Worm resistance in sheep: a growing concern in Ireland
There is a growing problem of parasitic worm resistance in livestock in Ireland. The problem can quietly wreak havoc on farms, often going unnoticed by farmers until it severely impacts production. However, there are ways to manage and control resistance through appropriate worm treatment and flock management strategies.
Understanding worm resistance
Worm resistance in sheep is not limited to Ireland; it is a global challenge. Unfortunately, it often goes unrecognised in sheep flocks until it has reached significant proportions. By the time resistance is clinically apparent, it has become well and truly established in the flock and on the pasture. At that stage, resistance-prevention strategies may not be effective, while treatment with most classes of commonly-used wormers may be of little value, as resistant worm-eggs dominate the pasture. These resistant eggs remain as a reservoir 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 this approach aimed to reduce parasite burden, they unintentionally selected for work resistance. 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 perfect environment for resistance to flourish. 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 resistant worms to thrive and multiply, becoming the dominant strain on the pasture. In extreme cases, these resistant eggs become the only source of new infestation amongst naive sheep. In this way, the level of pasture contamination with resistant eggs increases significant over time. Moreover, infested sheep may be unknowingly traded, spreading resistance to new farms that previously did not have the problem.
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 fail to thrive despite adequate nutrition or disease being present,
- Continue scouring in individual animals even after a broad-spectrum wormer 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 by appropriate laboratory tests.
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
Selecting the appropriate wormer can be challenging due to the number and variety available. Often, the same or very similar wormers are marketed under different brand names.
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, for example, 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 for example 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, 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 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:
- Are scouring, or
- Are suffering from ill-thrift, or
- 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,
- Rotate the chemical classes of wormers used each year, avoiding those to which resistance has been found,
- 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.
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.
If free of resistant worms allow, them to join the 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. 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.