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A change of direction for sheep worming

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01 October 2015, at 1:00am

RICHARD GARD reports from a recent meeting hosted by Zoetis at which the latest thinking on anthelmintic resistance was one of several topics to be discussed

THE whole topic of sheep worming has become much more urgent and complex. At a meeting sponsored by Zoetis, Fiona Lovatt (president in 2013/14 of the Sheep Veterinary Society) highlighted the difficulties that veterinary surgeons have with sheep keepers and the frustration that sheep farmers have with veterinary surgeons.

Some sheep folk consider that the vet saves money, are aware of disease risks and use the vet before problems arise. Many more farmers believe that the vet costs money and only turn to the vet in a crisis.

In a serious effort to bridge the veterinary-shepherd gap a new hub is now available “for vets who treat sheep but are not a specialist sheep vet” at www.sheepvet.net. Comments on the content and discussion areas will be welcomed.

Anthelmintic resistance (AR) has moved up a gear and is increasing much more rapidly than expected. In the content of the EBLEX Better Returns newsletter earlier this year, it was speculated that the increase in AR is due to the use of Group 3 macrolytic lactones (ML) (clear drenches) for sheep scab. With the demise of dipping for scab the 3-ML endectocides have become the main control measure.

As scab control often takes place during the winter, the larger population of the roundworms are living in the sheep rather than on pasture. As all the sheep need to be treated for scab it is resistant worm eggs that are then shed.

It has been accepted for some time that there is widespread worm resistance to Group 1 Benzimidazole (BZ white drenches) and a recognised problem with Group 2 Levamisoles (LV yellow drenches) but the resistance to moxidectin (3-ML) has come as a bit of a shock.

There are two other recognised anthelmintic categories, Group 4 amino acetonitrile derivatives (AD orange drenches) and Group 5 multi actives (SI purple drenches).

If this is all getting a bit convoluted then put yourself in the position of the sheep farmer where the sheep are a secondary enterprise. Reading the various leaflets and handouts can be quite confusing. For this reason it seems appropriate for the chair of the meeting, Ailsa Milnes (Zoetis vet adviser), to address the SQPs in the audience sitting among the veterinary surgeons, farmers and sheep management advisers.

Urgent need

Although the products for resistant worms may be POM-V there is an urgent need to promote the use of anthelmintics responsibly. It is expected that Suitably Qualified Persons located within farm outlets will communicate with vets and encourage the farmers to communicate with their vet. It was not said, but possibly implied, that more could be done by veterinary practices to actively communicate with SQPs in turn.

Dave Leathwick (AgResearch New Zealand) has been involved in large scale field trials to study anthelmintic resistance and how to manage the problem in commercial flocks. He explained that a great deal of work has centred on the use of multi active drug therapy (SI purple drenches).

The aim is to kill the worms resistant to other anthelmintics and to increase the dilution of the population with susceptible worms. Retaining susceptible worms on the farm is the key. The idea of refugia was discussed at length, providing a refuge for nonresistant worms, and a high level of refugia is needed for multi active drug therapy to be most effective.

A combination of two drugs allows worms that are resistant to the first to be killed by the second. Fewer residual worms surviving treatment has a significant impact on the health of the sheep. Most farms have been found to have multiple drug resistance in one worm species with several parasites on each farm.

A combination approach involving multi actives leads to fewer resistant worms but also the resistant worms are found to be less fit. The worms live for shorter periods on the pasture and contain less fat. Less fat means that the worms appear shorter.

The New Zealand work demonstrates that the repeated use of multi actives is the choice of therapy to overcome anthelmintic resistance and to prevent the development of anthelmintic resistance.

Influencing factors

Fiona Lovatt has experienced that farmers are influenced mostly by growth rates and weaning times. If these are unsatisfactory then help is sought to overcome the difficulties. Some shepherds only treat lambs with poor growth rates. Detecting anthelmintic resistance with faecal egg counts is effective but many farmers do not follow the sample collection rules.

Dung samples from at least 10 lambs in separate pots are required and it is most helpful to build a picture of the flock over time. This forms part of flock health planning. The development of flock health hubs, where groups of smaller producers engage a vet to discuss health and health plans, shares the cost of the vet.

Involvement in a health hub allows the vet to demonstrate areas for improved profitability and for the farmer to consider investing further in veterinary involvement.

There is considerable concern about resistant populations of worms being bought in to the farm. A discussion with all sheep keepers about their programme involving yarding on arrival, treatment, and quarantine before the sheep move onto pasture, appears a good starting point for a flock health plan. Fiona advises that the new anthelmintics should not be left on the shelf until resistance problems are evident.

Further discussions

The meeting was full, with veterinary surgeons attending from several counties and it was the third of a programme covering the livestock areas. Literature was available about a dual active worm control product combining Derquantel 10mg/ml (5-SI) and Abamectin 1mg/ml (3-ML). It is explained that “Derquantel is an acetyl choline receptor antagonist, causing flaccid paralysis and expulsion of nematodes.”

Abamectin has not been used in sheep in the UK before, with a high potency against some ML resistant strains of nematodes. Practical aspects include using the combination as a one-dose quarantine treatment, two or more doses at the start of the lambing season or at the end of the lamb production cycle.

It seems clear that worm resistance in the sheep areas has accelerated and is a major productivity and welfare issue for farmers. Farmers are having great difficulty this season to receive an effective price for their lamb and there is an issue of cheaper imports.

The need for a more effective approach to anthelmintic resistance and an increasing involvement by veterinary surgeons to make sense of refugia, resistance and management options appears proven.

The message from the New Zealand research comes across as “hit hard, don’t hold back”, a dramatic change from limited use of the more effective products. Veterinary surgeons will need to be clear which strategy they are to adopt in their practice.