Equis ISSN 2398-2977

Parascaris equorum infestation

Synonym(s): Roundworms

Contributor(s): Roberta Baxter, Christopher Brown, Joseph DiPietro


  • Parascaris equorumis the commonest equine roundworm.
  • It is a ubiquitous organism, rarely pathogenic in adults.
  • Cause:Parascaris equorum  Parascaris equorum  .
  • Signs: weight loss, ill thrift and respiratory symptoms to intussusception, intestinal impaction and death.
  • Diagnosis: confirmed by the presence of characteristic worm eggs in feces.
  • Treatment: endoparasiticide treatment.
  • Prognosis: guarded to excellent depending on severity of worm burden.



Predisposing factors

  • Presence of infective eggs on pasture, degree of challenge.
  • Lack of immunocompetence.


  • Adult roundwormParascaris equorum  Parascaris equorum  lives in the small intestine.
  • Larve migrate through liver and lungs as part of life-cycle.
  • Aberrent migration through other tissues can occur.
Small intestine
  • Lesions in the small intestine are rare. Large worm burdens may cause some inflammation. In serious cases abnormal gut motility may result leading to intussusception, which, if not recognized and treated may lead to death due to gut ischemia and toxemia.
  • Very large worm burdens can cause colic and even death due to gut rupture.
  • Less serious effects include weight loss and ill thrift due to decreased food absorption, and possibly chronic low grade colic.

Respiratory system

  • Respiratory symptoms may occur during migration through the lungs. These may be transient and mild (such as intermittent coughing and slight nasal discharge) or may be more severe (with dyspnea, pyrexia and pallor of the mucous membranes).

Liver disease

  • Although lesions can be seen in the liver, liver disease is rare.

Nervous system

  • Aberrent larval migration occurs rarely, causing lesions in the central nervous system. Neurologic symptoms can result.


  • Age-dependent immunity develops at 6-12 months in most foals.
  • The pre-patent period is 3 months.


  • The life cycle ofParascaris equorumis direct, ie there is no intermediate host.
  • Adult roundwormParascaris equorumlives in the small intestine -> millions of eggs are passed daily in feces and are very resistant in the environment -> they develop into infective stage (takes 10-14 days) -> ingested and hatch in the intestine -> larvae migrate through liver, lungs and occasionally other tissues for 2-4 weeks -> return to small intestine and mature.
  • The whole cycle takes approximately 3 months.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • 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.
  • Getachew A M, Innocent G T, Trawford A F et al (2008) Equine parascarosis under the tropical weather conditions of Ethiopia: a coprological and postmortem study. Vet Rec 162 (6), 177-180 PubMed.
  • Morgan E R, Hetzel N, Povah C & Coles G C (2004) Prevalence and diagnosis of parasites of the stomach and small intestine in horses in south-west England. Vet Rec 156 (19), 597-600 PubMed.
  • Proudman C J (1999) The role of parasites in equine colic. Equine Vet Educ 11 (4), 219-224 VetMedResource.
  • Lyons E T et al (1996) Natural superinfection of Parascaris equorum in stall-confined orphan horse foal. Vet Parasitol 66 (1-2), 119-123 PubMed.
  • DiPietro J A et al (1989) Efficacy of ivermectin in the treatment of induced Parascaris equorum infection in pony foals. JAVMA 195 (12), 1712-1714 PubMed.
  • Boraski E A et al (1987)Efficacy of ivermentin against Parascaris equorum. JAVMA 191 (3), 278 PubMed.
  • Vandermyde C R et al (1987) Evaluation of fenbendazole for larvacidal effect in experimentally induced Parascaris equorum infections in pony foals. JAVMA (2), 353-370 PubMed.
  • DiPietro J A et al (1987) Evaluation of ivermectin paste in the treatment of ponies for Parascaris equorum infections. JAVMA 190 (9) 1181-1183 PubMed.

Other sources of information

  • Parasitic pneumonitis in horses. Comp Cont Educ. 20, 378-383 (Diagnosis and management of pneumonitis due to Parascaris and Dictyocaulus).
  • Rose R J and 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.