Teagasc has teamed up with industry partners to highlight and combat the alarming rate at which anthelmintic resistance is developing on farms.

The initiative, carried out in tandem with the Department of Agriculture and UCD, with support from Chanelle, Elanco, Norbrook and Zoetis, has already been set in motion but is set to be ramped up significantly throughout 2021.

Farmers will have received an information leaflet with their 2020 Annual Sheep and Goat Census which outlines the importance of paying heed to anthelmintic resistance, along with setting out a simple four-point plan designed to maintain susceptible worm populations and extend the efficacy of commonly used anthelmintics.

These farms are in a difficult position

Michael Gottstein, head of the Teagasc Knowledge Transfer Sheep Programme, said: “Anthelmintic resistance is not new at farm level but it is a topic that is challenging to get farmer buy-in to and this is clearly reflected in the worrying trend of increasing resistance. On some farms there is now resistance to group 1-BZ (white), group 2-LV (yellow) and group 3-ML (clear) anthelmintic classes. “These farms are in a difficult position as they are greatly limited in their choice of product and risk a significant reduction in lamb performance. Farmers need to take heed before it is too late.

“The information leaflet in the sheep census was the first step of a comprehensive programme planned to hopefully drive change at farm level. This includes discussions at events/webinars, information leaflets/factsheets and practical videos and dialogue circulated from industry supporters.”

Four-point plan

The four actions include:

  • 1 Refraining from treating adult ewes for worms unless there is a demonstrated need.
  • 2 Use only products from group 1 – BZ (white drenches) to treat lambs for nematodirus.
  • 3 Quarantine treatment to prevent buying in resistance through the arrival of new sheep on to the farm.
  • 4 Drench test/faecal egg test to determine drench efficacy and the need to treat respectively.
  • The first action is timely with lambing stepping up a gear and farmers tending to target worm treatment pre-lambing, while there is also sometimes a temptation where treating ewes for liver fluke to use a combination flukicide/wormer.

    The advice from Teagasc is crystal clear – healthy mature ewes should not receive treatment for worms unless there is a demonstrated need. Doing so will increase the rate at which the efficacy of wormers breaks down and resistance develops.

    Teagasc researcher Orla Keane explains that farmers often think they are bringing about two benefits by dosing ewes in spring – directly clearing the ewes of any worm burden and reducing pasture contamination.

    Healthy ewes will develop immunity to the normal range of worms and therefore treatment in this regard is a waste of time and money. Reducing the level of pasture contamination is also not necessarily a good thing if it is reducing the population of susceptible worms, leading to resistant worms dominating and multiplying over time.

    Enhancing refugia

    Increasing the population of susceptible worms is known as enhancing refugia.

    Figure 1 demonstrates what pasture contamination is likely to look like for a mid-season lambing flock. The level of pasture contamination from worms will typically start to decline early in the year, with mild weather the precursor for worms naturally dying.

    The worm burden in healthy ewes does increase in the final weeks of gestation and for the first six weeks of lactation. This is due to a relaxation of the normal immune system of healthy ewes but this will quickly fall off, as detailed in the graph.

    This source of pasture contamination is important to increase the level of susceptible worms present before the onset of a sharp increase in pasture contamination. If this occurs in the absence of a susceptible population of worms, then animals will ingest a higher percentage of resistant worms, which will significantly advance the rate at which these worms dominate the pasture and resistance develops.

    Exempted treatments

    The exception to this advice for treating ewes is where there are individual sheep or small numbers of sheep present which may be immunocompromised or where the flock is located in an area with a known haemonchus contortus (barbers pole worm) issue.

    A compromised immunity may be caused by ewes being in poor body condition or suffering from underlying health issues, while it may also take longer for yearling hoggets who rear lambs to develop natural immunity with treatment generally warranted during lactation.

    Orla advises care should be taken if taking faecal samples from batches of ewes containing sheep with compromised immunity as it can sometimes give the impression of a high worm count in a group of ewes which appear healthy.

    In such cases, Orla says ewes should be segregated for FEC testing while seeking advice from your vet will help identify any predisposing health issues contributing to high FECs.

    Valuable guidance

    Where a decision is taken to treat ewes, then there are a number of critical elements that can be followed to negate some of the downsides to treating ewes and hopefully reduce the rate of resistance developing.

    The first of these is to make sure you are using a class of anthelmintic which you are sure works on the farm so that there is not an added advantage given to resistant worms.

    Long-acting anthelmentics can increase the risk of resistance developing due to the treatment potentially fading out to a low level at the end which is not capable of bringing about a sufficient kill. While on the topic, it is important to remember there are times when animals can be treated for worms where not intended or required and this only serves to increase the rate of resistance.

    As mentioned already, combination flukicide-wormer products are one common route while the other is the use of macrocyclic lactones for treating sheep scab. Teagasc advises the only treatment for sheep scab where at all possible should be plunge dipping.

    The second piece of advice includes ensuring the weight of ewes is accurately assessed and dosing guns are calibrated properly to deliver the correct volume. If ewes receive treatment while still outdoors then they should be retained on swards which are likely to have a worm burden and not moved to clean ground post-treatment.

    Finally, avoid using wormers from the same anthelmintic class and make strategic use of the recent additions – amino-acetonitrile derivative (4-AD), available as Zolvix, and spiroindole (+Abamectin), when the latter is available again.