Parascaris equorum infestation in Horses (Equis) | Vetlexicon
equis - Articles

Parascaris equorum infestation

ISSN 2398-2977

Synonym(s): Roundworms


  • 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.

Presenting signs

  • Respiratory symptoms such as nasal discharge, dyspnea, pallor of the mucous membranes.
  • Pyrexia.
  • Severe abdominal signs such as severe colic   Abdomen: pain - adult  due to intussusception or intestinal blockage.
  • Liver disease.
  • Neurologic symptoms.

Acute presentation

  • Rare - colic or even sudden death with gut compromise and possibly rupture.

Age predisposition

  • Young animals - foals from 3 months of age.
  • Up to 2 years of age.
  • Very rare in adults, may occur if immunocompromised.

Cost considerations

  • Colic surgery may be required.
  • Can be treated and prevented with routine anti-helmintic treatment.



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.


Presenting problems

Client history

  • Young horse (over 3 months, under 18 months of age).
  • Weight loss/ill thrift.
  • Colic-spasmodic/recurrent.
  • Diarrhea.
  • Intermittent mild respiratory disease.
  • Severe colic.
  • Neurologic disease.

Clinical signs

  • Non-specific clinical signs such as weight loss   Weight loss: severe  /poor weight gain.
  • Diarrhea.
  • Colic - spasmodic - intermittent bouts with mild increase in pulse/respiratory rate; generally concurrent with diarrhea.
  • Pallor of the mucous membranes   Conjunctiva: pallor 01  .
  • Respiratory symptoms - typical greyish-white nasal discharge, increased respiratory sounds, also possibly coughing, pyrexia.
  • Severe colic - signs including tachycardia, tachypnoea, pyrexia, circulatory disturbances, decreased borborygmi, collapse, toxemia, death.
  • Neurologic abnormalities.
  • Liver disease - obstructive jaundice.

Diagnostic investigation

  • Fecal analysis   Feces: parasitology  .
  • Microscopic examination of feces for characteristic brown thick-shelled eggs; passed intermittently.
Fluid/aspirate analysis


  • Examination of abdomen for presence of an intussusception.


Electrolyte assays   Blood: biochemistry - potassium    Blood: biochemistry - sodium    Blood: biochemistry - calcium  can be useful to detect imbalances.

Confirmation of diagnosis

Discriminatory diagnostic features

  • History.
  • Clinical signs.
  • Fecal examination   Feces: parasitology  .
  • Bacteriology.
  • Hematology.
  • Biochemistry.
  • Electrolyte assay.
  • Ultrasonography.
  • Peritoneal fluid evaluation.

Definitive diagnostic features

  • Fecal analysis.

Gross autopsy findings

  • Large numbers of large (15-50 cm) roundworms in small intestine, possibly associated with intussusception or gut rupture.
  • Areas of hemorrhage and greyish-green nodules in lungs.
  • White spots in liver.

Histopathology findings

  • Widespread petechial and echymotic hemorrhages.
  • Sub-pleural grayish-green nodules approximately 2 mm in diameter, containing larvae.


  • Focal areas of hemorrhage and fibrosis.
  • Lymphocytic infiltrate.

Small intestine

  • Inflammation.

Differential diagnosis

Weight loss



  • Other causes of diarrhea.

Respiratory disease

Neurologic disease



Standard treatment

  • Supportive treatment.
  • Antibiotics   Therapeutics: antimicrobials  if needed to treat secondary infection.
  • Anti-parasiticides - ivermectin 0.2 mg/kg PO as a single dose   Ivermectin  or fenbendazole 10 mg/kg PO daily for 5 days   Fenbendazole  .

Effective against migrating larvae.

  • Pyrantel pamoate   Pyrantel  6.6 mg/kg for adults and intestinal larvae.

Not effective against migrating larvae.

  • 2.64 mg/kg daily is effective in preventing migration.
  • Levamisole   Levamisole  10 mg/kg.

Narrow therapeutic index in horses. Rapid onset of action means that it can cause ascarid impaction in foals.


  • Clinical response.
  • Parasitology - worm egg count.

Subsequent management


  • Initiate appropriate regular deworming regime.
  • In particular worm foals every 4-6 weeks.



  • Initiate appropriate regular deworming regime   Therapeutics: parasiticides  .
  • Disinfect foaling boxes well before use.
  • Worm mares regularly every 4-6 weeks.
  • Worm foals at 8 weeks of age and every 4-6 weeks.
  • Mare and foal turnout should be on clean pasture.


Group eradication

  • Apply deworming regime to all horses kept together.



  • Excellent - provided worm burden not severe.
  • Guarded - if severe worm burden due to possibilities of gut rupture post-treatment due to simultaneous death and passage of large numbers of worms.
  • Guarded if neurologic or hepatic disease.

Expected response to treatment

  • Signs of colic should abate with 24 hours of treatment.
  • Diarrhea, weight loss, and respiratory symptoms may take several weeks to improve.

Reasons for treatment failure

  • Immunocompromise.
  • Severity of disease at time of diagnosis.

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.