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Anthelmintic resistance in Australian sheep nematode populations
D J Overend1, M L Phillips, A L Poulton
1Department of Food and Agriculture, Benalla, Victoria.
Australian Veterinary Journal
|April 1, 1994
Summary
Gastro-intestinal nematode resistance to common sheep anthelmintics is widespread in Australia. Few farms achieved effective parasite control with available treatments, highlighting a significant challenge for sheep health.
Area of Science:
- Veterinary Parasitology
- Sheep Health Management
- Anthelmintic Resistance
Background:
- Gastro-intestinal nematodes pose a significant threat to sheep health and productivity worldwide.
- Anthelmintic resistance in nematodes is an escalating global concern, impacting effective parasite control strategies.
- Previous studies indicated emerging resistance, necessitating a comprehensive national evaluation.
Purpose of the Study:
- To assess the prevalence and spectrum of anthelmintic resistance in gastro-intestinal nematodes affecting sheep across Australia.
- To identify the primary nematode species contributing to resistance.
- To investigate potential correlations between resistance patterns and farm management practices or environmental factors.
Main Methods:
- A nationwide survey of 881 sheep farms was conducted between 1991-1992.
- Faecal egg count reduction tests were performed to evaluate the efficacy of benzimidazole, levamisole, and combination anthelmintics.
- Faecal cultures were used to identify dominant nematode species.
Main Results:
- Widespread anthelmintic resistance was confirmed, with 85% of farms showing resistance to benzimidazole and 65% to levamisole.
- Resistance to combination (benzimidazole+levamisole) products was observed on 34% of farms; ivermectin resistance was not detected.
- Only 9% of farms demonstrated effective control (≥95% egg count reduction) across all anthelmintic groups.
- Teladorsagia circumcincta, Trichostrongylus spp., Chabertia ovina, and Haemonchus contortus were the principal species identified.
- Resistance to combination products was significantly higher (almost double) in areas with >500 mm annual rainfall.
Conclusions:
- Anthelmintic resistance in sheep nematodes is a pervasive issue in Australia, severely limiting treatment efficacy.
- The widespread resistance necessitates urgent development and implementation of integrated parasite management strategies.
- Environmental factors, such as high rainfall, may exacerbate the prevalence of resistance to certain anthelmintic classes.