How high burdens of small redworm harm your horse

By Jacqui Matthews on 25 September 2020

Don’t let small redworm turn your horse’s gut into a leaky tea bag

Every horse over approximately 6 months of age should be blood tested or treated for encysted stages of small redworm in November/December using an effective wormer, to prevent the dangerous and undetected build-up of larvae. This should be done even if your horse’s FWEC is negative.

Don’t let this happen to your horse - follow our checklist to help keep him safe

  • Remember encysted small redworm won’t show up in a faecal worm egg count: Horses can harbour several million larvae yet show negative or low faecal egg counts.1
  • Treat or blood test every horse for encysted small redworm once a year: Ideally treat in the late autumn or early winter each year but certainly before the spring.2 Alternatively, your vet can carry out a specific blood test which will show if encysted small redworm are present.3
  • Use the right wormer: A single dose of moxidectin or a five-day course of fenbendazole.
  • Remember youngsters are at the highest risk: Be extra vigilant with horses of less than five years old.
  • Beware of resistance: There is widespread evidence of resistance in small redworm to fenbendazole, including the five-day dose so a resistance test is recommended before using it.4
  • Keep redworm under control in summer: Regular faecal worm egg counts from early March until October and treating according to the results will help keep redworm under control and reduce the risk of large hidden encysted burdens forming.
  • Use faecal egg count reduction tests during the grazing season: The best way to ensure that your wormers are working properly is to ask your vet to perform a faecal egg count reduction test. This involves taking a FWEC immediately before and two weeks after worming to assess the level of worm eggs being shed.
  • Be rigorous with pasture management: Daily poo-picking, regular rotation and resting of fields and cross grazing with sheep or cattle will help keep pasture worm burdens under control.
  • Seek veterinary advice: If you have a vulnerable young horse showing any clinical signs it is important to speak to your vet before using a wormer.


  1. Dowdall SM, Matthews JB, et al. Antigen-specific IgG(T) responses in natural and experimental cyathostominae infection in horses. Vet Parasitol. 2002;106(3):225-42.
  2. JB. Matthews, An update on cyathostomins: Anthelmintic resistance and worm control. Equine Vet. Educ. (2008).
  4. Steinbach T, Bauer C, et al. Small strongyle infection: consequences of larvicidal treatment of horses with fenbendazole and moxidectin. Vet Parasitol. 2006;139(1-3):115-31.




Professor Jacqui Matthews, a qualified veterinarian, has worked in livestock/equine helminth research and education for more than 25 years. Her group works on roundworms and their research spans sub-unit vaccine development, anthelmintic resistance and epidemiology. She has published over 125 peer-reviewed research papers and numerous lay articles, as well as given many presentations to industry, stakeholder and scientific audiences. She has won over £13 million in external funding, with highlight outputs being discovery of a sub-unit vaccine for control of brown stomach worm in sheep and development of an diagnostic test for cyathostominosis in horses. Professor Matthews has taught or examined at most UK veterinary schools and holds a Ministerial appointment as Parasitology Expert on the UK Veterinary Products Committee. She is currently based at Moredun, Edinburgh, and is Honorary Professor at the Royal (Dick) School of Veterinary Studies, University of Edinburgh.

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