Timing everything when tackling encysted small redworm in horses

Small redworm larvae can encyst within the horse’s gut wall throughout the year – especially in autumn and winter.
Small redworm larvae can encyst within the horse’s gut wall throughout the year – especially in autumn and winter. © Zoetis

A warning has been issued to northern hemisphere horse owners to ensure they deworm their animals with a product that will treat encysted small redworm.

Using a dewormer that does not specifically treat the encysted stages during late autumn or winter can increase the risk of a horse with a high encysted small redworm burden developing larval cyathostominosis, which can be fatal, said Wendy Talbot, a veterinarian with animal health company Zoetis.

It was not a case of “any wormer will do”, Talbot said. “Wormers such as those containing ivermectin, pyrantel or single-dose fenbendazole will remove the adult stages of small redworm from the gut which then leaves room for the encysted stages in the gut wall to emerge and take their place.”

It is this en masse emergence that can cause severe damage to the gut resulting in clinical signs such as diarrhoea, colic and even death, she said.

All horses of more than six months of age should be blood tested by a vet to determine the presence of an encysted small redworm burden or treated with a wormer that is licensed to treat encysted small redworm in autumn/winter, regardless of their faecal worm egg count.

Moxidectin is the only active ingredient licensed to treat encysted small redworm in a single dose. Because moxidectin acts against the encysted stages before they have a chance to emerge, it is less likely that larvae in the gut wall will be able to cause this effect.

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.

“A practical and cost-effective solution to targeting parasites of concern over winter, may be to combine a bot treatment with encysted small redworm and tapeworm treatments using a single dose of moxidectin and praziquantel,” Talbot said.

She advises horse owners to always seek advice from their vet or medicines prescriber.

» More on “timing it right” over winter. Here’s what to do over spring/summer.


1. AAEP (2019) Parasite Control Guidelines
2. Reinemeyer CR and Nielsen MK (2013) Handbook of Equine Parasite Control.p87. Hoboken, NJ: John Wiley & Sons.
3. Rendle D (2017) De-worming targeted plans. Vet Times, Equine, Vol.3 Issue 1 p16-18
4. Matthews JB (2008) Equine Vet Educ, p 552-560
5. Hallowell- Evans C and Hallowell G (2017) Vet Times, April 24
6. Austin Davis Biologics Ltd. Small Redworm Blood Test Press Release. https://www.austindavis.co.uk/small-redworm-blood-test. Accessed 11th October 2019
7. Austin Davis: Elisa Kits. https://www.austindavis.co.uk/elisa-kits


Latest research and information from the horse world.

Leave a Reply

Your email address will not be published.