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

Parascaris equorum

ISSN 2398-2977

Contributor(s) :




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


  • Worldwide - common in foals (and yearlings).


  • Large numbers of migrating larve may cause clinical liver damage   Parascaris equorum infestation  .
  • Large numbers of adult worms can cause stunting, reduced weight gain, poor coat condition, pendulous abdomen, diarrhea/constipation in foals and may even block the small intestine if present in sufficient numbers.

Active Forms

Active Form 1


  • Large nematodes.
  • Male 15-28 cm.
  • Female up to 50 cm.
  • Mouth surrounded by 3 fleshy lips.
  • Tail of the male curved ventrally, with small caudal ale.


  • White.


  • In vivoonly.
  • Inhabits the small intestine.


  • Grow rapidly once in the intestine at 2.5 weeks (17 days) post-infection (the worms measure 2 mm at this stage).
  • Pre-patent period after infection is approximately 80 days.
  • Eggs laid in host intestine - passed out onto pasture.
  • Like other ascarids, femaleParascarisare fecund.
  • One foal can produce thousands of eggs per day.
  • Often expulsion of adult worms occurs when foals are approximately 6 months of age.

Active Form 2


  • L2 hatch from egg once ingested.
  • Small, ascarid larvae.


  • In vivoonly.


  • 2 mm by time return to intestine.
  • Grow rapidly after that and at maturity can be 15-20 cm long.

Resting Forms

Resting Form 1


  • Almost spherical - 100 x 90 mm.
  • Thick wall, typical of ascarids.
  • Rough outer coat ensures adherence in environment.
  • Once embryonated, contains single L1 then L2 larvae.


  • Brownish.


  • Will survive for several years in the environment at a wide range of environmental temperatures.
  • Tolerant of range of humidities.


  • Larvae develop in about 10 days at 27°C.
  • May take weeks or months at lower temperatures.


  • Will survive for several years.

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.


Useful samples

Specimen storage

  • Refrigerate at 4°C.
  • If this is not possible, fecal sample can still be examined after a number of days as the eggs will remain.

Transport of samples

  • Sealed container.

Field diagnosis

  • Clinical appearance of foal or young horse may aid diagnosis.
  • Large, white worms that are circular in cross-section and that do not taper at one end may be seen in feces - particularly of foals when a few months old - these will beParascaris.

Laboratory diagnosis

  • Fecal flotation and observation of eggs in feces.
  • Worms may be identified on the basis of size and morphology.
  • Disease associated with migrating larvae may require liver and/or lung workup.

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.