Wendy Short talks to vet Ben Strugnell on tackling possible worm infestations in sheep.

THE dry start to the grazing year has probably meant low worm burden challenges for the majority of sheep in the North of England. However, prolonged wet weather could change the situation and producers must devise a clear strategy on how to tackle the issue of worm infestation and wormer resistance, according to vet, Ben Strugnell.

There is still a belief that a policy of annually rotating wormer types will help to eliminate wormer resistance issues and that a ‘dose and move’ strategy offers a complete solution, but Mr Strugnell stressed that these techniques are no longer recommended.

“The only way to identify wormer resistance is to carry out rigorous, on-farm testing to assess the efficacy of individual products,” said Mr Strugnell, who runs the Farm Post Mortems service in Hamsterley, County Durham.

“Unfortunately, a positive test result will only prove that the specific wormer is effective at that particular time of year. A yellow drench may be effective in September, but the same product could encounter resistance if dosing is carried out in June. This is because different worm species, with differing resistance levels, may be active at varying times of year.”

A faecal egg count reduction test (FECRT) is one of the most useful tools in the battle against wormer resistance and further steps will be needed if a problem is highlighted, he explained. The FECRT can in theory be conducted for all three wormer types simultaneously, but usually only one product at a time is trialled for each lamb group.

To set up a FECRT, a group of ewes and lambs suspected of carrying worms is gathered in, for example. As they move through the race for worming, the ten lambs with the highest visible level of faeces on their back ends are selected. This group should be marked, have faeces samples taken, and a faecal egg count performed. It is important that lambs are weighed, and dosed for the heaviest lamb.

“The marked lambs will need to be gathered again after a certain period, depending on product type,” said Mr Strugnell. “For yellow drenches, the timing is seven days, with 12 days for a white drench and 14 days for a clear drench. A second sample, ideally from the same lambs, is taken at this second gather, as well as another worm egg count.

“It would be convenient if both sample sets could be sent off for testing at the same time, but the practice is not recommended as the material tends to degrade after a couple of days.”

If no eggs are found in the second test samples, the conclusion can be drawn that the wormer has been effective. If eggs are found, however, wormer resistance is highly likely and the advice is to avoid using that wormer type at that time of year in the future. This regime may sound drastic, he commented, but it is the only way to safeguard flock health. If new sheep are introduced to the farm they should be wormed with a quarantine wormer (possibly a Group 4 or 5 product; consult your vet) and confined for 48 hours.

“It can probably be assumed that once proven, the efficacy of a wormer will apply to the whole farm. If all three of the cheaper wormer types have resistance issues, the only option is to choose a group four or five wormer type and these are generally more expensive. Nevertheless treating lambs with an ineffective wormer is also expensive, because worms are not destroyed and flock performance is impaired.

The alternative is a drench test; less labour-intensive but with limitations, he added.

“This test involves worming the lambs, waiting for the required interval and taking only the second faeces samples in the FECRT method. The flaw in this system is that the producer will not have a clear picture of how many worms were present at the time of the initial drenching.

“If the sample results taken at the appropriate interval after drenching indicate high egg numbers, it can be assumed that resistance is present for the wormer type. If there are no eggs in the faeces, there is a possibility that the test sheep were not carrying worm eggs at the initial dosing.”

A lot will depend on rainfall figures for the coming months. In dry conditions, worm eggs will often hatch and die, while high rainfall levels can result in a population explosion.

“Keeping on top of sheep wormer resistance is an extremely complex matter and there is no likelihood that a simple solution will be found any time soon, so producers must remain vigilant,” said Mr Strugnell.

Farmers who wish to assess whether wormers are effective on their farms should contact their vet or advisor to organise a FECRT. More information about worming can be found of the Sustainable Control of Parasites website at www.scops.org.uk.