NEMATODE PARASITES OF ECONOMIC IMPORTANCE IN SHEEP IN NEW-ZEALAND

Sheep were introduced into New Zealand in significant numbers after 18 14 principally from Britain and Australia. As the indigenous mammalian fauna of New Zealand comprised only three species of bats, the parasi te fauna present today is primarily composed of those parasites that s urvived the journey in introduced domestic stock. Twenty-six species o f gastrointestinal nematodes and three species of lungworm have been i dentified from sheep. All are found throughout the country, though not all are equally common in all areas. Individual sheep usually harbour several species at various sites in the gastrointestinal tract and lu ngs. The most numerous and pathogenic of these are Haemonchus contortu s, Ostertagia spp., and Trichostrongylus axei in the abomasum, and Tri chostrongylus spp., Nematodirus spp., and Cooperia spp. in the small i ntestine. Clinical parasitism is usually only found in young lambs up to 9-10 months of age, though adult stock can be affected during times of stress. Most lambs progressively develop resistance to the establi shment and effects of parasitism and usually show few clinical signs o f their worm burdens beyond the first year of life. Control of parasit es to prevent stock losses has been a problem since the introduction o f sheep to New Zealand. Early remedies, such as copper sulphate and ni cotine sulphate mixture, were relatively ineffective and often highly toxic both to the sheep and to the operator. They were largely replace d by phenothiazine in the 1940s and 1950s. However, it was not until t he introduction of thiabendazole and levamisole in the 1960s that effe ctive control of a wide spectrum of sheep nematodes was achieved. Thia bendazole was followed by a number of other benzimidazole compounds in the 1970s and, more recently, by a third group of anthelmintics, the milbemycin/avermectin compounds. The high efficacy and relatively low cost of modern broad-spectrum anthelmintics allowed many farmers to re ly almost exclusively on heavy drench usage to control nematode parasi tes in their flocks. As a consequence, resistance has emerged to both benzimidazole and levamisole drenches. Although milbemycin/avermectin resistance has as yet not been recorded in sheep in New Zealand, it ha s been found in goat parasites, which are also able to infect sheep. I n order to maintain sheep production at present levels, farmers must b e persuaded to adopt management strategies that will enable them to re duce the number of drenches they give to their flocks and thus preserv e the efficacy of currently available drugs.