Keep encysted small redworm on your radar this autumn

By Wendy Talbot on 25 September 2020

Responsible and sustainable worm control is crucial to help keep your horse healthy and performing at his best. While faecal worm egg counts (FWECs) can help you keep control of worms during the grazing season some pesky parasites need special attention during the autumn and winter, particularly encysted small redworm.

During the autumn, the larval stages of the small redworm can stop developing inside the horse’s gut and enter a type of hibernating state known as encysted small redworm (ESRW). Large numbers of encysted small redworm larvae can be present and because larval stages do not produce eggs they cannot be tested for using faecal worm egg counts. 1

Large burdens of encysted small redworm can cause a condition known as larval cyathostominosis when they emerge from their hibernating state. The resulting diarrhoea, colic, and severe weight loss can be fatal, especially in young horses.

All horses of more than six months of age should be tested or dosed with a wormer that will treat for encysted small redworm in autumn/winter time regardless of their FWEC. 1,2

  • Moxidectin is the only active ingredient licensed to treat encysted small redworm in a single dose.
  • A five-day course of fenbendazole is also licensed to treat encysted small redworm but there is widespread evidence of small redworm resistance to fenbendazole, including the five-day dose so a resistance test is recommended before using it. 1
  • Treating with a wormer that does not specifically target the encysted stages (ivermectin, pyrantel or single dose fenbendazole) during late autumn and winter can increase the risk of a horse with a high ESRW burden developing larval cyathostominosis. 3

Always talk to your vet or RAMA

Make sure you choose the right worming methods and products for your horse’s circumstances by discussing your worming programme with your prescriber.



  1. Matthews JB (2008) Equine Vet Educ. Oct. 552-559
  3. Reinmeyer and Neilsen (2013) Handbook of Equine Parasite Control




Wendy graduated from Bristol University in 1999. She then went on to complete a residency at Liverpool University and holds a European Diploma in Equine Internal Medicine. After working in practice for 13 years, she joined Zoetis in 2012 as the National Equine Veterinary Manager.

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