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A can of worms: Farmers losing battle against dose-resistant parasites

Sharp rise in resistance to ivermectin on sheep farms


The rising resistance to worm doses is the biggest threat to Irish sheep farmers.

The rising resistance to worm doses is the biggest threat to Irish sheep farmers.

Getty Images/Flickr RF

The rising resistance to worm doses is the biggest threat to Irish sheep farmers.

Farmers are losing the battle against resistance to worm doses, with the latest research identifying further declines in the effectiveness of key products.

The problem is being exacerbated as many farmers do not realise they have resistance problems until it is too late.

Orla Keane of Teagasc Grange told the National Sheep Conference in Killarney last week that stomach and gut worms have great propensity to develop resistance. She said farmers have been using products in a way that encourages resistance.

To investigate the extent of anthelmintic resistance, faecal egg count reduction tests were performed on 19 farms in 2018-19.

The research found that white doses were not working on every farm tested, on 17pc of farms yellow doses were not working and on 56pc of the farms ivermectin was not working.

Keane said the huge increase in the number of farms where ivermectin is failing is of particular concern as farmers rely heavily on this product.

"We need to stop this. We rely heavily on anthelmintic drugs to control these worms. As the drugs get used more and more, you get a build-up of resistant parasites until the whole population is resistant," she said.

"Lamb performance is optimised when 95pc of these worms are killed. The problem is when that falls to 60-70pc.

"While farmer might be still seeing an improvement in performance and think the dose is working, they don't realise they have resistance, and quite often they don't realise they have a problem until its too late.

"It's vitally important that it's detected early, when farmers can do something about it."

4 key steps every farmer can take

1 Don't dose mature ewes unless there is demonstrated need

Older stock has generally developed good immunity to gut worms and so mature ewes should not require dosing for them. Lactating yearling ewes or thin or immunocompromised ewes may require treatment but this should be targeted to individual animals on the basis of demonstrated need.

2 Only use white drench for treating Nematodirus

There is widespread resistance to 1-BZ (white drenches) among mid-season worms. However, resistance to 1-BZ products among Nematodirus has not been detected. So only use products from group 1-BZ to treat Nematodirus in lambs.

3 Treat stock coming onto the farm

If you don't have resistance, don't buy it in. A good biosecurity protocol for all bought-in animals should be implemented to prevent bringing resistant worms onto the farm. Animals should be treated with one of the new anthelmintics (proscription) and housed for 24-48 hours. They should then be turned out to contaminated pasture recently grazed by sheep.

4 Monitor faecal egg count

Use anthelmintics when necessary based on indicators such as flock-level faecal egg count. Monitoring for gut worms is important and should be an integral part of a flock health strategy. Worm burden can be monitored using faecal egg counts. In lambs a group faecal egg count of greater than 500-600 eggs per gram may have an impact on performance and may indicate a need to treat for gut worms.

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