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Avoiding Anthelmintic resistance in Sheep Flocks

How widespread is anthelmintic resistance?

Surveys carried out suggest that nearly all lowland flocks and the majority of upland/hill flocks in England (80%) have some resistance to white drenches. There is also widespread resistance to Levamasole wormers and to a lesser extent Ivermectin based wormers. Worms resistant to all three classes of wormer are present in the UK.

What are the consequences of anthelmintic resistance?

With resistance levels of less than 5% there will be no obvious reduction in effectiveness of the wormer or in lamb production. With resistance levels of up to 20% performance will still appear to be acceptable although if not addressed the level of resistance will only increase and wormers will appear to be less effective.

Why has anthelmintic resistance developed?

Intensive worming regimes have worked well for production but are selective for anthelmintic resistance which reduces productivity. The challenge for the future is to develop worming strategies that maintain productivity but are not selective for anthelmintic resistance.

SCOPS - Sustainable Control of Parasites in Sheep

Aim to give sheep enough exposure to worms to allow natural immunity to develop without encouraging further resistance.

- When worming sheep/lambs do not dose and move onto clean pasture: This will increase the proportion of resistant worm eggs on the pasture and therefore encourage resistance. This occurs because the only worm eggs passed onto the clean pasture after worming are those that were resistant to the wormer used, there are no other worm eggs present on the land to dilute the intake by the sheep. The sheep then go on to ingest the resistant worm eggs whilst grazing and the adult worms that develop from these eggs are also all resistant to the wormer used. When these worms breed to produce the next generation of eggs, only resistant eggs are produced and so the cycle continues. By dosing and turning out onto dirty pasture, the resistant eggs that are passed from the sheep are mixed with eggs that remain susceptible to the wormer just used. The adult worms that develop are therefore from a mixture of both resistant and non-resistant eggs and when they mate will produce a range of susceptible and resistant eggs, diluting the resistance.

- Quarantine all bought in sheep and then turn back onto dirty pasture after 24-48 hours. At the moment the advice for quarantine dosing is to dose sequentially with a levamasole and an ivermectin based wormer, although with the advent of a new wormer group (Zolvix) this advice may change.

- Always use the correct dosage of wormer. Ensure the dosing gun is correctly calibrated and that the sheep/lambs are given the correct amount of wormer for their weights. Dose according to the heaviest animal in the flock not to the average weight; underdosing encourages development of anthelmintic resistance.

- Maintain the pasture population of susceptible worms. When dosing a group of animals it is a good idea to leave 10-20% of the animals (selecting the healthiest in appearance and weight) untreated to maintain the population of susceptible worms to dilute down the resistant ones.

- Target treatments to when the animals need treating rather than when you have traditionally wormed in the past. Adult ewes have a natural immunity to worms and therefore should not be wormed repeatedly, once per year around lambing time should be all that is needed (if that). Worm egg counts can be used to determine whether lambs need worming through the summer for parasite gastroenteritis (Ostertagia).

- Avoid repeated use of wormers with a persistent (long acting) action as these encourage development of resistance.

Faecal Egg Counts

- Life cycle of roundworms in sheep takes three weeks before egg laying commences.

- Faecal egg counts give an indication of the worm burden an animal is carrying.

- Very useful for lambs in the summer months (Ostertagia)

- Very useful for ewes pre-tupping.

- Limited use for Nematodirus in lambs or ewes around lambing.

- Faecal egg counts; less than 300 eggs per gram the burden is unlikely to be significant and there is no need to worm.

- Faecal egg counts; over 500 eggs per gram, a significant worm burden and it is beneficial to worm.

Monitoring for anthelmintic resistance

- Select groups of lambs with pooled faecal egg counts over 500 eggs per gram.

- Worm the group and then repeat the pooled faecal egg counts as follows:

For white drench re-sample after 10-14 days

Levamasole based products re-sample after 7 days

Ivermectin based products re-sample after 14-16 days or after persistency finished

- Faecal egg counts from post treatment samples should be reduced by at least 95% to show efficacy of the wormer. If reduction falls below 95% then resistance to the wormer is present.


- Make use of natural immunity to worms and only dose when necessary (ewes once per year and leave at least 10% untreated).

- Quarantine dose all bought in sheep.

- Don't underdose, don't dose and move onto clean pasture.

- Perform faecal egg counts on lambs during summer.


Worming strategically can save you time and money on unnecessary doses without affecting performance while allowing lambs to build immunity and avoid anthelmintic resistance.

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