Nematode parasites of economic importance in sheep in New Zealand

Abstract

Sheep were introduced into New Zealand in significant numbers after 1814 principally from Britain and Australia. As the indigenous mammalian fauna of New Zealand comprised only three species of bats, the parasite fauna present today is primarily composed of those parasites that survived the journey in introduced domestic stock. Twenty‐six species of gastrointestinal nematodes and three species of lungworm have been identified 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 lungs. The most numerous and pathogenic of these are Haemonchus contortus, Ostertagia spp., and Trichostrongylus axei in the abomasum, and Trichostrongylus spp., Nematodirus spp., and Cooperia spp. in the small intestine. 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 establishment and effects of parasitism and usually show few clinical signs of their worm burdens beyond the first year of life.

Control of parasites to prevent stock losses has been a problem since the introduction of sheep to New Zealand. Early remedies, such as copper sulphate and nicotine sulphate mixture, were relatively ineffective and often highly toxic both to the sheep and to the operator. They were largely replaced by phenothiazine in the 1940s and 1950s. However, it was not until the introduction of thiabendazole and levamisole in the 1960s that effective control of a wide spectrum of sheep nematodes was achieved. Thiabendazole 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 rely almost exclusively on heavy drench usage to control nematode parasites 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 has been found in goat parasites, which are also able to infect sheep. In order to maintain sheep production at present levels, farmers must be persuaded to adopt management strategies that will enable them to reduce the number of drenches they give to their flocks and thus preserve the efficacy of currently available drugs.

Keywords:

• sheep
• nematode parasites
• control
• anthelmintics
• production
• New Zealand
• anthel‐mintic resistance