Anthelmintic resistance in worms

INTERNAL parasites, in particular gut worms, are a major challenge to sheep production systems.

Grazing sheep are naturally exposed to stomach and gut worms. A large number of different worm species can infect sheep, but most follow a similar life cycle with both free-living and parasitic phases.

Worm larvae, as part of their life cycle, accumulate on pasture over the grazing season, where they are generally a greater problem later in the summer.

Nematodirus, Teladorsagia and Trichostrongylus species are the main gut worms infecting sheep.

Nematodirus becomes an issue on farms after a prolonged period of cold weather in spring, followed by a period of warmer weather, which allows eggs to hatch.

Fortunately lambs develop strong immunity to Nematodirus by approximately 12 weeks of age.

Broad – Spectrum Anthelmintic’s (wormers) are administered to sheep for the control of gut worms.

The main class of wormers used in Ireland are Benzimidazole (white), Levamisole (yellow) and Macrocyclic Lactones (clear).

However, nationally, anthelmintic resistance, which is the ability of parasites to survive a dose that should kill them, is increasing for these wormers.

When animals are treated with an anthelmintic at the correct dose rate, all susceptible worms are killed, allowing only resistant worms to survive, which results in the resistant worms making up a greater proportion of the worm population in subsequent generations.

Therefore, the continuous use of anthelmintics can lead to the development of anthelmintic resistance.

The sheep welfare scheme was introduced to contribute to the continued development in animal and health welfare in sheep, with parasite control - Faecal Egg Count – as one of the actions available.

Faecal egg count should be carried out from weaning onwards to determine the need for dosing.

A fully effective anthelmintic dose reduces egg count to zero after administration.

If the egg count reduction is less than 95 per cent, then anthelmintic resistance is considered to be present.

Use anthelmintics when necessary, based on the results of faecal egg count. Farmers must help maintain a susceptible worm population and prolong the efficacy of commonly used anthelminthic products.

Adult ewes have a good immunity to gut worms so don’t need dosing unless advised to do so by a vet. Undosed ewes help maintain a susceptible worm population within a farm.

To effectively treat Nematodirus in lambs, only use products from group one Benzimidazole (white Drench) as no resistance has been detected with this treatment.

Other products should be used later in the season for treatment against other stomach worms.

A good biosecurity protocol for all bought-in animals should be implemented to prevent bringing resistant worms onto the farm.

Sheep must be administered the correct dosing rates and dose to the weight of the heaviest animal.

Unfortunately anthelmintic resistance is a heritable trait, which means resistant worms pass on genes conferring anthelmintic resistance to their offspring.

If you have doubt about the best course of action to take in the treatment of stomach and gut worms, seek advice from your vet or advisor