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Call for action as first case of resistance to group 4 wormer (monepantel) is reported in the UK

A collaborative study between Moredun and the University of Edinburgh’s Royal (Dick) School of Veterinary Studies has identified the first case of resistance to monepantel (trade name Zolvix®) in the UK. The recent publication* has led to a call for action from the Sustainable Control of Parasites in Sheep (SCOPS) group in order to prolong the life of all groups of anthelmintics.
Speaking on behalf of SCOPS, sheep consultant Lesley Stubbings says: “This report highlights the need for sheep farmers to follow best practice advice, but also reveals the dangers of not integrating the newer wormer groups into on-farm control programmes before the other groups fail.
“SCOPS has been advising that the group 4-AD (orange) and 5-SI (purple) wormers should be carefully incorporated into control programmes on sheep farms for the last eight years (when Zolvix® was launched) as a quarantine drench and a mid/late season treatment for lambs. That advice was given in order to avoid this sort of situation where the other three groups are no longer effective and the group 4AD is relied upon.”
SCOPS advice on orange (4-AD) and purple wormers (5-SI):

  • The two newest wormer groups (4-AD and 5-SI) should be incorporated into worm control programmes on all sheep farms, not left ‘on the shelf’ until the others are no longer effective. Their real value is in prolonging the life of 1-BZ, 2-LV and 3-ML groups.
  • There are only two occasions when a group 4-AD or 5-SI should be used and SCOPS suggests farmers alternate between the two groups. The two occasions are quarantine, and mid/late season as a ‘one off’ annual drench for lambs.
  • Groups 4-AD and 5-SI should only be used at other times under veterinary direction and then only if the full anthelmintic resistance status of the farm is known.
  • Effectiveness of products used should be monitored carefully.
  • Best practice must always be followed.

When using any wormer, regardless of the group, best practice guidelines should always be followed:-

  • Ensure the correct dose rate (by weighing animals and treating to the heaviest in a group), calibrate the gun and administer correctly, over the back of the tongue.
  • If moving to low challenge pasture after treatment, sheep must either be left on the dirty pasture for four to five days before moving, or at least 10% of the animals left untreated.
  • Check the efficacy of wormer treatments on a regular basis.

* The publication of the first UK case of resistance is in Veterinary Parasitology 257 (2018) 48-53: Lack of efficacy of monepantel against trichostrongyle nematodes in a UK sheep flock by Kim Hamer, Dave Bartley, Amy Jennings, Alison Morrison and Neil Sargison.