Farm management practices associated with macrocyclic lactone resistance on sheep farms in New Zealand

Abstract

AIM: To identify farm practices associated with the presence of resistance to a macrocyclic lactone (ML) anthelmintic on sheep farms in New Zealand.

METHODS: A cross-sectional study was conducted to test for associations between the presence of resistance to an ML anthelmintic (ivermectin) and management practices on sheep farms in New Zealand. Selection of farms was both random (n=80) and purposive (n=32; being farms with a history of suspected ML resistance). Resistance was inferred from faecal nematode egg count (FEC) reduction (FECR) tests (FECRTs) when there was <95% reduction in FEC 7–10 days after treatment with a half dose of ivermectin (0.1 mg/kg). A logistic regression model was built to identify farm-level factors that were associated with the presence or absence of ML resistance.

RESULTS: Of the 112 flock managers that were approached for interview, 103 (92%) returned useable questionnaires. The odds of ML resistance were increased: on farms that had used long-acting ML products in ewes as a pre-lambing treatment for ≥3 of the previous 5 years (odds ratio (OR) = 7.2; 95% confidence interval (CI) = 1.7–30.3); on farms where <70% of the total stock units mid-winter were from sheep (OR=6.5; 95% CI=1.6–25.6); on farms which over the year purchased ≥10% of the number of sheep present mid-winter (OR=7.1; 95% CI=1.5–34.7); and on farms where the average wool diameter of the main flock was <37 (OR=4.1; 95% CI=1.1–14.7) microns. The model provided a good fit to the data (pseudo R²=0.64; Hosmer-Lemeshow statistic = 0.38).

CONCLUSIONS: Explanatory factors identified as associated with the presence of ML (ivermectin) resistance on farms included the use of long-acting anthelmintic formulations in ewes pre-lambing, sources of refugia of unselected parasites on the farm, breed of sheep and their requirements for anthelmintic treatments, and the importing of resistant parasites with purchased stock. The study provides support for controls that aim to provide refugia of susceptible worms and that minimise the risk of introduction of resistance through effective quarantine-drenching.

KEY WORDS:

• Sheep
• anthelmintic resistance
• macrocyclic lactone
• ivermectin
• FECRT
• associations
• quarantine-drench
• refugia
• modelling

Acknowledgements

This work was jointly funded by the MAF Sustainable Farming Fund, Meat & Wool New Zealand, and Schering-Plough Animal Health Ltd. Merial NZ Ltd, Schering-Plough Animal Health Ltd, Ancare NZ Ltd, Allflex, and PGG Wrightson Ltd, kindly donated products.

The field work was supported by veterinarians in practice throughout New Zealand during a busy period of the year. The farmer participants are acknowledged for their willingness to support an initiative that has established important benchmarks for the entire industry; they worked through the inconvenience of screening sampling as well as the management inconvenience of testing through what were, at times, challenging climatic conditions. AgResearch staff Rebecca Alexander, Anne Tunnicliffe and Brenda Batchelar carried out all the FEC. Anne-Maree Oliver and Barbara Adlington conducted all the faecal cultures and counted and identified all the larvae.

Notes

¹ J Hurst, Merial NZ Ltd, Manukau City, New Zealand