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
|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
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
The buccal cavity contains 2 ear-shaped teeth. Mature adult worms are about 2cm long.
The buccal cavity is devoid of teeth. Mature adult worms are 4-5cm long.
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.
|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.