Large Strongyles of Horses

Species and morphology

The large strongyles are members of the subfamily strongylinae which includes two genera in domestic horses and donkeys, the genus Strongylus and the genus Triodontophorus.

Strongylus species  are commonly found throughout the world and they are of particular importance in horses in North America. The predilection site for all species is the cecum and colon and because of their size they are readily visible at necropsy as seen in the image on the right. The three species are  Strongylus vulgaris, Strongylus edentatus and Strongylus equinus. All three species are important pathogens of horses. Adult worms are destructive feeders on the mucosa of  the cecum and colon and  parasitic larvae undergo extensive migrations in their equine hosts causing significant damage to organs during their migratory routes. They are especially pathogenic in susceptible foals and yearlings.

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Image courtesy of Merial Inc

Mature adults of these species can usually be identified by their relative sizes but more precise identification, especially  immature adults and L4s,  is based on microscopic features such as the shape of the buccal cavity and the presence or absence of teeth. These features of strongylus species are illustrated in the table below.

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Strongylus vulgaris.
The buccal cavity contains 2 ear-shaped teeth. Mature adult worms are about 2cm long.
Strongylus edentatus.
The buccal cavity is devoid of teeth. Mature adult worms are 4-5cm long.
Strongylus equinus
The buccal cavity contains 3 teeth - 1 large and 2 small. Mature adult worms are 5cm long.

Several Triodontophorus species are found in horses but they appear to be less pathogenic than Strongylus because no migrations occur in the host although the buccal cavities of adults contain teeth so damage does occur in the large intestine due to their feeding activities. Triodontophorus appears to be the more common species and it is usually seen at necropsy gathered in clusters around ulcers in the colon.

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Triodontophorus adults feeding at an ulcer in the colon of a horse. Image courtesy of Dr. Harold Drudge Head end of Triodontophorus showing its large buccal cavity with teeth at the base and fringed by a prominent leaf crown around the opening.












Parasites and Parasitic Diseases of Domestic Animals
Dr. Colin Johnstone (principal author)
Copyright 1998 University of Pennsylvania
This page was last modified on January 24, 2000