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Strongyle infestation: large


Synonym(s): Redworms Cyathostome


  • This is the most potentially pathogenic endoparasitic disease of horses.
  • Cause: a variety of species of equine redworms, divisible into large and small strongyles.
  • Signs: ill thrift, diarrhea, colic, and can be serious or even fatal.
  • Disease is seen in animals of all ages but may be particularly severe in young animals.
  • Diagnosis: clinical signs and presence of strongyle eggs in feces. See also:
  • Treatment: endoparasiticides are used, and good pasture management is also necessary for control.
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Large strongyles

  • Strongylus vulgaris- common.
  • Strongylus edentatus- common.
  • Strongylus equinus- rare.

Small strongyles

Predisposing factors

  • Overstocking.
  • Poor pasture management.


  • Adult worms in large intestine feed by permanent attachment and can cause loss of protein and blood, as well as changes in gut motility.
  • Large strongyle larve migrate through the body potentially causing damage to gut walls, arteries and other tissues, eg liver, brain.
  • Small strongyle larve become encysted in the gut wall causing damage and inflammation, particularly when they emerge in large numbers from the cysts.
Large strongyles
  • Adults in large intestine consume protein and cause bowel wall damage, anemia and hypoproteinemia, and can cause a direct effect on bowel motility by release of certain chemicals.
  • Eggs are passed in feces and develop on pasture to infective larve, particularly in autumn and spring, and during summer months. A percentage of infective larve can survive the winter on pasture.
  • Horses ingest infective larve; these migrate from the gut through the submucosal arterioles and into the arterial system. In the case ofStrongylus vulgaristhey migrate via the cecal, colic, and cranial mesenteric arteries. This migration causes inflammation within the blood vessels which can result in vascular occlusions and aneurysms, and thus inflammation, ischemia and infarction, particularly of the distal small colon, cecum and ventral colon. This can cause colic due to loss of blood supply to portions of bowel, and blood loss through damaged arteries may occur.Strongylus edentatusmigrate via the portal vein, liver and peritoneum. Hepatitis and peritonitis can also result in the case ofS. edentatus.
  • Larve then return to the gut where they develop to adults.

Small strongyles

  • Adults live in the large intestine, cause inflammation and damage and pass eggs in the horse's feces.
  • These develop into infective larve on pasture.
  • The horse ingests infective larve, and they migrate from the gut lumen into the gut wall where they become encysted, causing gut wall damage via damage to the lamina of the tubule glands and inflammation and goblet cell hypertrophy. They can overwinter in this state.
  • Emergence of large numbers of larve from the gut wall in spring leads to further inflammation (eosinophilic) of the gut wall and often diarrhea.


  • Prepatent period ofS. vulgarisis 6 months; that ofS. edentatusis 11 months and that ofS. equinusis 9 months.
  • Prepatent period of small strongyles is 6-12 weeks.


  • Strongyles have direct life cycles - no intermediate host is needed for transmission.
  • Overwintering of infective larve can occur to a limited extent on pasture, larve can survive up to 2 years. Small strongyles can overwinter as encysted larve.
  • Emergence of large numbers of infective larve in spring can lead to severe disease.
  • Infection levels vary with location and season. Highest numbers in Spring and Autumn. Also high risk during Summer (Northern US) and Winter (Southern US).


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Kaplan R M, West E M, Norat-Collazo L M & Vargas J (2014) A combination treatment strategy using pyrantel pamoate and oxibendazole demonstrates additive effects for controlling equine cysthostomins. Equine Vet Educ 26 (9), 485-491 WileyBlackwell
  • Conwell R (2009) A case of larval cysthostominosis. UK Vet 14 (4), 4-8 WileyBlackwell.
  • Lloyd S (2009) Effects of previous control programmes on the proportion of horses shedding small numbers of strongyle-type eggs. Vet Rec 164 (4), 108-111 PubMed.
  • Schumacher J, Livesey L, DeGraves F et al (2009) Efficacy of moxidectin against cyathostomins after long-term use in a large herd of draught horses with a high stocking density. Vet Rec 164 (21), 652-654 PubMed.
  • Molento M B, Antunes J, Bentes R N & Coles G C (2008) Anthelmintic resistant nematodes in Brazilian horses. Vet Rec 162 (12), 384-385 PubMed.
  • Roetting A K, Freeman D E, Constable P D et al (2008) The effects of Strongylus vulgaris parasitism on eosinophil distribution and accumulation in equine large intestinal mucosa. Equine Vet J 40 (4), 379-384 PubMed.
  • Bairden K, Davies H S, Gibson N R et al (2006) Efficacy of moxidectin 2 per cent oral gel against cyathostomins, particularly third-stage inhibited larvae, in horses. Vet Rec 158 (22), 766-768 PubMed.
  • Kaplan R M & Little S E (2000) Controlling equine cyathostomes. Comp Cont Educ Pract Vet 22 (4), 391-395 VetMedResource.
  • Fernandez S et al (1999) A new isolate of the neamtophagous fungus Duddingtonia flagransas biological control agent against free-living larvae of horse strongyles. Equine Vet J 31 (6), 488-491 PubMed.
  • Lind O E, Hoglund J, Liungstrom B L, Nilsonn O & Uggla A (1999) A field study on the distribution of strongyle infections of horses in Sweden and factors affecting fecal egg counts. Equine Vet J 31 (1), 68-72 PubMed.
  • Proudman C J (1999) The role of parasites in equine colic. Equine Vet Educ 11 (4), 219-224 VetMedResource.
  • Craven J, Bjorn, Henriksen S A et al (1998) Survey of anthelmintic resistance on Danish horse farms, using 5 different methods of calculating fecal egg count reduction. EVJ 30 (4), 289-293 PubMed.
  • Duncan J L, Bairden K & Abbott E M (1998) Elimination of mucosal cyathostome larvae by five daily treatments with fenbendazole. Vet Rec 142 (11), 268-271 PubMed.
  • Thamsborg S M, Leifsson P S, Groendahl C, Larsen M & Nansen P (1998) Impact of mixed strongyle infections in foals after one month on pasture. Equine Vet J 30 (3), 240-245 PubMed.
  • Murfy D & Love S (1997) The pathogenic effects of experimental cyathostome infections in ponies. Vet Parasitol 70 (1-3), 99-110 PubMed.
  • Herd R P & Coles G C (1995) Slowing the spread of anthelmintic resistant nematodes of horses in the United Kingdom. Vet Rec 136 (19), 481-485 PubMed.
  • Van Loon G, Deprez P, Muylle E & Sustronck B (1995) Larval cyathostomiasis as a cause of death in two regularly dewormed horses. Zentralbl Veterinarmed A 42 (5), 301-306 PubMed.
  • Herd R P (1986) Epidemiology and control of equine strongylosis at Newmarket. Equine Vet J 18 (6), 447-452 PubMed.

Other sources of information

  • Rose R J & Hodgson D R (1993) Manual of Equine Practice. Saunders. ISBN 0 7216 3739 6.
  • Urqhart G M, Armour J, Duncan J L et al (1988) Veterinary Parasitology. Longmann Scientific and Technical. ISBN 0 5824 0906 3.

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