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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.2

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
Anthelmintic resistance


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:

Sheep wormer resistance stats"/></p>
<img src="/images/Detailed resistance.png" alt="Detailed resistance" /></p>
<img src="//assets-eu-01.kc-usercontent.com/ca66ff62-0736-01f3-ea9a-aa3a66d97272/6eb84fc9-b917-41fb-b36a-8610a8459afe/Zolvix stats.png" alt=

Detailed resistance

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.

    As one of the newer groups, Zolvix™, the group 4-AD wormer, should be used in every flock, every year. Zolvix™ should replace one dose of wormer to help preserve the efficacy of the other 3 older wormers. For maximum benefit, it is recommended as a:

  • One off mid-late season break dose for all lambs (after July depending on worm burden)
  • Routine farm protection (quarantine) dose for all incoming sheep to avoid buying in resistant worms in otherwise healthy animals
  • For farmers with confirmed resistance, Zolvix™ may be required to be used at different times of year. Veterinary advice should always be consulted.
  • Zolvix

    See SCOPS for full details on effective quarantine treatments.

  • treating sheep when the refugia population (ie those worms not exposed to the wormers) is small: for example, dosing animals and moving immediately to clean pasture; dosing the whole group in the winter, eg ewes at lambing time, when the worm population on the pasture is low

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.

Act now: find out more about how you can incorporate Zolvix™ in your worm control strategy, for your mid-late season break dose or as part of a routine quarantine treatment for all incoming and returning animals.


1 Wales Against Anthelmintic Resistance Development (WAARD) Final Report 2015.
2 Preventive Veterinary Medicine, Vol 182, September 2020, 105103
3 SCOPS Internal Parasite Technical Manual
4 SCOPS Using Group 4-AD and Group 5-SI anthelmintics
Zolvix™ 25mg/ml oral solution is a broad spectrum anthelmintic for the treatment and control of gastro-intestinal nematode infections and associated diseases in sheep including lambs, hoggets, breeding rams and ewes. POM-VPS in UK. Information regarding the side effects, precautions, warnings and contra-indications can be found in product packaging and leaflets; further information can also be found in the Summary of Product Characteristics. Advice should be sought from the medicine prescriber prior to use. Zolvix, Elanco and the diagonal bar logo are trademarks of Elanco or its affiliates. Use medicines responsibly (http://www.noah.co.uk/responsible).PM-UK-21-0466