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Mild Winter May Increase Small Redworm Risks

Mild winter may increase small redworm risks warns Zoetis.

Early spring is the high-risk time for larval cyathostominosis, a potentially fatal syndrome caused by the mass emergence of small redworm from their dormant, encysted state. Worming experts at Zoetis are warning that this year, the risk of disease may be higher than usual, following the UK’s exceptionally mild, wet winter.

Encysted small redworm (ESRW) are one of the most harmful parasites to affect horses in the UK. They are larval stages of the small redworm that hibernate in the lining of the gut and don’t show up in a standard faecal worm egg count. They usually ‘wake up’ in the early spring and their mass emergence can lead to larval cyathostominosis, causing diarrhoea and colic with up to a 50% mortality rate 1.

Normally, during winter months, lower environmental temperatures prevent worm eggs and larvae from developing on the pasture, meaning that re-infection of horses does not occur to a significant extent until the following spring. However, during unusually mild, wet winters such as this year’s, worm eggs and larvae can develop on the pasture and grazing horses can become re-infected 2. Even horses that have been treated for encysted small redworm in late autumn/early winter may still be at risk of re-infection, particularly if they have been turned out on heavily used pasture.

“It is recommended that all horses receive a treatment for ESRW during the late autumn/winter, regardless of their faecal worm egg count 3,” said Wendy Talbot, Zoetis veterinary surgeon. “In some circumstances, such as if the preceding winter has been especially mild, then it is advisable to consider a second ESRW dose in the spring for those horses most at risk.”

All horses can develop larval cyathostominosis but those at particular risk are youngsters, old or immune-compromised horses (such as those with Cushing’s disease), those with an unknown or sub-optimal worming history and those that were not dosed correctly in late autumn/early winter. If you have a horse showing signs possibly related to a worm burden such as loss of condition, sudden weight loss or diarrhoea, it’s important to contact your veterinary surgeon as soon as possible.

To find out more about year-round worm control visit: www.wormingyourhorse.info.

You can also visit: www.esrw.co.uk to test your knowledge of encysted small redworm.



  1. Dowdall S.M.J. et al (2002) Veterinary Parasitology 106, 225-242
  2. Reinmeyer CR and Nielsen MK (2013) Handbook of Equine Parasite Control. 45-53
  3. Nielsen (2012) Veterinary Paristology. 185. 32-44
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