11 January 2002

First case of multiple sheep worm resistance

By Marianne Curtis

WORM resistance to all three classes of sheep anthelmintics has been confirmed for the first time in a UK flock and producers must take extra care to prevent the problem becoming more widespread.

In a letter to Vet Record, Neil Sargison senior lecturer at the Royal (Dick) School of Vet Studies Large Animal Practice, Edinburgh, describes how ill-thrift in Suffolk lambs led to an investigation which highlighted a wormer resistance problem.

"In our practice we are beginning to see benzamidazole or levamisole resistance on many farms and on a small number, resistance to both." Warning signs are lambs not thriving, despite being drenched regularly.

"But although worms in a flock may be resistant to one anthelmintic group, they can usually be controlled by switching to one of the remaining two groups," says Mr Sargison.

Problems arise when resistance to two groups occurs and a farm is forced to rely on one, usually ivermectin. "Using ivermectin year in, year out, increases the selection pressure for worms resistant to the drug and once multiple resistance to all three anthelmintic groups happens, anthelmintics can no longer be used in the conventional way to control worms."

In the case reported by Mr Sargison, despite being grazed on adequate pasture, lambs failed to reach slaughter weights by October, gaining only 120g a day over summer to reach 34kg. They had been orally drenched with ivermectin at four-weekly intervals from 4-6 weeks old.

Resistance may have easily gone undetected, but vets at his practice stress to clients the importance checking drenches are working. However, many farms may be tempted not to investigate, accepting lamb ill-thrift as a fact of life."

All sheep producers should monitor drenches, he says. "Once a year, 10 days after worming, collect faecal samples from 7-10 animals. Take them to your vet for a faecal egg count." Where resistance is suspected, vets will advise on further action.

Although resistance to one or two anthelmintic classes can be controlled, resistance to all three is serious, warns Mr Sargison. "Other farms must be careful they do not enter this situation; on a wide scale it could make sheep production unsustainable."

Andy Forbes of animal health company Merial – which markets ivermectins, but not the particular product used in this case – believes it was only a matter of time before resistance to this class of drugs was reported. "Although animal health companies actively look for new anthelmintic compounds, there are none on the horizon, so producers cannot afford to relax over wormer resistance issues."

Although multiple resistant worms can be treated using moxidectin, resistance to this may also develop. The other mode of treatment is full dose combinations – using a full dose of each of two classes of anthelmintic – which becomes expensive, says Mr Sargison.

With possible new treatments a long way off, preventing multiple resistant worms from becoming more widespread depends on careful management.

"Bought in sheep are a major potential source of resistant worms. Drench these with moxidectin or a full dose combination to treat any resistant worms and keep them away from pasture for 48 hours to eliminate eggs."

Also look at ways to reduce anthelmintic use, while still controlling worms, he advises. "High use increases the selection pressure for resistant worms. Where possible, use clean pasture by alternating annually between cattle, crops and sheep grazing. &#42

Underdosing may also select for wormer resistance, so it is important to ensure dosing guns are accurate and sheep are correctly dosed for the weight of the heaviest in the group.

Last, anthelmintic resistance is common in goats, which share the same worm species as sheep. Grazing sheep and goats on the same pastures should, therefore, be avoided, advises Mr Sargison.

WORMER RESISTANCE

&#8226 Ivermectin resistance confirmed.

&#8226 Threatens sustainability of sheep production.

&#8226 Management steps essential.