; the annual frequency of anthelmintic treatments (n=38) were 6.1 drenches to lambs/hoggets; 1.4 to two-tooths and 1.8 to mature ewes; about 71% farmers used macrocyclic lactones for lambs/hoggets on at least one occasion either alone or in combination with other anthelmintics; a majority (61%) of farmers followed a planned annual drench family rotation; about 76.5% (29/38) farmers had never carried out a test for drench resistance and about 57%(4/7) of those farmers who had tested did identify resistance on their farms to either the benzimidazole or combination (benzimidazole + levamisole) drench. Worms were considered "very important" by 55% farmers (n=38) for the cause of economic losses in their farm, whereas 42% farmer considered it "important". Drench resistance was considered as a "serious problem" today for the industry by 47% farmers (n=38), whereas, 34% farmers believe it as a problem but "not serious one today" for the industry.