Encysted small redworm in horses

Posted by Wormers-direct on 15th Jan 2020

Encysted small redworm in horses

A survey (Equine Health Survey (NEHS) May 2012) of horse owners showed that nearly half were not worming their horses correctly when it came to treating encysted small redworm.  In more recent times with the extensive use of targeted and strategic worming we need to be even more aware of the danger of mass eruption which often happens in early spring.  This mass emergence can lead to a disease syndrome known as “Larval cyathostominosis” causing diarrhoea and colic with up to a 50% mortality rate1.

The increased use of FWEC (Faecal Worm Egg Counts) and how to interpret the results is adding to the need to understand the importance of treating for encysted small redworm.  These inhibited encysted are the larval stages of the small redworm.

Encysted small redworm (small strongles/cyathostomes) are the most common worms found in horses today and may account up to 90% of the redworm burden in the horse2.  They are also the most pathogenically significant equine parasites3, in fact many horses can be infected all of their lives 4.

Studies have shown that many horse owners are failing to worm with an effective product for the control of encysted small redworm, often assuming that the products they were using did treat for encysted small redworm when in fact they do not.  Some owners simply do not treat at all.  The most common reason for not treating for encysted small redworm was that the horse had had a clear faecal egg count.

Encysted small redworm don’t show up in a standard faecal worm egg count because the larval stages are dormant and buried within the gut wall, even if the horse has shown a negative or low count it could still be harbouring several million encysted small redworms5, a potential fatal health risk to the horse.

Encysted small redworm can remain dormant inside a horse for up to 2 years, but usually develop and emerge from the gut wall all at the same time in the early spring.  It is most important to use a wormer containing moxidectin or a 5 day fenbendazole course licensed to treat encysted small redworm.  It is important to remember that there is now widespread resistance to fenbendazole in parasite populations4, whereas moxidectin has been shown to be effective against benzimidazole resistant worms.  Treating with products that do not specifically treat for encysted small redworm can increase the risk of larval cyathostominosis6.

So when you next need expert advice on an animal health plan that includes worming and testing your horse, make sure you enlist the help and advice of an SQP (Suitably Qualified Person).

SH Wetherald E-SQP
(with thanks to Zoetis for their input and guidance)

1Dowdall S. et al (2002) Veterinary Parasitology 106, 225-242

2 Bairden K. et al (2001) Veterinary Record 148, 138-141

3 Love S. et al (1999) Veterinary Parasitology 85, 113-122

4 Matthews JB (2008) An update on cyathostomins: Anthelmintic resistance and worm control. Equine Vet. Education20 552-560

5 Dowdall S. et al (2002) Veterinary Parasitology 106, 225-242

6 Craig R. Reinemeyer and Martin K. Nielsen. Handbook of Equine Parasite Control