Monday, February 26, 2024

Drench resistance bombshell hits cattle sector

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Decades of warnings about drench come to pass.
Reading Time: 4 minutes

Triple drench resistance in cattle, once considered only a distant threat, is now a clear and present danger to the New Zealand beef and dairy industries, and only likely to get worse.

A research paper by AgResearch senior parasitologist Dr Dave Leathwick and colleagues has identified triple resistance to drenches in cattle on four farms studied. 

The detection of resistance to the three main anthelmintic types comes 16 years after widespread double resistance left only the levamisole active as the last drug standing in countering Cooperia parasites in cattle. 

It also comes as growing drench resistance in sheep is costing that sector over $100 million a year in lost earnings.

“It is quite scary because we did not really go looking for these parasites, this simply dropped into our laps,” Leathwick said.

The study on four farms identified, from drench testing, that in only three of 20 tests taken did drench efficacy exceed 90%. 

On its own the previously effective levamisole active recorded efficacy of only 44%-70% against Cooperia, far below the 95% required.

“Every case we got involved in was because the farmer saw clinical signs, he went to the vet and got a drench test and then came to us,” Leathwick said. 

One farmer had already grappled with widespread drench resistance in his sheep flock. 

Cattle performing poorly for no apparent reason had been the prompt for farmers to suspect there was a drench problem.

“One vet in the South Island was investigating 1000 dairy heifers that would not grow, were scouring and looked awful.”
Despite being a clinical scientific paper, the document does not hide the authors’ frustration after 20 years of trying to alert farmers to the inherent risks of relying solely upon drench products to keep parasites at bay – at the risk of resistance developing.

It states: “Despite years of advice and recommendations to change farming practices from intensive monocultural systems, many farmers have continued with the practice and some are now faced with the very real possibility of being unable to control cattle parasites on their farms. 

“In 2006 Professor Bill Pomroy [a Massey parasitologist] wrote in the New Zealand Veterinary Journal that with widespread resistance to mectin and benzimidazole actives already present, it was only a matter of time before levamisole failed as well. That time is now.”

Leathwick said if anything he is surprised it has taken this long for cattle parasites to develop a resistance trifecta.

He lambasted animal health companies for continuing to market products as “reliable and effective” drench solutions for cattle.

“No, they are not, they cannot now be relied upon at all.”

However, he sheeted much of the blame onto farmers, who have been warned about resistance for years but have failed to adjust their management practices to help delay resistance development, or even test that their drenches were working.

The findings come hard on the heels of the “vast majority” of sheep farmers also failing to recognise resistance risk. They are also now having to deal with the consequences.

“The problem is farmers will not recognise resistance as a problem until they find dead stock in the paddock.” 


Dave Leathwick says he has spent the past two decades warning farmers about the risks of widespread resistance developing to all major cattle drenches, and that has arrived.

Leathwick has worked with farmers who have experienced this, and who have had to instigate some major management changes on their properties. 

“One guy lost $100,000 of capital stock in three months. He was very motivated to change his practices.”

This can include monitoring faecal egg counts regularly rather than simply drenching every four weeks, mixing stock classes and types up more, better feeding, and paying more attention to what stock are run where on farm.

While Cooperia resistance in cattle is an issue, Leathwick predicted even more dire consequences with triple resistance developing in Ostertagia parasites, a far more populous and aggressive internal worm. 

One of the farms under Leathwick’s study had Ostertagia with triple resistance – and “triple resistance there is almost a global first”.

The operations where resistance was detected tended to have 100% cattle run in intensive grazing and cell-type systems and included dairy heifer operations.

He estimates the cattle sector is only five to 10 years behind the sheep sector in terms of the resistance becoming widespread, and there is no new chemical saviour on the horizon.

“We have a psyche in NZ where parasite control comes in a drum. Most farmers have only ever farmed with drench as their sole control.

“The reality is that in future farmers will have to move away from total reliance upon drenches. The only ones doing it at the moment are those hit with triple resistance.”

He was, however, heartened by the minority of farmers who had taken heed of warnings years ago and made moves to reduce their drench reliance.

“I have seen farmers with very big operations who listened then and have no drench resistance issues. They are reaping the benefits now.”

MORE: Want to share your thoughts on this issue? Text us on 027 226 8553 with the keyword RESISTANCE followed by your comments.

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