Strongylus edentatus

The nematode Strongylus edentatus 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. edentatus 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. edentatus involves larval migrations in the portal veins from the intestine to the liver, thence beneath the peritoneum to the mesentery and back to the gut via the mesentery.  The prepatent period is one year.

Strongylus edentatus 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, for example the epididymis or in a cryptorchid testis.  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. edentatus.

Strongylus edentatus is not known to be zoonotic.

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

Strongylus edentatus, together with S. vulgaris 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. edentatus measure up to approximately 30 mm (males) and 45 mm (females) in length. They have a prominent buccal capsule with small teeth around its opening, a dorsal gutter, but no 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. edentatus 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.

Strongylus edentatus occurs in horses and other equids throughout the world. The prevalence and intensity of S. edentatus may be decreasing in some areas following decades of the use of highly effective antiparasitic drugs.

Adult S. edentatus 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 the intestinal wall and then migrate in the portal veins to the liver, where they spend several weeks. The larvae then leave the liver, enter the connective tissue beneath the peritoneum, and return to the large intestine via the mesentery. They then break into the intestinal lumen and complete development to adults. The pre-patent period for S. edentatus is approximately ten to twelve months.

Life Cycle: Strongylus edentatus

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.

Neither adult S. edentatus, which "plug feed" on intestinal mucosa, nor migrating larvae, are associated with distinct clinical signs. The larvae migrating beneath the peritoneum and subsequently penetrating the intestinal wall often cause nodular, haemorrhagic lesions which can be several cm in diameter, but these do not appear to be clinically significant. Very rarely, larvae of S. edentatus may wander off course and cause problems, for example in cryptorchid testes.

Diagnosis of the presence of adult S. edentatus is usually based on the detection of eggs in feces using a flotation technique. The 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 cyathostomes.

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

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

Drug(s)	Product(s)
Fenbendazole	PANACUR, SAFE-GUARD
Ivermectin	EQVALAN,  BIMECTIN, EQUELL, PANOMEC
Ivermectin with Praziquantel	EQVALAN GOLD, EQUIMAX
Moxidectin	QUEST
Moxidectin with Praziquantel	QUEST PLUS
Oxibendazole	ANTHELCIDE
Piperazine	VARIOUS
Pyrantel pamoate	STRONGID T, STRONGID P, EXODUS

 

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. edentatus 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.

Strongylus edentatus is not recognized as zoonotic.

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.