Worm resistance in sheep

There is a growing problem of parasitic worm resistance in sheep in Ireland. Even if it is often not immediately recognisable by farmers because of its insidious nature, it can lead to significant production-loss in sheep in affected farms. However, resistance can be controlled through appropriate worm treatment and flock management strategies.

Background to worm resistance in sheep

Resistance amongst sheep to treatment for roundworms has been identified as a growing not only in Ireland but in many countries where sheep are produced. Often the problem is not recognised in sheep flocks until it has reached significant proportions, as the loss of production may not be noticed until such time as a threshold or tipping point is reached. By the time resistance is clinically apparent, it has become well and truly established in the flock and on the pasture.  By then, the application of resistance-prevention strategies might not be effective.  At that stage too, treatment with most classes of commonly-used wormers may be of little value, as resistant worm-eggs have become dominant on the pasture, where they remain as a reservoir for future generations of sheep which may graze on them.

The resistance problem can be traced to the practice of worming sheep year-on-year with wormers that contain chemicals of the same chemical class. Such routine preventative disease approaches for worm control that were practiced for the past number of decades are now out-of-date scientifically.  These approaches were aimed at reducing the parasitic burden in individual animals and in the flock at large. However, they had the effect of selecting for resistance amongst the parasitic worms ingested from the pasture. In particular, the practice of treating all sheep in a farm at the same time and with the same wormer and moving them immediately after treatment to a clean pasture, and repeating this practice at frequent intervals during the grazing season, has created the ideal scenario for the development of worm resistance. The reason for this is that once susceptible worms are killed in the sheep, they can no longer shed eggs which are passed out in the dung to the pasture. This means that the way is clear for eggs from resistant worms to take over; in accordance with the established life-cycles of the worms concerned, the resistant worms lay eggs that contain the genes coding for resistance to the class(es) of wormer being used become the dominant strain on the pasture. In extreme cases, these resistant eggs become the only source of new infestation amongst naive sheep. Once ingested by the animal they take up residence, develop into worms and begin to lay eggs. In this way, the level of pasture contamination with resistant eggs increases significant over time. Moreover, infested sheep may be unknowingly traded and can act as incubators to spread resistance to new farms that previously did not have the problem.

Problem identification

Stock owners and keepers should ideally monitor the status of their flock for worm resistance as this will influence the choice of appropriate treatment and control strategies. Resistance to wormers can be suspected where:

  • The overall liveweight gain of the flock is falling year-on-year for no obvious reason,
  • Individual animals in the flock fail to thrive despite adequate nutrition or obvious disease being present,
  • Individual animals continue to scour even after treatment with a broad-spectrum wormer.


Resistance can be determined by faecal testing. This can be done either on:

  • A representative sample of animals in the flock,
  • Individual animals e.g. bought-in animals.


Where a problem is suspected, professional advice from a veterinary practitioner or other animal health specialist should be sought and presence of resistance confirmed by appropriate laboratory tests.

Treatment goals

Effective treatment and control strategies are based on:

  1. Targeted treatment of selected animals with an appropriate drug,  
  2. Biosecurity measures to prevent the introduction of resistance into the flock, e.g. by appropriate treatment and quarantine of new animals (e.g. rams) before they join the flock,
  3. Monitoring the health and production of the flock, and
  4. Managing the pasture to minimise the threat of the resistant worm-eggs becoming dominant over susceptible worm-eggs.


The goal for modern worm strategies is primarily to manage the pasture worm burden, balancing the need of individual animals for suitable therapy with the goal of ensuring the survival of susceptible parasites which can competitively inhibit the number of resistant parasites available on the pasture. In this way animals at pasture will not be exposed exclusively to resistant worm eggs but rather will ingest susceptible worm eggs.  These animals can be treated before their worm burden leads to significant production loss. In practice, this means that treatment is targeted towards:

  • Scouring or soiled animals only, or
  • Animals with confirmed high infestations, or
  • Animals with markedly poor body condition. 


Animals with light worm infestations of susceptible parasites should not be routinely treated (so as to maintain the pasture supply with susceptible worm eggs).

Choosing an appropriate wormer

There are a considerable number of veterinary medicines authorised for the treatment of worms in sheep and it can be difficult for a lay-person to determine which to use.  This difficulty is compounded by the fact that the same, or very similar, wormers are marketed under different brand names.  If the user were to switch between brands of drug from the same chemical class of chemicals, (s)he might not in fact be switching to an appropriate drug, but might conversely be accentuating the problem.


The objective in choosing an appropriate wormer is to pick one which will kill the parasites that are present and avoiding those for which resistance has been identified e.g. so-called white drenches are not effective to control parasites that are benzimidazole-resistant.  Therefore where benzimidazole resistance has been identified, users who switch from e.g. albendazole to fenbendazole containing brands will be wasting their money and adding to the problem of resistance on their farms. Likewise where resistance to other chemical classes has been identified.


To help users decide on the appropriate chemical class of chemical to use, HPRA has elaborated an initiative to label products with an appropriate symbol. At present, some five chemical group symbols are foreseen as outlined at the bottom of this page.

The initiative to display the chemical symbols on sheep wormers in Ireland is a voluntary one, which is intended for broad-spectrum products only. No symbol is required for narrow spectrum anthelmintics e.g closantel. A closantel/mebendazole combination drench will display the 1-BZ symbol only, while a combination of fenbendazole and levamisole will contain both the 1-BZ and the 2-LV symbols. The symbols recommended by us are the same as those already in existence in the UK. Through an initiative known as SCOPS (Sustainable Control of Parasites in Sheep) various recommendations for the use of anthelmintics aimed specifically at reducing the threat of anthelmintic resistance in sheep have been developed for the UK, and are being developed/adapted by the Irish Department of Agriculture to fit the national situation.


We expect that the Department will issue updated advice on strategies to contain anthelmintic resistance in sheep once the new symbols have been introduced onto the product labels.  At present, the initiative is intended for sheep wormers only, although the symbols can be introduced to packs which have a joint indication for use in sheep and cattle.  It should be noted that the initiative undertaken by the Department of Agriculture relates to anthelmintic resistance and does not extend to resistance control in liver fluke at this time.

It should also be noted that branded wormers containing drugs of the benzimidazole, levamisole and macrocyclic lactone classes are categorised by us for supply to farmers as Licensed Merchant (LM) supply status.  This means that they are available from licensed merchants, pharmacies and veterinary practices, with or without a prescription. However, veterinary medicines containing the newest chemical classes of wormers, i.e. the amino-acetonitrile derivatives and the spiroindoles, are restricted under EU legislation to prescription control. This means that a veterinary prescription is required for products containing these drugs.

Recommendations for using wormers

Whatever the chemical class of the drug that is to be used, sole reliance on that chemical as the only wormer will, inevitably, hasten the development of worm resistance in a flock. The following general principles apply to minimise the development of worm resistance:

  1. Use a wormer from a chemical class which is likely to be effective.  Where resistance to a particular chemical has been confirmed, avoid using that class of chemicals again.
  2. Treat only those animals in the flock that:
    1. Are scouring, or
    2. Are suffering from ill-thrift, or
    3. Have a high level of parasitic infestation as confirmed by faecal tests.
  3. Weigh the animals before treatment and apply the correct dosage. Too low a dosage increases the chances of resistance developing.
  4. Rotate the chemical classes of wormers used each year, avoiding those to which resistance has been found.
  5. Monitor the productivity of the flock during the year. Where poor thrive is suspected, discuss the position with your veterinary practitioner or animal health advisor.


Recommendations for worming new sheep prior to joining the flock

Sheep that are purchased to join an existing flock, which is known to be free of resistant worms, may contain worms that are resistant and therefore act as a source of anthelmintic-resistant eggs which could infect the flock.  Two control mechanisms exist:

  1. The imported sheep may be quarantined in a shed or concrete yard and tested for the presence of resistant worms. Only if the sheep is found not to harbour resistant worms, should it be allowed to join the flock.
  2. Where the imported sheep have either not been tested for the presence of resistant worms, or have tested positively for resistant worms, they should be quarantined in a shed or concrete yard and wormed with an appropriate wormer.  In this case the appropriate wormer is one that is:
    1. Of a different chemical class to that of which resistance has been found,
    2. From the newer classes of wormers authorised since 2009 i.e. containing a drug of the chemical class Amino-acetonitrile derivative or Spiroindole.  These drugs, being of a new chemical class, are subject to prescription-control by a veterinary practitioner.


In this case, the sheep should be left to stand in a concrete yard for 24 hours following treatment (to ensure that any worm-eggs in the contents of the animal’s intestines are expelled). The new sheep may then be allowed to join the flock.  The dung should be collected from the yard following this and not spread onto the pasture in which it is intended to graze sheep or goats for that year.

Worm Resitance - Vet Special Topics HPRA