Hit worms early to stop resistance developing

Sheep producers are being urged to get on top of worm burdens early this year to prevent resistance and stunted lamb growth.

Independent sheep consultant Lesley Stubbings says, so far, there has been little worm activity this spring. But she warns producers to be on the lookout for sudden outbreaks, particularly of nematodirus, should it rain soon.

And with this in mind Ms Stubbings advises farmers not to treat lambs unnecessarily and to ensure they undertake faecal egg counts to assess infection before drenching.

Coccidiosis is also a risk and particular attention should be paid when grazing is tight. “Where ewes aren’t milking well and grass is short lambs could become infected, but level of risk will vary from farm to farm, so keeping a close eye on lamb growth is essential.”

Staffordshire sheep farmer Andrew Blenkiron is ever-conscious of the changing worm burden. “Lambs are routinely brought in at 6-8 weeks to be wormed, while we rotate our wormers on a yearly basis to reduce resistance levels. This year we are using benzimidazoles, or white drenches.

Wider scale

“By regularly taking faecal egg counts of a couple of fields throughout the year, we have a good idea of what is going on. If there is a problem in one field, there are often problems on a wider scale, so everything recording more than 300/g is wormed.”

Any resistance to wormers is recorded 14 days after worming, enabling Mr Blenkiron to assess whether the dose has been effective. “We also keep a close eye on email forecasts which highlight potential problems, while keeping in close contact with our vet.”

The importance of measuring wormer effectiveness cannot be underestimated, adds SAC vet manager Brian Hosie. “There is little point worming sheep when the drench is only 55% effective.”

Parasitic worms in most flocks are resistant to one or more of the three wormer families, he adds. “Thissuggests that potentially, the wormer fails to kill more than 95% of worms present at the time of dosing.”

Resistance is a problem in many flocks, says Dr Hosie. “The BZ, or white drench, is no longer effective on most Scottish farms, while a recent survey suggests five of the 60 farms surveyed found no wormer to be effective.

Prevent resistance

“Find out which group the wormers belong to and rotate use to help prevent resistance. Ensure dose given is adequate, weighing ewes and lambs and calculating the dose based on the heaviest animal. Also make sure dosing guns are calibrated correctly, as giving lower doses contributes to the problem.”


Under certain climatic conditions, particularly sudden increases in temperature after a cold spell, nematodirus can cause stunted growth and mortality in lambs rapidly.

Risk factors include:

  • Lambs grazing pasture that carried lambs the previous spring

  • A sudden, late cold snap followed by warm weather

  • Lambs that are old enough to be eating significant amounts of grass

  • Groups where coccidiosis is likely to be a challenge

  • Lambs under stress, such as triplets, fostered or on young or old ewes

For those who consider themselves to be at risk, SCOPS advises treatment with a BZ, or white drench, as these are highly effective due to low levels of resistance in this particular species.