Anthelmintic Resistance in Sheep

By James Kelly, Drystock Advisor, Roscommon Town

Anthelmintic Resistance or Wormer Resistance is the ability of stomach worms to fight and survive a worm dose. Anthelmintic Resistance is becoming a growing problem in Irish sheep farms which is resulting in significant financial losses through lack of thrive and money spent on products which are ineffective. Once sheep are dosed 99% of worms should be eliminated but this is not always the case due to a build-up of resistance to commonly used drenches. The 3 main types of drenches widely used are as follows;

1. Benzimidazole: commonly known as a white drenches
2. Levamisole: commonly known as a yellow drenches
3. Macrocyclic lactone: commonly known as a clear drenches

Resistance to the above drenches can develop due to continuous year-on-year use of the same product. At first the problem is not apparent but overtime it becomes clearly visible. It is therefore vital for a sheep farmer to halt this build up in resistance

Below are 3 key areas sheep farmers can focus on to prevent resistance,

• Faecal Monitoring (dung sampling before dosing) - this will identify when worms are present in sufficient numbers to justify using a dose. It prevents overuse of drenches and also reduces the annual cost of such products. It is more accurate than visual assessment.
• Faecal Egg Count Reduction Test – this will determine if worms are resistant to a particular product. It involves dung sampling twice, once before dosing and secondly after a certain period of time post dosing. It will allow a farmer see how effective a particular dose has been at eliminating worms.
• Not dosing mature ewes as they will have built up immunity to gut worms (only thin or lactating ewes should be considered for dosing)

In summary, worms are one of the major causes of production inefficiency in sheep flocks reducing lamb growth rates. For the economic viability of any sheep production it is essential that stomach worms are controlled. Faecal monitoring (dung sampling) is an essential component of any successful worm management plan.

Teagasc provides a Local Advisory and Education service to farmers. They have offices based in Longford Town (Tel: 043 3341021), Roscommon Town  (Tel: 090 6626166) and Castlerea (Tel: 094 9620160). You can find us on Facebook @Teagascroscommonlongford and twitter @teagascRNLD.