Anthelmintic resistance (AR) is currently one of the biggest problems confronting the UK and Irish sheep sectors.

It is also an issue that could potentially impact the cattle sector over the coming years.

Against this background it is evident that identifying and then breeding animals that are inherently tolerant or resistant to the impact of gut worms and other parasites must be a priority for livestock sectors.

Co. Down sheep farmer John Martin believes that “such an approach is possible”.

He said: “Estimated Breeding Values (EBVs) are already available for rams demonstrating these traits.

“These can then be crossed with specific ewes within a flock that have been identified by individual farmers with the same trait.

“Essentially, these are animals within a flock that have always thrived from birth.”

Anthelmintic resistance

Martin farms on the outskirts of Greyabbey on the Ards Peninsula and is one of the seven livestock farmers from across Northern Ireland who have taken part in a trial designed to identify ways of addressing the challenge of AR.    

The work was co-ordinated by a team of research scientists from Queen’s University Belfast (QUB).

The project is a European Innovation Partnership (EIP) scheme, which has garnered support from Brussels and the Department of Agriculture, Environment and Rural Affairs (DAERA) in Northern Ireland.

Martin recently held a 'Parasite Control: A Whole Farm Approach' farm walk which was co-hosted by AgriSearch and the Nature Friendly Farming Network (NFFN).

Speakers at the event also included Professor Eric Morgan and Dr. Chris McFarland from QUB.

Prof. Eric Morgan, from Queen’s University Belfast

During the farm walk it was discussed that while improved genetics holds out a long-term prospect of the livestock sectors getting to grips with anthelmintic resistance, there are a number of steps that farmers can take immediately to allow them address the issue.

One of these is the mixed grazing of cattle and sheep.

It has been shown that the level of contamination on a pasture can be reduced by grazing cattle and sheep together. This approach reduces the stocking density of the host species: Cattle and sheep worms are different species.

The QUB experts also introduced the principle of ‘refugia’ on the farm walk.

This approach ensures populations of nematodes not recently exposed to treatment are deliberately allowed to survive.

They highlighted that progeny from the unselected parasites provide a source of less-resistant worms which can dilute resistant worms surviving anthelmintics, and hence reduce the rate of resistance development.

This can be achieved by either modifying strategic treatment regimens to ensure the survival of infective worm larvae on pasture, or by avoiding treatments to individual animals identified as best able to cope with parasites.

These strategies also include “targeted treatment” (based on estimates of worm burdens) and “targeted selective treatment” (based on indications of parasitic effects).

There is now strong evidence that shows that comparatively small changes to present practices can provide substantial refugia benefits.

There are also indications that appropriate resistance management approaches can be developed for different situations.   

The recent EIP project in Northern Ireland specifically looked at ways of reducing AR.

The work determined the feasibility and practicality of implementing targeted selective treatment (TST) of helminths (worms) on Northern Ireland’s commercial farms.

Group members designed, utilised and evaluated relevant targeted TST strategies to better understand the feasibility of their widespread use on farms.

The project commenced in November 2020 and finished at the end of June 2023.

Seven farms

There were seven farms involved ( three sheep, two dairy and two beef) and both TT and TST was trialled over the 2021 and 2022 seasons.

QUB’s Chris McFarland and Prof. Eric Morgan led on the majority of project activities alongside AgriSearch, the Agri-Food and Biosciences Institute (AFBI) and Animal health and Welfare Northern Ireland (AHWNI).

These projects included: The determining of suitable TST anthelmintic approaches for each participant farm, implementing TST approach on each participant farm and assessing the impact of implementing a TST approach.

Assessing the feasibility and practicality of undertaking wide-scale targeted, selective treatment of anthelmintics on farms across Northern Ireland were among the other priorities for the projects.

Meanwhile, helminth (worm) infections are a hugely significant drain on production efficiency in grass-based ruminant systems such as those that prevail across the island of Ireland.

At present, the control of parasitic helminths relies upon anthelmintics, but their widespread application at whole-herd level is leading to AR, reducing their effectiveness.

Problems with AR are well documented globally on sheep farms and increasingly by the cattle industry.

In Northern Ireland, AR has been detected on the majority of sheep farms, while no data is yet available for cattle.

A recent study in the Republic of Ireland found poor efficacy on most beef and dairy farms, including all 16 farms on which ivermectin was assessed (with both Cooperia and Ostertagia surviving).

To tackle AR, successive EU-funded research projects have investigated new strategies for sustainable worm control, primarily through changes to routine whole-herd / whole-flock treatment protocols. These focus on TST.

This entails targeting anthelmintics at the right time to maximise epidemiological benefits, and avoid unnecessary treatments.

Recently weaned ewes on the Co. Down farm of John Martin

Alternatively, a proportion of the flock or herd can be left untreated. This principle makes use of the fact that most worms are concentrated in a few individuals.

Removing them will have large effects on worm transmission and herd health while reducing the number of treatments needed.

A number of research projects have investigated this TST approach in recent years and concluded that they could be suitable for use on commercial farms.

However, success will largely depend on its on-farm practicality and wider economic benefit and prior to the recent EIP project, this approach has not yet been evaluated in Northern Ireland.