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Understanding worms

Anthelmintics (wormers) are vital to control worm infections and protect against productivity losses. At present, resistant and multi-resistant worms are threatening the efficacy of existing anthelmintics.

  • Resistance is the ability of worms to survive the normal dose of a wormer and pass that ability on to its offspring.
  • Worms can be resistant to one class or several classes of wormer.
  • 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 level¹.
  • A low to moderate worm burden can reduce growth rates by up to 50% without any obvious clinical signs¹. This is the reason why we need to act now and put a sustainable worm control programme in place.
  • In the last 10 years the reports of resistance to the three older classes has been increasing.

There are five main groups of wormers available to sheep farmers in the UK.

The worm lifecycle

The lifecycle starts on pasture where stage three infective larvae wait to be eaten inside water droplets on blades of grass. Once ingested larvae travel into the gut and develop into stage four larvae and then adult worms which then begin to produce eggs.

Sustainable worm control

For more information on sustainable worm control, visit
Learn more about Elanco's worm control products:
Use Rycoben for Nematodirus control
Use Zolvix at mid-late season and for all incoming stock


Watch farmer Simon explain how he has incorporated Zolvix into his worming strategy


Calibration of dosing guns and administration of Zolvix

¹SCOPS manual

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 (