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Also known as: Cyathostominosis


Cyathostomins are also known as small strongyles, and are one of the most important gastro-intestinal parasites of the horse. Over 40 species and several genera have been identified. The small, thread-like worms inhabit the caecum and the large colon, with their developmental stages confined to the mucosa of the large intestine. Unlike the large strongyles, they do not undergo any extra-intestinal migration during development.

Both acute and chronic forms of larval cyathostomosis occur in horses. The acute form occurs due to the re-emergence of 'encysted' (hypobiotic) third stage larvae from the intestinal wall, and horses with this type of disease are usually affected during late winter or early spring.

The chronic form occurs when ingested third stage larvae develop to the fourth stage without interruption, leading to inflammation and thickening of the intestinal mucosa. Larval emergence may occur throughout the summer and autumn, leading to the clinical signs associated with a 'wormy' horse. This type of cyathostomosis is more commonly seen in younger horses who have not yet developed full immunity against the parasite.


All horses may be affected but most are less than five years old. Geriatric animals also have an increased susceptibility. Clinical disease commonly occurs in late winter or early spring and usually affects only one or two horses in a herd. A history of an unthrifty or 'poor do-er' young horse, combined with identification of larvae in faeces may be sufficient to arouse clinical suspicion of cyathostomosis.

Clinical Signs

Acute disease:

  • Acute onset, profuse diarrhoea
  • Sudden weight loss
  • Pyrexia

Chronic disease:

  • Poor quality coat
  • Diarrhoea
  • Loss of condition
  • Stunted growth
  • Increased or decreased appetite
  • Recurrent colic

Other clinical features of the disease include ventral oedema due to a protein-losing enteropathy. Adult and larval worms may cause damage to the intestinal mucosa through plug feeding, resulting in ulcer formation.


The clinical features of cyathostomosis are relatively non-specific and confirming a diagnosis may be difficult. Faecal egg counts may not be useful as large numbers of eggs in the faeces do not always indicate a pathological worm burden. Affected horses may show the following abnormalities on haematology and biochemistry:

  • Hyperglobulinaemia (especially IgG)
  • Anaemia (often microcytic)
  • Neutrophilia
  • Hypoalbuminaemia
  • Increased ALP
  • Hypoproteinaemia
  • Metabolic acidosis (in horses with severe diarrhoea)

Gross examination of faeces may reveal large numbers of small, thread like larvae. Larvae may also be found on the plastic sleeve following rectal examination.

A definitive diagnosis of cyathostomosis may be made following identification of larvae in the gastrointestinal tract; either in the faeces, from an intestinal biopsy via laparotomy, or at post mortem. Biopsy of the intestine may occasionally reveal a massive eosinophilic infiltration of the mucosa.


Post mortem examination of the large intestine may reveal larvae in the mucosa, seen as small grey to red mucosal nodules (the characteristic 'pepper-pot' lesion). Mass emergence results in mucosal erosions, ulcers and fibrinous exudate. A catarrhal and haemorrhagic colitis and typhlitis may be seen, with hyperaemia or necrosis of the tissue. In chronic cases, thickening of the intestinal wall may be present. Adult worms (approximately 1.5cm in length) may be seen within the intestinal lumen.


Treatment is indicated for horses with faecal egg counts greater than 200 eggs per gram. Cyathostomins are widely resistant to anthelmintics, particularly benzimidazoles. Moxidectin PO is effective against these resistant strains and also against encysted larvae. Five daily doses of Fenbendazole followed by a single dose of Ivermectin may be administered as an alternative. Refractory cases may benefit from concurrent corticosteroid therapy with prednisolone.


Approximately 40% of cases with severe signs of cyathostomosis recover over a period of 2 or 3 months following treatment with prednisolone and anthelmintics. The prognosis is poorer if weight loss and/or diarrhoea have been present for more than three weeks prior to initiation of treatment.


Good grazing management such as regular manure removal, harrowing, pasture rotation and grazing horses with ruminants all aid in reducing cyathostomin pasture contamination. Anthelmintic programs should only be used as an adjunct to good hygiene and management practices.

Cyathostomosis Learning Resources
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Cyathostominosis publications


  • Fox, M and Jacobs, D. (2007) Parasitology Study Guide Part 2: Helminths Royal Veterinary College
  • White, N.A., Edwards, G.B. (1999) Handbook of Equine Colic Reed Educational and Professional Publishing Ltd
  • Knottenbelt, D.C. A Handbook of Equine Medicine for Final Year Students University of Liverpool
  • Edward Robinson, N and Sprayberry, K. A. (2009) Current Therapy In Equine Medicine Sixth edition Saunders Elsevier
  • Merck & Co (2008) The Merck Veterinary Manual (Eighth Edition) Merial
  • Giles, C.J., Urquhart, K.A. and Longstaffe, J.A. (1985) Larval cyathostomiasis (immature trichonema-induced enteropathy): A report of 15 clinical cases Equine Veterinary Journal 17, 196-201
  • Murphy, D., Keane, M.P. and Goulding, R. (1997) Cyathostome associated disease in the horse: investigation and management of four cases Equine Veterinary Journal 9 247-252

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