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Anthelmintic Resistance

About Anthelmintic Resistance

What is anthelmintic resistance?

Anthelmintic resistance refers to worm populations surviving a correctly-applied, standard dose of anthelmintic (anti-worm treatment).

Anthelmintic resistance is:

  • Genetic: Resistant worms produce resistant offspring
  • A pre-adaptive phenomenon: Genes that lead to resistance are already present in the worm population
  • Irreversible: No effective return to susceptibility even after removal of selecting anthelmintic.

Resistance to all the main anthelmintic classes has been exhibited by nematode populations in most sheep-rearing countries over the last twenty years.

It is important to note:

  • that it is the worms that are resistant to anthelmintics, NOT the sheep.
  • that resistant worms produce resistant offspring.
  • that sheep are transporters of resistant worms.
  • that it should be considered where pasture contamination is greatest.
  • that it should be considered what treatments have been administered to which animals, what class of animals were treated and where animals have grazed post-treatment.

Worms are resistant to anthelmintics ——- Sheep carry anthelmintic-resistant worms

Key issues in sheep

Benzimidazole resistance has been reported in Teladorsagia circumcincta, Haemonchus contortus, Cooperia curticei and Trichostrongylus spp. Additionally 1-BZ resistant Nematodirus battus has been detected in a small number of flocks across the UK. A survey showed that the genes for resistance were found in around 1 in 4 flocks but generally at very low levels (on 2% of the individual parasites examined but at higher prevalence on some farms). See our Nematodirus page for more information.

  Levamisole resistance has been reported in T circumcincta, C curticei and Trichostrongylus spp.

  IVM resistance has been reported in T circumcincta and Trichostrongylus species in a number of sheep flocks in parts of Great Britain.

There have also been a reports of moxidectin (MOX) resistance. In the early stages of selection, this usually manifests as a reduced period of persistency.

  Monepantel resistance has been reported in T circumcincta, Trichostrongylus vitrinus and Oesophagostomum from a single flock in the UK.

  Resistance has not currently been reported to 5-SI.


Multiple resistance

The emergence of ‘triple resistance’ is becoming greater, with the majority of farms examined in surveys containing parasite populations that are resistant to more than one anthelmintic class.

These populations are a cause for concern and presents a challenge in terms of correct advice and management.

Recent prevalence survey report findings of anthelmintic resistance from across the UK

Key issues in cattle

  Benzimidazole resistance has been reported in Ostertagia ostertagi in UK herds.

  IVM resistance has been reported in Cooperia oncophora in a number of cattle herds.


Testing for resistance has a number of benefits:

  • Improves health of stock by ensuring treatment has worked
    Improves productivity in stock, as reduced drug efficacy can lead to lower weight gains
  • Saves money on buying and administering ineffective treatment next time
  • Helps better decision making Highlights higher risk areas where contamination is going to be greatest
  • Allows decisions to be made on pasture management or stock rotation.

Consideration should also be given to the species of nematode concerned, since efficacy results will vary according to the species and hence also the time of year the test is carried out. It may be that drug efficacy changes throughout the season as different roundworm species become more prominent.

Early detection of reduced efficacy means that efforts can be concentrated on reducing selection pressures to help maintain efficacy for longer. However, under field conditions anthelmintics will apparently continue to give clinical responses in parasitised sheep when the reduction in faecal egg count (FEC) is substantially less than 95%. Consequently, sheep farmers remain unaware that resistance to an anthelmintic is present until the reduction reaches a high prevalence.

Resistance has been shown to result in significant production losses in affected animals, particularly young stock but also in older stock with concurrent infections.

Available tests

Post-Drench Efficacy Check (PDEC)

Simply, are there any eggs left following treatment?

FEC is carried out post-treatment (timings for assessment in Table 1).

Any eggs present suggest that the drug is not working effectively.

Faecal Egg Count Reduction Test (FECRT)

True efficacy testing FEC carried out before AND after treatment Timing of FEC post-treatment depends on class of chemical used (see Table 1).

If treatment has been effective, there should be a >95% reduction in FEC.

Timing of post-treatment FECs for each anthelmintic group

Managing resistance in roundworms

  • Avoid under-dosing animals and ensure you follow advice on handling, storage and administration of anthelmintics
  • Adopt stringent quarantine measures for imported stock
  • Checking anthelmintic efficacy every time they are used provides the best option for optimising use, but any testing for efficacy of these drugs is better than none. It may be that drugs are still useful at different times of the year when different roundworm species predominate on farm
  • Monitor performance, grazing history and egg counts to reduce treatment frequency
  • Target treatments effectively (targeted treatment/targeted selective treatments)
  • Breeding for resistance/resilience (may take time to achieve results)
  • Use parasite forecasting aids (SCOPS/NADIS) to help determine best time for treatment
  • Reduce treatment frequency and eliminate unnecessary or wasted treatments e.g. routine use of FECs to ensure animals are only treated when there will be a clinical benefit
  • Consider management options and, if you need to treat, use right drug at right time on right animals at right dose
  • Ensure nutrition is correct
  • Work with your vet and animal health advisor to devise sustainable worm control strategies tailored to your individual farm
  • Reduce pasture contamination to reduce stock exposure to roundworms:
    • Consider stocking densities
    • Consider grazing management options (use of ewes or cattle as parasite mowers)
    • Re-seeding pastures – this may not fully resolve the issue but will help reduce contamination

Other causes of anthelmintic failure

Anthelmintic resistance is not the only reason that anthelmintics sometimes appear to fail to control worm parasites.

Other causes:

  • Dosing with insufficient anthelmintic due to:
    • underestimation of the animal’s weight
    • poorly maintained dosing equipment
    • poor administration technique
  • Failure to follow the manufacturer’s instructions:
    • incorrect storage of products
    • using products beyond their use-by date
    • mixing anthelmintics with other products
    • incorrect formulation used for your host species
  • Rapid re-infection of animals after treatment from highly infective pastures, often seen with Nematodirus battus where patent (egg laying adults) re-infection can occur in less than 14 days
  • Use of the incorrect drug for the target worms and/or mis-diagnosis

Ongoing research

Research being undertaken at Moredun is focused on improving our understanding of a variety of parasitic diseases along with the mechanisms of resistance and the way in which resistance develops.

The aims of this research is to provide improved means of diagnosis and develop effective management strategies that can be used to conserve the efficacy of our current anthelmintic families.

Published research:

  • Turnbull, F., Devaney, E., Morrison, A. A., Laing, R., & Bartley, D. J. (2019) Genotypic characterisation of monepantel resistance in historical and newly derived field strains of Teladorsagia circumcincta. International Journal for Parasitology: Drugs and Drug Resistance, 11, 59-69.
  • Bartley, D. J., Hamer, K., Andrews, L., Sargison, N. D., & Morrison, A. A. (2019) Multigeneric resistance to monepantel on a UK sheep farm. Veterinary Parasitology: X, 1, 100003.
  • Avramenko, R. W., et al. (2019) Deep amplicon sequencing as a powerful new tool to screen for sequence polymorphisms associated with anthelmintic resistance in parasitic nematode populations. International Journal for Parasitology, 49(1), 13-26.
  • Bartley, D. J., Meslé, M., Donegan, H., Devin, L., & Morrison, A. A. (2016) Phenotypic assessment of the ovicidal activity of monepantel and monepantel sulfone on gastro-intestinal nematode eggs. Veterinary parasitology, 220, 87-92.

Our Experts

Dave Bartley


Fiona Kenyon


Philip Skuce

(BSc, PhD)

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