Farm Ireland

Friday 23 March 2018

Fears grow over rising resistance to drenches

Darragh McCullough

Darragh McCullough

Sheep worm parasites are almost 100pc resistant to traditional white drenches based on benzimidazole, while resistance to ivermectin and moxidectin products, such as Cydectin, is present in 25-50pc of flocks, according to the latest research out of Northern Ireland.

Veterinary Ireland's Peadar O Scanaill admitted that similar trends are emerging in flocks south of the Border.

"Sheep farmers here are not as proactive on this issue as we'd like to see and there is the risk that the range of effective products will be drastically curtailed in the very near future," he warned.

Industry experts say that ineffective worm control due to resistance is costing the sector here hundreds of thousands of euro a year.

The most effective products in the trials, which were run separately by Northern Ireland's agricultural R&D council and Queen's University, were the yellow drenches based on levamisole or orange drenches based on aminoacetonitrile.

The latter generated almost zero levels of resistance in the flocks where it was used.

The researchers recommended leaving up to 10pc of the fittest ewes or those with singles untreated to keep a mixed population of susceptible and resistant strains of worms on the farm.


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They also recommended that long-acting moxidectin products be used sparingly in lambs. They also proposed grazing management and routine faecal egg count monitoring to fine tune worm control in lambs. They also discouraged the use of long-acting moxidectin products in flocks purely for the control of sheep due to the exposure that this gives the flock's worm population to the drug, leading to an increased risk of resistance.

The research also highlighted the need for care when using combination drenches, such as a flukicide and wormer combination, when only one of the components is actually being used at the optimum time.

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