Mild winter may increase small redworm risk

The ground may be drying up, but the effects of this year's wet and mild winter are still all too apparent.

Spring is on it’s way and with it comes the pleasure of turning our horses out onto fresh pasture.  However we also need to be conscientious about parasite management at this time of year: early spring is the high-risk season for larval cyathostominosis (small redworm). This potentially fatal disease syndrome is caused by the mass emergence of small redworm from their dormant, encysted state and this year the risk may be higher than usual, given the exceptionally mild, wet winter we have just experienced.

What are encysted small redworm?

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 have buried into the lining of the gut where they lie dormant for a period of time.  They can pose a potentially fatal health risk but won’t show up in a standard faecal worm egg count. It’s important to make sure that encysted small redworm are treated properly during late autumn or winter otherwise they may develop and emerge en masse from the gut wall in the early spring and could lead to larval cyathostominosis, causing diarrhoea and colic with up to a 50% mortality rate1. It is recommended that all horses receive a treatment for ESRW during the late autumn/winter, regardless of their faecal worm egg count.2,3. If the preceding winter has been especially warm then in some circumstances it’s advisable to consider a second ESRW dose in the spring, prior to turnout onto clean grazing.

Why does warm weather increase the risk?

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. 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.

Are all horses susceptible?

Horses of any age can develop larval cyathostominosis. 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 speak to your vet.

Can larval cyathostominosis be treated?

Treatment of larval cyathostominosis is difficult and not always successful once the full symptoms have started. It’s important to contact your vet as soon as possible if you have any concerns - the sooner it is treated the better the prognosis.


To find out more about year-round worm control visit You can also visit to test your knowledge of encysted small redworm. Stable Mate, the horse health management App from Zoetis, is available from the iPhone App Store and Google Play Store: text Stable Mate to 80800 to download.


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




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