Parascaris equorum

The ascarid nematode Parascaris equorum occurs in horses around the world, including Canada.


The ascarid nematode Parascaris equorum occurs in horses around the world, including Canada.  In general its prevalence and abundance are greater in young (less than one year old) and in aged animals than in other age groups.  Adult P. equorum live in the small intestine.  The life cycle is direct and typical of ascarids.  Eggs are passed in the faeces and the infective stage for horses is the egg containing a second-stage larva.  Parasitic development of P. equorum involves an tracheal migration by the larvae   The pre-patent period is approximately 10 weeks.

Many horses infected with P. equorum never show any clinical signs but heavy infections in young animals can significantly interfere with growth and development and in older animals can result in loss of condition and energy and sometimes gastro-intestinal signs.  Rarely large numbers of adult parasites will obstruct the intestine (this can happen after treatment with an antiparasitic drug), and single adults can obstruct the bile duct or  pancreatic duct.  Horses of any age that are immunosuppressed and kept in sub-optimal environments are particularly susceptible to the development of heavy P. equorum infections.

Although there are several drugs that are effective against the life cycle stages of P. equorum in the gut lumen, treatment and control can be problematic.  First, the adult females can produce very large numbers of eggs and the eggs are sticky and very resistant to adverse environmental conditions.  Second, ivermectin resistance has been detected in P. equorum in Europe, the United States and eastern Canada, resistance to moxidectin has been demonstrated in Canada and elsewhere, and resistance to the macrocyclic lactones and pyrantel in the southern United States..

Parascaris equorum is not known to be zoonotic.


Phylum: Nematoda
Order: Ascaridida
Superfamily: Ascaridoidea
Family: Ascarididae

Among parasites of veterinary importance related to P. equorum are Ascaris suum of pigs, and Toxocara species and Toxascaris leonina of dogs, cats and other carnivores. Another close relative is the human ascarid, Ascaris lumbricoides. All these ascarids live as adults in the small intestines of their hosts and all have similar life cycles.

Note: Our understanding of the taxonomy of helminth, arthropod, and particularly protozoan parasites is constantly evolving. The taxonomy described in wcvmlearnaboutparasites is based on that in the seventh edition of Foundations of Parasitology by Larry S Roberts and John Janovy Jr., McGraw Hill Higher Education, Boston, 2005.


Adult P. equorum are very large and stout, measuring up to approximately 28 cm (males) and 50 cm (females) in length. Three prominent lips are obvious microscopically at the anterior end, but other structures are not usually visible.

 Eggs of P. equorum measure approximately 100 µm in diameter, and when freshly passed are yellow-brown and have a thick, rough outer shell layer and contain one or two cells. Sometimes during fecal flotation the outer rough layer is lost, and the eggs appear smooth.

Host range and geographic distribution

Parascaris equorum occurs in horses and other equids throughout the world. It is common in horses in Canada, particularly in the young and the aged.

Life cycle - direct

Adult P. equorum live in the small intestine, and eggs pass in the feces. A second-stage infective larva develops within each egg; in ideal environmental conditions, this takes approximately three weeks. Infection of the horse is by ingestion of the larvated eggs. The larvae that hatch migrate into the intestinal wall, travel in the portal veins to liver, thence in the vasculature to the heart and then the lungs. Here the larvae break out from the branches of the pulmonary artery into the airways, are coughed up and swallowed. This is a tracheal migration. The pre-patent period is approximately10 weeks.

Life Cycle: Parascaris equorum


The prevalence and intensity of P. equorum in horses is heaviest in young and in aged horses, probably because of the development, and subsequent waning, of a protective immunity. Parasite transmission among horses is enhanced by the high fecundity of the adult females, by the stickiness of the eggs, and by the ability of the eggs to remain viable for long periods (at least months), even in very unfavourable environmental conditions.

Pathology and clinical signs

Many horses with light infections with P. equorum show no obvious adverse effects, but heavier burdens may significantly depress growth and development in foals and other young horses, and cause non-specific clinical signs in older animals, for example diarrhea, flatulence and malaise. Occasionally, larvae migrating through the lungs may result in a nasal discharge. Rarely, large numbers of adults may cause intestinal obstruction and/or perforation, sometimes immediately following treatment.


Eggs of P. equorum are easily detected using a fecal flotation technique. The eggs are easily distinguishable from those of other enteric parasites of horses.

Treatment and control

There are several products approved in Canada for the treatment of Parascaris equorum in horses. Some are also effective for the migrating larvae. There is recent evidence from Europe and from North America and elsewhere that some P. equorum have developed resistance to ivermectin and/or moxidectin.  In addition, P. equorum resistant to macrocyclic lactones and pyrantel has recently been discovered in the southern United States.







Ivermectin with Praziquantel




Moxidectin with Praziquantel






Pyrantel pamoate



Additional information is on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

Successful control of P. equorum depends on treatments to remove the adult (and larval) parasites, and thus the source of eggs, and to create an environment for the horses that minimizes their exposure to infective eggs.

Public health significance

Parascaris equorum is not recognized as a zoonosis, but given the often low host specificity of ascarid nematodes, rare human infections may occur.


Reinemeyer CR (2012) Anthelmintic resistance in non-strongylid parasites of horses. Veterinary Parasitology 185: 9-15.

Reinemeyer C (2009) Diagnosis and control of anthelmintic resistant Parascaris equorumParasites and Vectors 2 (Supplement 2) S8.

Slocombe JOD et al. (2007) Macrocyclic lactone-resistant Parascaris equorum on stud farms in Canada, and effectiveness of fenbendazole and pyrantel pamoate. Veterinary Parasitology 145: 371-376.