The Sustainable Control of Parasites in Sheep (SCOPS) group is encouraging sheep farmers to monitor ewe body condition scores to reduce the doses of wormer the administer this lambing season.

The group has said that new evidence supports this approach as a way to cut the proportion of ewes wormed around lambing time, without detrimental effects on lamb performance.

Speaking on behalf of SCOPS, independent sheep consultant Lesley Stubbings said the stress of late pregnancy can cause a ewe’s immune system to wane and allow adult worms in her gut to produce more eggs.

“This is known as the peri-parturient rise and the worm eggs, passed out in the ewe’s dung, contaminate the pasture grazed by their lambs later in the season,” he said.

“The objective of using a wormer at lambing time is to reduce the level of pasture contamination and the subsequent challenge to lambs. It normally has nothing to do with the health of the ewe.”

Wormer resistance

SCOPS warned that worming all ewes at lambing time in the hope of reducing the risk to their offspring is high-risk in terms of accelerating the development of resistance to wormers on a farm.

Reducing the proportion of ewes wormed reduces this risk, SCOPS said, and helps slow the development of resistance.

“For some years, SCOPS has recommended at least one in 10 of the fittest ewes are left untreated to reduce the selection for resistance in the worm population,” Stubbings said.

“However, as resistance levels increase, we need to better identify those ewes likely to produce the highest level of contamination and when this if likely to occur, so treatment can be timed more accurately.”

SCOPS said it has been watching a research project in Wales that has shown a significant variation in the extent, timing and duration of the peri-parturient rise, both between individual ewes and between farms.

“This latest research suggests, rather than just leaving the fittest or single rearing ewes untreated, you can use loss of body condition in your ewes to identify those under the most nutritional stress and therefore more likely to produce a high number of worm eggs in their dung,” Stubbings said.

“In most flocks this means the proportion treated can be reduced. This is critical to address concerns over the blanket use of long-acting moxidectin 2% wormers in ewes.

Stubbings said that while these products reduce egg output over an extended period, and therefore can reduce pasture contamination, they also mean the worm population is exposed to the active for longer.

“Taking this new approach means we can target only those ewes likely to produce the most contamination, harnessing the potential of moxidectin while ensuring minimum selection pressure is exerted,” he said.