Strongylus vulgaris

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


The nematode Strongylus vulgaris 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. vulgaris 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. vulgaris involves larval migrations in the cranial mesenteric artery to its root adjacent to the aorta.  Here the larvae continue their development, causing a verminous arteritis, until they return to the gut in the artery and establish as adults in the gut lumen.  During the upward migration in the artery, the larvae are held within thrombi closely applied to the endothelial surface.  On the return they are in the arterial lumen.  The pre-patent period is approximately six months.  Sometimes the migrating larvae of S. vulgaris enter the aorta and are distributed in a variety of sites, for example the renal arteries, the endocardium and the CNS, sometimes with clinically apparent effects.

Many horses with S. vulgaris never show clinical signs but in some animals the parasite is associated with disturbances in intestinal function, notably colic.  The exact mechanisms by which this parasite causes colic are not fully understood.  Possibilities include compromised blood flow to the intestine, blockage of blood flow - with possible infarction of the affected intestinal wall, and interference with the autonomic ganglia adjacent to the root of the cranial mesenteric artery.   Experimental work suggests, however, that larvae newly arrived in the intestinal wall (prior to arterial migration) might be a significant cause of colic in some horses.  Many horses with arterial lesions caused by S. vulgaris never have colic, and many horses with colic do not have arterial lesions.  Recent opinion supports cyathostomins as a more important cause of equine colic than S. vulgaris.  Anthelmintic resistance does not seem to be a significant issue for S. vulgaris.

Strongylus vulgaris is not known to be zoonotic.


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

Strongylus vulgaris, together with S. edentatus and S. equinus, 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. vulgaris measure up to approximately 16 mm (males) and 24 mm (females) in length. They have a prominent buccal capsule with small teeth around its opening, a dorsal gutter, and two rounded 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. vulgaris 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. vulgaris 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 vulgaris occurs in horses and other equids throughout the world. The prevalence and intensity of S. vulgaris may be decreasing in some areas following decades of use of highly effective antiparasitic drugs.

Life cycle - direct

Adult S. vulgaris 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 the infective larvae. In the horse, the larvae penetrate into intestinal wall and enter small branches of cranial mesenteric artery. The larvae migrate up these branches, applied closely to the endothelium, to the root of the artery adjacent to the aorta, where they are present from approximately 14 days after infection. The larvae spend up to several months in this location, causing a verminous arteritis. Beginning approximately six weeks after infection the larvae, which are now visible to the naked eye, return in the arterial lumen to the gut wall, break into the lumen and complete development to adults. The pre-patent period for S. vulgaris is approximately six months.

Life Cycle: Strongylus vulgaris


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

Adult S. vulgaris are not usually associated with distinct clinical signs, although their "plug-feeding" may cause irritation to the intestinal mucosa. The major problems are associated with larval migrations in the gut wall and its arterial supply . In the past it was believed that S. vulgaris was responsible for many cases of colic, but recent evidence suggests that its importance may have been overestimated. There is no doubt that in some animals the larval migrations can result in interference with blood supply to the gut, sometimes with thrombi detaching from areas of arteritis and causing infarction (death) of areas of the gut wall. However, many horses with severe migration lesions never show colic, and many horses with colic show no evidence of S. vulgaris infection.

There is evidence from experimental infections that larvae of S. vulgaris can cause significant pathology, and clinical signs, early in their migrations – while they are first entering the small branches of the artery in the intestinal wall. Clearly, colic in horses can be caused by a variety of events that are not yet fully understood.

Lesions attributable to S. vulgaris larval migrations can be found on the endothelium of the arterial system in many parts of the body, including the endocardium, but the significance of these lesions is unknown. Rarely, migrating larvae of S. vulgaris go "off course", for example to the brain, where they can be a significant problem.


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

Diagnosis of the effects of the migrating larvae of S. vulgaris is difficult and complex, even at post mortem.

Treatment and control

There are several products approved in Canada for the treatment of adult Strongylus vulgaris, but only three are also approved for treatment of the migrating larvae: only two drugs are effective for migrating larvae of Strongylus vulgaris: ivermectin at normal therapeutic levels; and (extra-label) fenbendazole at high doses. All treatments for larvae require close veterinary supervision. Anthelmintic resistance, a significant issue with the cyathostomins 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. vulgaris 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 (targeted) 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.  It has recently been suggested, however, that these selective treatments - in which many horses are infrequently or never treated, could lead to the re-emergence of other parasites, for example Strongylus vulgaris. 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 vulgaris is not known to be 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 eggs are indistinuishable from those of the cyathostomins and of Strongylus spp.. The large non-migratory strongyles are not associated with any particular pathology, and for treatment and control are considered along with the cyathostomins.



Kaplan RM et al. (2010) An evidence-based approach to equine parasite control: it ain t the 60s anymore. Equine Veterinary Education 22: 306-316.