Management of Cattle Gastrointestinal Worm Resistance

Summary

​The Gastro-intestinal parasite challenge in beef cattle is reasonably predictable within different regions, although a heavy reliance on single active drenches has seen the emergence of drench resistance. The development of anthelmintic resistance in cattle has been slower than in sheep probably due to less frequent dosing and more refugia on pasture.  However, while the effects internal parasites have on animal health are well known, the development of anthelmintic resistance is an inevitable consequence of using the same drenches year on year. 

​​The aim of this producer demonstration site is to demonstrate the prevalence and scale of anthelmintic resistance in weaner cattle on a subset of properties across Victoria. This will then support producer decision-making with anthelmintic selection for parasite control on their farm whilst also aiding in slowing the emergence of drench resistance for these producers and, more broadly, the beef cattle industry across southern Australia.  

Objectives

By end June 2025, in the high country, central and south west Victoria, engaging 20-25 core producers and 40 observer producers: 

1. ​Conduct an entry survey of demonstration and observer producers to gain an understanding of current producer practices for controlling intestinal parasites in cattle. 
2. ​Use core demonstration producer properties to identify the prevalence and scale of anthelmintic resistance present on their individual properties by conducting worm egg count drench reduction tests (WECDRT’s) on 15-20 properties over 2 seasons. 
3. ​Minimum of eighty percent of core producers will increase their knowledge of cattle anthelmintics and the efficacy of the different drench actives on their individual farms.  
4. ​Conduct workshops each year with the core and observer producers to discuss the results of the drench resistance tests. 
5. ​Seventy five percent of core producers and 50 percent of the observer producers will intend to adopt/or change their current cattle worm management by completing/observing a WECRT’s and identifying the resistance status of drench products they commonly use on farm. 
6. Seventy five percent of core producers will continue farm monitoring of drench efficacy with post-drench WEC checks or a full WECRT’s every 4-5 years with the adoption of an effective annual drench rotation protocol for all cattle on their farm. 
7. ​Conduct field days each year to showcase the drench resistance monitoring and encourage adoption of resistance monitoring and other key practices to a wider group of 50 producers in both north east and south west Victoria. Post demonstration site surveys to measure knowledge and practice change.​ 

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Progress

The PDS project “Management of cattle gastrointestinal worm resistance” commenced in May 2024 and to date, eight worm egg count reduction tests (WECRT’s) across Victoria and southern NSW have been completed. It is coordinated by Ben Blomfield of the Mackinnon Project, University of Melbourne.

These drench trials have tested efficacy of several drench groups including: Ivermectin, Abamectin, Doramectin, moxidectin, a benzimidazole (BZ’s), a Levamisole and a triple active combination in either oral, injectable or pour on application.

Average worm egg reductions percentages of individual drenches to all worm species to date are:

Injectable Ivermectin: 56%

Injectable Abamectin: 89%

Injectable Doramectin: 70%

Pour on Moxidectin: 76%

Oral BZ: 97%

Triple combination: 100%

Key messages to date include: i) Drench resistance has been detected on all farms tested. ii) Cooperia is demonstrating resistance to all macrocyclic lactones. iii) Levamisole is demonstrating 100% efficacy against Cooperia spp. iv) It is recommended to use combination drenches in all weaner/yearling cattle.