Strongylus equinus

The nematode Strongylus equinus occurs in horses around the world, including Canada.


The nematode Strongylus equinus occurs in horses around the world, including Canada.  Possibly it is not as prevalent or abundant as it once was, in part because of the widespread use of the very effective macrocyclic lactone anti-parasitic drugs (ivermectin and moxidectin).  Adult S. equinus live in the large intestine.  The life cycle is direct.  Eggs are passed in the faeces and the first-stage larva that hatches from each egg moults twice to the third, infective stage.  Infection of horses is by ingestion of these larvae.  Parasitic development by S. equinus involves larval migrations across the peritoneal cavity from the intestine to the liver, thence in the hepatic ligament to the pancreas, and finally to the caecum.  The prepatent period is eight to nine months.

Strongylus equinus rarely causes clinical signs in horses, and when significant disease and/or lesions occur they are usually associated with the larval parasites in unusual locations.  In many cases, however, these aberrant larvae are simply incidental findings during surgery or post-mortem.  Anthelmintic resistance does not seem to be a significant issue for S. equinus.

Strongylus equinus is not known to be zoonotic.


Phylum: Nematoda
Class: Rhabditia
Order: Strongylida
Superfamily: Strongyloidea
Family: Strongylidae

Strongylus equinus, together with S. vulgaris and S. edentatus, are grouped as the migratory large strongyles of horses on the basis of the relative size of the adults and the extensive migrations of the developing larvae in the horse. The other groups of GI nematodes of horses are the large, non-migratory strongyles, and the small strongyles, also known as the trichonemes, cyathostomes, or more usually now, the cyathostomins.

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 S. equinus measure up to approximately 35 mm (males) and 45 mm (females) in length. They have a prominent buccal capsule with small teeth around its opening, a dorsal gutter, and a large tooth with a bifid tip and two smaller teeth at its base. The copulatory bursa of the male is also prominent, and some elements of the alimentary and reproductive systems are usually obvious microscopically.

Eggs of S. equinus are typical "strongyle" eggs, oval, with a thin, smooth shell and measure approximately 90 µm by 50 µm. In fresh feces each egg contains a small clump of cells (a "morula"). Eggs of S. edentatus cannot be differentiated microscopically from those of the other species of Strongylus, or from those of the non-migratory large strongyles, or of the cyathostomins.

Host range and geographic distribution

Strongylus equinus occurs in horses and other equids throughout the world. In Canada, S. equinus is probably the least common of the three species of Strongylus. The prevalence and intensity of S. equinus may be decreasing in some areas following decades of the use of highly effective antiparasitic drugs.

Life cycle - direct

Adult S. equinus live in the large intestine, and eggs pass in the feces. In the environment, a first-stage larvae develops in each egg, which then hatches. Over as little as a few days these larvae grow and develop to the infective third stage.

Infection of the horse is by ingestion of infective larvae. In the horse, the larvae penetrate into intestinal wall and travel across the peritoneal cavity and enter the liver, where they spend several weeks. The larvae then leave the liver and migrate across the pancreas to the caecum, from where they break into gut lumen and complete development to adults. The pre-patent period for S. equinus is approximately eight to nine months.

Life Cycle: Strongylus equinus


Other than the longer pre-patent periods of Strongylus spp. and the non-migratory large strongyles, their basic epidemiology is is in some ways similar to that of the cyathostomins.  Horses need access to pasture for significant parasite transmission to occur, and the longer pre-patent periods means that Strongylus spp. and the non-migratory large strongyles have essentially annual cycles (ingestion of infective larvae one year produces adults the next year), whereas adult cyathostomins could develop the same year and begin to contaminate the pastures with eggs towards the end of summer, depending on the local climate and its effects on the temperature-dependent egg development rates.  Under ideal conditions, the eggs of all these strongyles can develop to infective third-stage larvae in approximately one week. Where there is very little or no over-winter survival of the free-living stages of the parasites on pastures the major source of these parasites for grazing foals is the mares and other older and adult horses with which they are pastured. Where there is over-winter survival, the pastures could act as a source of infection.

Pathology and clinical signs

Neither adult S. equinus, which "plug feed" on intestinal mucosa, nor migrating larvae, are associated with distinct clinical signs, although making a specific diagnosis of disease associated with the larval migrations would be difficult, even at post mortem. Very rarely, larvae of S. equinus may wander off course and cause problems.


Diagnosis of the presence of adult S. equinus is usually based on the detection of eggs in feces using a flotation technique. The eggs of S. equinus cannot be differentiated microscopically from those of the other species of Strongylus, or from those of the large, non-migratory strongyles, or of the cyathostomins.

Diagnosis of the effects of the migrating larvae of S. equinus is not usually required.

Treatment and control

There are several products approved in Canada for the treatment of adult Strongylus equinus. Anthelmintic resistance, a significant issue with the cyathostomes of horses, is not thought to be a problem with Strongylus species.







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.

Control of S. equinus depends on the appropriate use of antiparasitic treatments and minimizing contacts between horses and infective larvae. An new approach for the optimal control of cyathostomins in horses depends on the aggregated distribution of the parasite population in a horse population - most of the parasites are in a few of the hosts.  The significance of this concept in parasitology was first articulated by Harry Crofton in 1971, and has since become one of the key concepts underlying helminth control programs for people in many areas of the world.  In horses the aggregated parasite distribution has led to the exploration of selective treatments of only animals with large parasite burdens - usually assessed by faecal egg counts - to achieve optimal control and to minimize the occurrence and significance of anthelmintic resistance. Whatever approach is used for the design and application of a control program for helminth parasites in horses, prerequisites for success include an understanding of the basic features of the parasites and the drugs, and of the management of the horses that are the subject of the program.

Public health significance

Strongylus equinus is not recognized as zoonotic.

Non-migratory large strongyles

Parasites within this group, comprising the genera TriodontophorusCraterostomum and Oesophagodontus, have a life cycle with free living and parasitic development similar to the cyathostomes, involving only a mucosal (or sub-mucosal) migration. The large non-migratory strongyles are not associated with any particular pathology (it is probably similar to that of the cyathostomes), and for treatment and control are considered along with the other large strongyles.


Kaplan RM et al. (2010) An evidence-based approach to equine parasite control: it ain t the 60s anymore. Equine Veterinary Ed