How big of a deal is anthelmintic resistance in sheep?

Anthelmintic resistance is one of the biggest challenges to the health and profitability of sheep and affects the entire sheep industry: a recent study found that parasitic worms has been estimated to cost the European livestock industry more than €1.8 billion per year, with drug-resistance costing at least €38 million per year in production losses and treatment costs.


Anthelmintic resistance is the ability of worms to survive the normal dose of a wormer and pass that ability on to its offspring. This results in the proportion of resistant worms in the population increasing over time. Worms can be resistant to one class or several classes of wormer.


The percentage of resistant worms, and the number that survive treatment, increases with time as illustrated:

Source – SCOPS

The impact of wormer resistance on the farm will not be immediately obvious. As resistance develops, there is a gradual increase in the number of worms surviving treatment. The worms left behind will hold the lambs back, decreasing growth rates by up to 50% before there are any visible signs that the wormer treatment has not been fully effective1. The level of challenge, and therefore the need to treat for these worms during the summer and autumn, is weather dependent.

  • Even low levels of resistance can leave enough worms behind to affect growth rates. Indeed, when the number of resistant worms is more than 10%, we start to gradually lose lamb performance because the product is not performing at its optimum level3.
  • A low to moderate worm burden can reduce growth rates by up to 50% without any obvious clinical signs3.


This is why we need to change current practices and adopt a more sustainable approach to worm control - even when there are no visible signs of decreased performance or lack of wormer efficacy.


In the last 10 years the number of reports of resistance to the three older classes have been increasing. Research from Wales Against Anthelmintic Resistance Development (WAARD) showed the majority of farms surveyed had a degree of resistance to all three older wormer classes1:

Source – Elanco (

Once anthelmintic resistance has developed, it is not reversible, so we need to do everything possible to slow the development of resistance.

The speed at which resistance develops depends on how carefully and sustainably anthelmintics are used. This means avoiding practices that drive resistance development, such as:

  • under-dosing: weigh the animals, calibrate the dosing gun and dose to the heaviest animal in the group.
  • over-using wormers: move from routine dosing to routing monitoring. Lambs should only be treated when their worm burden reaches the point where it starts to decrease their growth rate; the same wormer group should not be used repeatedly or for consecutive treatments; and the newer group 4-AD (Orange) and group 5-SI (Purple) wormers should be used in every flock4 at the right time.

It's always best to check how effective a treatment has been by doing a post-treatment drench check, with a faecal egg count, after every treatment.

As the impact of resistant worms remains invisible until resistance levels reach the tipping point, it is important to include newer actives in worm control plans now. Taking action before there are obvious problems on the farm will help maintain the activity of wormers for the future, as well as allowing lambs to reach their growth potential today.

For more information, please speak to your vet or visit

1st May 2022

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