Good Practice based on Current Knowledge

Reduce the frequency of drenching. Consult your veterinary surgeon or sheep advisor for advice on the optimum number of drenches needed per season. Frequent suppressive drenching has been shown to increase the rate of selection for anthelmintic resistance, as it allows only resistant worms to pass eggs onto the pasture.

Adopt a rotation of anthelmintic classes. Resistance to an anthelmintic is more likely to occur if the same class of wormer is used without any change. It is a good policy to adopt an annual (not more frequent) rotation between BZ and LV classes of drugs to reduce the selection pressure for development of resistance. However, it may be necessary to use specific drugs for the control of tapeworms and sheep scab at certain times of year.

Ensure the use of the correct dose rate. Follow the manufacturer's instructions and do not mix different wormers together. Increase the efficacy of the drench by applying it over the back of the tongue and not just in the mouth. This will ensure the anthelmintic enters the rumen instead of entering the abomasum via the oesophagial groove. Efficacy can also be improved by short-term withdrawal of feed. However, ewes in late pregnancy should not have their feed withheld, due to the risk of inducing pregnancy toxaemia.

Check that the drenching equipment is accurate. Calibrate regularly.

Check stock body weights and dose to the heaviest animals within the group being treated. It is better to slightly overdose than underdose.

Use grazing strategies to reduce pasture contamination/hot exposure. Provision of 'clean' or 'low-risk' pastures such as new leys or aftermaths and grazing using adult non-lactating ewes or cattle can reduce the parasite population on pasture and hence the requirement for frequent drug treatment.

Avoid importing resistant worms: Since most anthelmintic resistance in the United Kingdom is against the benzimidazole class of drugs, treat all purchased stock on arrival on your farm using drugs from the LV class of anthelmintics or with moxidectin. If possible, hold new stock in a yard for at least 24 hours after dosing before turning out onto pasture to reduce contamination of grass with worm eggs.

Do not graze sheep and goats together, as goats more commonly have resistant parasites.

Where resistance is suspected: Seek assistance to check the efficacy of your drench. The veterinary surgeon can organise a simple faecal egg count reduction test to check the efficacy of the anthelmintic. It is worth noting that continued scouring after treatment might indicate that other infections such as coccidia could be a contributory factor (see section on anthelmintic resistance).

- Discontinue the use of the class of drenches to which resistance has been demonstrated.

- Alternate annually between the remaining effective drench classes.

- Periodically test that these products are still effective.

- Use management strategies which minimise the number of treatments per season.

Defra is working with the National Sheep Association (NSA) in an industry led initiative for the Sustainable Control of Parasites in Sheep (SCOPS) which offers farmers and vets advice on how to combat the growing problem of anthelmintic resistance (AR) in the treatment of worm parasites

A Defra funded endoparasite workshop led to the production of a technical manual for Veterinary Surgeons and Advisors that was published in March 2004. Defra and the Scottish Agricultural College have contributed to the work of the group alongside industry and stakeholder groups such as SNFU, NOAH, AHDA, RUMA, CSL/VLA and SVS and have sponsored research and the production of the manual “Sustainable Worm Control Strategies for Sheep - Technical Manual for Veterinary Surgeons and Advisers”.                                                                               

The following leaflets have been produced by SCOPS in conjunction with Defra:

Are you buying in sheep? - Then don't get more than you bargained for”?

'The 1-2-3 of Good Drenching Technique' leaflet

SCOPS 10 things you should know if you want to keep farming sheep

SCOPS New worm control strategies for sheep - Taking the first steps

An evaluation of the practicality and effectiveness of the SCOPS guidelines for worm control strategies in sheep is currently being researched (Defra project OD0550 details available at

For further information on reducing the risk of anthelmintic resistance, see section Good Practice

Use of faecal egg counting

FECs should not be used solely when making decisions regarding parasite management. Other factors, including the age of the animal, grazing history, current performance and growth rates, clinical signs, previous treatments, the time since the last treatment, withholding times and the presence of intercurrent disease also need to be considered before a decision on when to dose can be made (McCoy et al., 2005). McCoy et al., 2005 consider that at least 20 lamb dung pats is representative of a batch of lambs when carrying out FECs.

Soil Association (2006) provide guidance on the interpretation of FECs:


Veterinary advice is normally required for treatment when FECs of 400-600 eggs per gram are observed, although the advisable treatment threshold is subject to parasite species, season and the immunological status of the animal.

 The Soil Association (2006) provide the following table of guidance for the interpretation of faecal egg counts, although it is stressed that this is not prescriptive and veterinary guidance should always be sought when interpreting FECs.

FEC (eggs per gram)



Treatment should not be necessary although Nematodirus risk in young lambs should be monitored


Treatment unnecessary unless there is a Nematodirus risk or early stage of Haemonchus is suspected


If these levels are stable, the animal could be dealing with the parasite challenge and immunity could be developing. Important to continue monitoring


Treatment may be necessary, particularly for lambs 3-4 months of age when immunity has not been acquired


Treatment likely to be required although age, performance and grazing risk should be accounted for


Treatment probably essential

From Soil Association (2006)

A suggested monitoring programme for roundworms, as recommended by the Soil Association (2006) proposes the key periods for faecal sampling of ewes is at lambing and pre-tupping and from 10 weeks of age and every month onwards for lambs with ewe replacements also being examined in early winter to evaluate worm infestation during autumn.  Store lambs should be sampled on arrival on the farm and monthly thereafter.