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Parasitic Roundworms (Sheep)

About Parasitic Roundworms (Sheep)

Roundworms are estimated to cost UK sheep producers £84 million pounds per annum in treatment and lost productivity.

Sheep become infected by ingesting the larval stage with pasture resulting in roundworms living in the stomach or small intestine where they can cause an infection of the gut known as parasitic gastroenteritis (PGE).  The severity of disease caused by these worms (also known as gastrointestinal nematodes) depends on the size of the challenges faced by the sheep and their susceptibility to infection.

Although roundworm infection can lead to overt clinical disease, in many cases these parasites result in sub-clinical infections in which sheep perform below their full potential.  Production losses are associated with reduction in live weight gain, as well as a reduction in both the quality and quantity of meat and wool.

The major roundworm species involved in disease in UK sheep flocks:

Scientific Name Common Name Season
Nematodirus battus Thread-necked worm Spring
Teladorsagia (Ostertagia) circumcincta Brown stomach worm Early summer
Haemonchus contortus Barber’s pole worm Early summer
Trichostronglus vitrinus Scour worm Late summer/autumn
Trichostrongylus colubriformis Black scour worm Late summer/autumn


Ill thrift in sheep may not be just as a result of roundworm infections and it is therefore important to identify the root cause of any problem.  Faecal worm egg counts (FWEC) can be a useful tool in not only confirming that roundworms are indeed the cause of the problem but can also provide information regarding the species of worm present, and can also be used to help monitor the effectiveness of any treatment.

Current control is largely achieved through the therapeutic and prophylactic use of drugs known as anthelmintics. However the development of resistance by gastrointestinal nematodes to the anthelmintics used to control them has become a major threat to the sustainability of sheep farming.

Watch our short animated film, War of the Worms, which highlights the important elements involved in the development and spread of anthelmintic resistance as well as the crucial messages on sustainable methods of worm control.

Key points

Parasitic Gastroenteritis

    • Parasitic gastroenteritis is arguably the most important disease affecting productivity in lambs but it is important to recognise and diagnose which of the other ‘ill thrift’ diseases may affect the performance of lambs.


    • Roundworms are estimated to cost UK sheep producers £84 million pounds per annum in treatment and lost productivity.


    • Five types of roundworm are commonly found in sheep in the UK (see table above).


    • Infections are dose limiting i.e. greater infections cause greater losses
      Production losses are associated with reduction in live weight gain, meat and wool quality/quantity.


  • Roundworms traditionally followed a typical seasonal pattern, but changes in climate and management practices have meant that disease is being reported year through.

Sustainable Control

    • Worm control is a year-round issue.


    • There are a number of worm species that can cause disease in sheep.


    • Anthelmintics (wormers) are one of the major control options available for the treatment of gastro-intestinal worms; however worms are developing resistance (link to AR page) to some of these anthelmintics.


    • It is essential to conserve the efficacy of new and existing wormers through correct usage.


    • Make best use of all available information – farm history, farm location, abattoir returns, diagnostic samples, on-farm risk factors, climatic conditions = informed decision making.


    • Use gathered data to set performance targets for your lambs
      Consider management options and, if you need to treat, use the right drug at the right time on the right animals at the right dose.


    • Plan a strategic worming programme with your vet/health advisor that will reduce unnecessary treatments. Avoid frequent drenching with anthelmintics by integration with grazing management.


    • Consider reducing treatment frequency further by using targeted treatments or by only treating identified individuals within the flock.


    • Treat bought-in stock and animals that have been grazed off farm with wormers from both the 4-AD and 3-ML classes of drugs or 5-SI class and, where possible, yard for at least 24-48 hours prior to turnout.


    • Turnout quarantine treated stock onto dirty pastures to ensure that any resistant surviving worms form only a minor proportion of the total worm population.


  • Make sure that the treatment has been effective using a post drench efficacy check.

Research at Moredun

    • Develop molecular tools for species identification of free-living stages in faeces and on pasture which will enable quicker identification of worm species present and their proportion in the worm population.  This will help to ensure correct diagnosis and identification of any anthelmintic resistance species


    • Following the successful development and commercialisation of Barbervax, a vaccine to control Haemonchus contortus, work continues to develop a vaccine to control Teladorsagia circumcincta


    • Continue to research and develop biological and molecular tools to identify potential markers that will enable the early detection of disease and characterisation of anthelmintic resistance


    • Investigate the ‘new’ anthelmintic classes (Startect and Zolvix) before resistance becomes a problem in the UK sheep flock


    • Examine Nematodirus populations for markers of benzimidazole resistance, a new emerging concern


    • Survey the incidence of benzimidazole resistance in Nematodirus populations


    • Investigate potential markers to identify which sheep need anthelmintic treatment to maintain growth rates throughout grazing season


    • Investigate the role that maintaining a worm population unexposed to anthelmintic plays in maintenance of anthelmintic efficacy


    • Improving control strategies (how to integrate the use of persistent anthelmintics, such as moxidectin, into grazing strategies whilst reducing the selection pressure for anthelmintic resistance)


    • Further work on development of sensitive treatment criterion for small scale farmers


    • Undertaking risk factor analyses to examine correlations between the presence of anthelmintic resistance, animal movement and general farming practices


    • Further investigate motivators/barriers for adoption of best practice advice, particularly with respect to worm control


  • Investigating co-infections and how they may interact. For example, the interactions between roundworms and gut bacteria

Roundworm and Fluke FEC

Our Experts

Dave Bartley


Fiona Kenyon


Alasdair Nisbet


Tom McNeilly


David Smith


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