As well as an increased risk of developing anthelmintic resistance on your farm, treating lambs where it is not needed is also costing you money and unnecessarily increasing your workload.

Anthelmintic resistance is not a commonly used term or discussion topic among flockowners.

It does, however, pose a serious threat to sheep enterprises with animal performance significantly reduced. The simplest explanation of worm resistance developing means worms will not be killed when dosed with products from the main anthelmintic classes.

Studies to date show significant resistance problems with the roundworm species of Teladorsagia (occurs from June onwards) and Trichostrongylus (occurs from late summer/autumn onwards) and the Benzimadazole (white drench) and Levamisole (yellow drench) anthelmintic classes. Resistance has also been confirmed in Northern Ireland to Macrocyclic Lactones.

At last week’s Teagasc sheep farm walk in Roscommon, Frank Hynes, Teagasc sheep specialist, detailed a number of approaches farmers can take to reduce the rate of resistance developing.

These include:

Treating lambs on the recommendation of faecal egg count analysis.

Administering the correct dose rate for the weight of lamb treated. Avoid under-dosing by treating to the heaviest lamb in the group.

Where there is a wide weight variation, split the flock into two weight groups to avoid underdosing. Running lambs through a weigh scales is the best approach available for accurate dosing.

Ensure the dosing gun is calibrated correctly and is delivering the desired dose rate.

Dose correctly by administering the liquid at the back of the tongue of the animal. Be careful to hold the lamb steady so that damage will not be caused to the lining of the mouth and throat.

The recommended management practices post treatment have also changed. Dosing and moving to clean pasture is thought to be highly selective for resistance worms. The advice now is to delay moving animals to fresh pasture for a period of four to seven days post treatment. Leaving about 10% of lambs in the group untreated is also seen as beneficial to reduce the rate of resistance developing.

Adhere to withdrawal dates and read manufacturers recommended guidelines.

Alter product usage between different anthelmintic classes. Be careful to check the main active ingredient or anthelmintic class as numerous products on the market have the same active ingredient.

Testing for anthelmintic resistance

Faecal egg tests pre-dosing will determine if and when dosing is required. A drench test to check that the dose you are using is working will also pay dividends in checking if resistance is a problem. Faecal egg tests can be carried out by collecting faecal samples from individual animals in a group. A number of vets have started analysing samples, or, if not, they can make arrangements to have them sent to a laboratory for analysis.

To carry out a drench efficacy test, you will need to collect faecal samples pre and post treatment.

The length of time between drenching and post treatment faecal egg test analysis will depend on the product used.

The recommended time frame is one week for levamisole products and two weeks for benzimidazoles and macrocyclic lactones. Drench test failure is one of the best indicators of worm resistant issues.