Owners are urged to ensure they choose the right wormer this winter – or increase the risk of potentially fatal disease.
Products that specifically target encysted small redworm must be used at this time of year, to treat the parasites in their dormant stage.
“You may think any wormer will do for the treatment of the dangerous encysted small redworm but using one that does not specifically treat the encysted stages during late autumn or winter can actually increase the risk of a horse with a high encysted small redworm burden developing larval cyathostominosis, which can be fatal,” said Zoetis vet Wendy Talbot.
All horses aged over six months should be blood-tested by a vet to check for the presence of encysted small redworm, or wormed with a product licensed to treat this stage of the parasite, regardless of their faecal worm egg count results.
“It’s imperative that the wormer used is indicated for encysted small redworm,” Ms Talbot said.
“This is because 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.”
Moxidectin is the only active ingredient licensed to treat encysted small redworm in a single dose; it acts against the encysted stages before they have a chance to emerge from the gut wall.
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,” Ms Talbot said.
Owners are advised always to seek the advice of a vet or medicines prescriber.
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