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Encysted small redworm and the dangers to your horse.

Encysted stages of small redworm are one of the most dangerous parasite threats to horses, yet almost 1 in 5 horse owners have never even heard of them[i]. The parasite is picked up while the horse grazes and will then burrow into the gut wall to form a cyst (encysted stage). This can lead to potentially serious problems such as diarrhoea and if the burden is large enough even fatal colic.

Encysted stages of small redworm are one of the most dangerous parasite threats to horses, yet almost 1 in 5 horse owners have never even heard of them[i]. The parasite is picked up while the horse grazes and will then burrow into the gut wall to form a cyst (encysted stage). This can lead to potentially serious problems such as diarrhoea and if the burden is large enough even fatal colic.

Controlling small redworm during spring, summer and early autumn is therefore essential to help reduce the risk of the formation of the encysted stages. The best way to do this is to carry out regular faecal worm egg counts. These tests use a sample of the horse faeces and identify eggs produced by the adult female small redworm as well as other worm types. Although this test will not show the encysted stages of small redworm, as no eggs are produced whilst the parasite is encysted, by controlling the adult population during the spring, summer and early autumn you will reduce the number which will encyst once the weather becomes colder.

Once the small redworm have formed a cyst, they are effectively in hibernation, and as such will not show in a faecal worm egg count as they are not producing eggs. The only way to test for their presence once encysted is with a specific blood test[ii] carried out by your veterinary surgeon. It is therefore much simpler to control the small redworm population before they begin to encyst.

If the small redworm population goes uncontrolled and a large number are able to encyst in the gut wall this can lead to potentially fatal consequences. Once the weather begins to warm up again, any small redworm which have encysted, will begin to develop and emerge into the gut lumen as adults. Mass emergence can cause severe damage to the gut resulting in colic, diarrhoea and sometimes death. Statistics from the Veterinary Medicines Directorate show that almost 100% of the cases reported to them had resulted in the death of the animal.

According to the National Equine Health Survey 23% of owners did not intend to treat for this potentially fatal threat. The reasons given included low egg count results and their horses healthy appearance. Unfortunately, a healthy appearance can be deceptive, and the potential for a worm egg count to mislead by not showing encysted redworm burden could lead to serious problems. Every horse over 6months should be treated for encysted stages of small redworm in November/December using an effective wormer treatment or have a blood test carried out in order to prevent an undetected build up or larvae even if the faecal worm egg count is negative.

Advice should be sought from your Vet, Pharmacist or SQP as to the most appropriate treatment for your horse. In terms of licensed treatments there are horse wormers for encysted small redworm; Moxidectin horse wormers will treat the encysted stage whilst still encysted in the gut wall. Fenbendazole given over a five day period at the specified dose rate is also licensed to treat encysted stages. However, as there is widespread resistance now to Fenbendazole horse wormers it is advisable to use this only after faecal egg reduction tests have shown that it remains effective.

Recent research has suggested that the treatment immediately before and during the cold weather with wormers except Moxidectin/Fenbendazole may actually increase the risk of serious illness or even death from larval cyathostomiasis (small redworm disease).

A targeted treatment with a Moxidectin or (if suitable) Fenbendazole based wormer (or blood test) during the colder months is essential to avoid a potentially fatal situation from developing and Ivermectin horse wormers/Pyrantel horse wormers products should be avoided at this time. Please find a full list of horse wormers here.


[i] Data from the National Equine Health Survey 2016, supplied by the Blue Cross.

[ii] Matthews JB. An update on cyathostomins: Anthelminticresistance and worm control. Equine Vet Edu 2008; 20(10): 552–560.

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