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.

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

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.

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.

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.

Parasites within this group, comprising the genera Triodontophorus, Craterostomum 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.