Strains of sheep scab that are resistant to injectable wormer products are present in NI, according to a leading expert on the disease.

Speaking to reporters on Tuesday, Dr Stewart Burgess from Moredun Research Institute said the issue was first identified along the English-Welsh border in 2018.

Sheep scab is caused by mites, and injectable wormers which have the active ingredient macrocyclic lactone (ML) are a common treatment.

“We are having solid lack of efficacy reports in all regions of the UK, and I can now include NI in that as well. The mites are resistant to all the MLs, so this is not just a single product that they are resistant to,” Burgess said.

Two separate trials on ML-resistant mites have been conducted at Moredun’s laboratories in Scotland, and both experiments confirmed that the parasites are still susceptible to an organophosphate (OP) based dip product.

“It was a real relief to everybody that OP dip is 100% effective against the ML resistant mite. We can control those populations confidently using a properly administered OP plunge dip,” Burgess said.


Sheep farmers are being warned to use OP-based dip responsibly so that mites do not develop resistance to ?Reporters were told that the main concern is the use of OP-based dip in mobile shower units.

Lesley Stubbings, an independent consultant, explained that mites can be present in areas of sheep which are not covered by showers, such as deep in the wool at the skin.

“For the dip to work, it has to get down to the skin to reach the scab mites. When a sheep is in fleece, there is no way the shower will be able to do that,” she said.

Optimal exposure

Stubbings said mobile shower units will give mites “sub optimal exposure” to OP-based dip, and this leads to “a major, major risk” of mites developing resistance to the product.

The only OP-based dip available in the UK is Gold Fleece. The product is manufactured by Bimeda and is only ?for use in plunge dippers.

Bimeda’s general manager, Mark Cokayne, said there has been “a very large shift” to using Gold Fleece for the control of external parasites in sheep in recent years.

“Last year, there was just under 20,000 five-litre cans sold. Three years ago, it was 12,000 units, so it has significantly increased over the past few years,” he explained.