Worm resistance in sheep: a growing concern in Ireland

Anthelmintic resistance (AR) is a growing problem in Irish livestock, which can quietly wreak havoc on farms. Resistance development is typically an insidious process, going unnoticed by farmers until it severely impacts production. However, strategies have been developed to manage and control resistance through appropriate worm treatment and flock management.

Understanding anthelmintic resistance

Anthelmintic resistance (AR) is the heritable ability of a parasite to tolerate a normally effective dose of a wormer. A parasite is considered resistant if it survives exposure to the standard recommended dose of the anthelmintic, and this ability to survive is passed on to the parasite’s offspring.

Anthelmintic resistance in sheep parasites is not limited to Ireland; it is a global challenge. By the time resistance is clinically apparent, it has become well and truly established in the flock and on the pastures they graze. At that stage, resistance prevention strategies may not be effective as eggs from resistant worm populations dominate the available grazing platform. These eggs remain as a source of infection for future generations of sheep which may graze on them.

The root of this problem lies in the traditional practice of worming sheep using wormers from the same chemical class year after year. While such approaches were implemented with the aim of reducing parasite burden, they inadvertently contributed to the development of anthelmintic resistance. In particular, the practice of treating all sheep in a farm at the same time (whole flock treatment) 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 actually created the perfect environment for resistance to flourish.

The practice of whole flock treatment results in susceptible worms within the sheep being killed, meaning they can no longer shed eggs. This creates an environment in which resistant worms can thrive and multiply, becoming the dominant strain on the pasture in the absence of competition from susceptible strains. In extreme cases, resistant eggs may in time become the only source of new infestation amongst naïve sheep. 

It is important to consider that the development of resistance is not only a cause for concern on the farm directly affected, but also poses a significant threat to other flocks when sheep carrying resistant parasites are sold and may act as reservoirs of infection.

Identifying the issue

To tackle the issue, livestock owners and keepers should monitor their flock for signs of worm resistance. Suspect resistance if you notice:

• The overall liveweight gain of the flock is falling year-on-year for no obvious reason,
• Individual animals failing to thrive despite adequate nutrition and husbandry (e.g. up-to-date vaccination status, free of co-morbidities),
• Continued scouring in individual animals despite anthelmintic treatment.

Testing for resistance

Resistance can be confirmed by dung testing. This can be done either on:

• A representative sample of animals in the flock,
• Individual animals, such as 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. There are a number of laboratory testing methods available for the detection and confirmation of AR; ask your veterinary practitioner for guidance on selecting an appropriate test.

Effective treatment goals

Effective treatment and control strategies require:

• Targeted treatment of selected animals with an effective drug,
• Biosecurity measures to prevent the introduction of resistance, such as appropriate treatment and quarantine of new animals (e.g. rams) before they join the flock,
• Monitoring flock health and production,
• Managing pasture to minimise the risk of resistant worm-eggs becoming dominant over susceptible ones.

The over-riding aim is primarily to manage the pasture worm burden, so that animals at pasture will not be exposed exclusively to resistant worm eggs but rather will ingest susceptible worm eggs. 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.

Choosing the right wormer

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

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, such as 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 used 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 adapted by the Irish Department of Agriculture, Food and the Marine to fit the national situation.

Recommendations for using wormers

To minimise resistance development:

• 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,
• 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.

• Weigh animals before treatment and use the correct dosage. Too low a dosage increases the chances of resistance developing,
• 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.

It is critical to preserve susceptible worms within an ‘in refugia’ population which are not exposed to anthelmintic treatment. Increasing the size of the population in refugia serves to “dilute” the proportion of worms carrying resistance genes in the overall population. This is a key concept in delaying the development of anthelmintic resistance (AR). Thus, caution is advised when administering treatments where animals are on pastures which have a low level of contamination. 

Historically, rotation of wormers from different chemical groups was considered a useful strategy to delay resistance development. However, considering the widespread resistance to Group 1 – BZ (white) and increasing incidence of resistance to the Group 2 - LV (yellow) and 3-ML (clear) chemical groups, current knowledge suggests that rotation is unlikely to be effective. Reversion to susceptibility is unlikely to occur once resistance has been detected in a worm population. Furthermore, rotation of wormers from different chemical groups should certainly not take precedence over other more important decisions regarding selection of appropriate anthelmintics.

Worming new sheep prior to joining the flock

When introducing new sheep to a resistant-free flock:

Quarantine new sheep in a shed or concrete yard and test for the presence of resistant worms. The quarantine period should ideally be a minimum of 2 weeks, to ensure the absence of infectious/contagious diseases (including anthelmintic resistant strains of parasitic worms) in purchased stock.

If free of resistant worms, allow bought-in animals to join the destination flock.

If sheep have either not been tested for the presence of resistant worms, or have tested positively for resistant worms, quarantine them in a shed or concrete yard and worm them with an appropriate wormer(s), to avoid contamination of pastures with potentially resistant strains of worms. In this case, the appropriate wormer is one that from a different chemical class to that of which resistance has been found.

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). Collect the dung from the yard and do not spread on grazing pastures that year.

For further information on control of worms in sheep in Ireland, visit Animal Health Ireland.

Note: From 1st December 2025, all antiparasitic veterinary medicinal products for use in food-producing animals in Ireland are subject to veterinary prescription (POM) under the Veterinary Medicinal Products Regulations 2024.