Equis ISSN 2398-2977

Parascaris equorum

Contributor(s): Joseph DiPietro, Maggie Fisher, Sheelagh Lloyd




  • Phylum: Nematoda.
  • Order: Ascarididae.
  • Superfamily: Ascaridoidea.
  • Genus:Parascaris.
  • Species:equorum.

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Clinical Effects



  • Eggs passed into feces.
  • Second stage larvae develops within egg.
  • Embryonated eggs ingested.
  • L3 and L4 larvae carry out hepatotracheobronchial migration.
  • Adult worms in small intestine.


  • Sticky surface of eggs allows them to adhere to the udder and teats of mares so infect suckling foals from the first few days of life.
  • Most important factor isfoal to foal transmission from year to year.
  • Pastures are contaminated with eggs that develop in the summer months and overwinter.
  • Infection occurs by ingestion of embryonated eggs.
  • No evidence for prenatal infection.

Pathological effects

  • Hosts become sensitized to antigens of migrating larvae - eosinophils and other inflammatory cells infiltrate the areas of migrating larvae.
  • Marked increase in circulating eosinophils with heavy infection.
  • Migrating larvae associated with fibrous tracts in liver.
  • Trapping of larvae may occur especially after a heavy infection, associated with lymphocytic nodules.
  • Presence in tracheobronchial tree gives rise to frothy mucus and nasal discharge.
  • Adult worms in the small intestine cause ill thrift and reduced weight gain.
  • Large numbers may obstruct the intestine.
  • Parascarishas been associated with verminous pancreatitis (incidental finding at post mortem).
  • Parascarismigration has ben associated with verminous pneumonia.

Other Host Effects

  • Following ingestion of eggs larvae (L2) hatch in gastrointestinal tract.
  • Larvae burrow through the small intestine and are carried to the liver via caudal vena cava then carried via the blood stream to the lungs, where a further moult occurs in the alveoli.
  • Larvae carried out of the lungs in the mucus of the tracheobronchial tree, are coughed up and swallowed.
  • Mechanical damage can be done to liver and lung tissue by migrating larvae.


Control via chemotherapies

  • Anthelmintics:
    • Moxidectin: 400 µg/kg effective against adult worms.
    • Ivermectin   Ivermectin  : 200 µg/kg - effective against adult and immature worms.
    • Fenbendazole   Fenbendazole  : 7.5 mg/kg - effective against adult and immature worms.
    • Pyrantel tartrate: 6.6 mg/kg daily   Pyrantel  - effective against adult worms and immature worms.
    • Oxibendazole   Oxibendazole  : 10 mg/kg - effective against adult worms.
    • Mebendazole   Mebendazole  : 5-10 mg/kg - effective against adult worms.

    Treatment should be given at 2, 4 and possibly 6 months.

Control via environment

  • Reduction of prevalence in foals will reduce egg output, hence level of environmental infection for foals born in subsequent years.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Geurden T et al (2013) Determination of anthelmintic efficacy against equine cyathostomins and Parascaris equorum in France. Equine Vet Educ 25 (6), 304-307.
  • 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.
  • Proudman C J (1999) The role of parasites in equine colic. Equine Vet Educ 11 (4), 219-224.
  • 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.
  • Ihler C F et al (1995) The distribution of Parascaris equorum eggs in the soil profile of bare paddocks in some Norwegian studs. Vet Res Commun 19 (6), 495-501 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 ivermectin against Parascaris equorum. JAVMA 191 (3), 278 PubMed.
  • Vandermyde C R et al (1987) Evaluation of fenbendazole for larvaecidal effect in experimentally induced Parascaris equorum infections in pony foals. JAVMA (2), 353-370.
  • 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.