Vets warn of encysted small redworm infection risk


Vets are reminding horse owners of the risks posed by not treating small redworm infections.

In the spring small redworm larvae erupt from hibernation inside the horse’s gut in large numbers, breaking and damaging the lining of the intestinal wall.

Symptoms include diarrhoea, weight loss and colic. This condition (known as larval cyathostominosis) can be fatal. Young horses (under 6 years old) have the highest risk although all ages may be affected.

Negative worm egg counts

Zoetis vet Wendy Talbot said: “Over the past few weeks I have heard of a number of cases of larval cyathostominosis. Unfortunately people think their horses are safe from this parasite if they have had a recent, negative faecal worm egg count (FWEC) but this is not the case.

“Because encysted small redworm are hibernating, they won’t show in faecal worm egg counts. A horse could actually have a burden of several million encysted small redworm larvae yet show a negative or low FWEC.” 

Currently there is no effective test for encysted stages of small redworm. All horses of more than six months of age should be dosed for it ideally during the late autumn/early winter and certainly before the spring arrives.


There are only two active ingredients licensed to treat encysted small redworm: a single dose of moxidectin or a five-day course of fenbendazole. However, there is widespread evidence of resistance to fenbendazole, so a resistance test is recommended before using it.

Moxidectin has shown to have high efficacy against adult small redworm including encysted mucosal larvae.

Always make sure you choose the right worming methods and products for your horse’s circumstances by discussing your worming programme with your prescriber.

Find out more about worming drugs here.


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