Large Strongyles of Horses

Pathogenesis of adult worms in the large intestine

The pathogenic effects due to large strongyle infections in horses can be divided into those caused by  adult worms  in the large intestine  and those caused by larvae and immature adults during their extensive migrations in other organs.

The pathogenic effects due to  adults in the cecum and colon of infected horses are similar in all three species of large strongyles because they all provoke the formation of nodules in the gut wall as they pass through in completing their parasitic migrations and their methods of feeding are similar.

The formation of nodules in the gut wall appears to be a mechanism to allow immature adults to migrate from the serosal to the mucosal side without the leakage of gut contents into the peritoneal cavity.  After the worms have escaped  into the lumen of the gut, they leave behind ulcerated nodules  that appear to heal quickly.  It is generally accepted that although these nodules may each be several centimeters in diameter, they are of  of minor clinical consequences in horses with large intestines that measure 12 feet or more in length depending on the size of the horse.


cj030sm.jpg (20463 bytes)

H117sm.jpg (29017 bytes)

Large nodule (N) caused by an immature adult of Strongylus edentatus (arrow) migrating through the wall of the large intestine onto the mucosal surface. Image courtesy of Dr. Harold Drudge and Hoechst Roussel Nodules casued by immature adults of Strongylus vulgaris migrating through the wall of the cecum onto the mucosal surface
Image courtesy of Dr. Harold Drudge and Hoechst Roussel

All three strongylus species have large buccal cavities and are aggressive feeders. They attach to the mucosa of the large intestine by their mouth openings and draw a plug of mucosa into the buccal cavity where it is ground up by teeth, if present,  digested by secreted enzymes then drawn into the intestine by the sucking action of the muscular esophagus. Adult worms then move to a fresh site leaving behind small bleeding ulcers that may be seen as red spots at necropsy. Although these worms do not appear to be actual blood suckers, damage to mucosal blood vessels during feeding may cause significant blood loss particularly if the damage extends to the level of the muscularis mucosa. Pathophysiological studies by Duncan and Dargie have shown that 30ml of blood may be lost per day by foals infected with 75-100 Strongylus vulgaris adults. These ulcers appear to heal readily after the feeding worms move on. However, if the damage is deep into the muscularis, as is often the case with the largest of the three strongyles (Strongylus edentatus), granulation tissue will be formed and healing will result in a circular scar at each feeding site.

cj037sm.jpg (31337 bytes)

H026sm.jpg (25618 bytes)

H023asm.jpg (24349 bytes)

Strongylus vulgaris feeding by drawing a plug of mucosa into its buccal cavity.
Image courtesy of Dr. Harold Drudge
Strongylus edentatus feeding by drawing a plug of mucosa into its buccal cavity.
Image courtesy of Merial Inc
Large strongyles attached to the mucosa. Red dots are sites of previous attachments.
Image courtesy of  Merial Inc



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