Large Strongyles of Horses


Strongylus equinus - life cycle

The preparasitic phase of this life cycle is virtually identical to that of Strongylus vulgaris described previously.

Of the three large strongyles, we know the least about the parasitic phase of the life cycle of Strongylus equinus because it is less common than the others and so has been little studied. Infection is by ingestion of third stage larvae and exsheathment occurs in the small intestine. Exhseathed L3s invade the wall of the small intestine, cecum and colon and become encapsulated in nodules in which they molt to L4s by 12-14 days after infection. L4s leave their nodules and cross the peritoneal cavity to the liver, the right lobe of which is in close aposition to the cecum.  Most L4s will have reached the liver by 19-20 days after infection and will remain there for at least 12 weeks. Finally, they begin to migrate back to the large intestine by leaving the liver and crossing the abdominal cavity directly or by first passing through the pancreas ( which is closely associated with the right lobe of the liver) and then the abdominal cavity. The final molt to immature adults (L5s) occurs about 15 weeks after infection during their migration back to the large intestine. L5s penetrate the gut wall and enter the lumen of the large intestine via the formation of nodules. Reproduction takes place and the prepatent period is approximately 9 months.

Strongylus edentatus - life cycle

Again, the preparastitc phase of this life cycle is virtually identical to that of Strongylus vulgaris described previously.

Ingested L3s exsheath in the terminal portion of the small intestine as well as the cecum and right ventral colon, invade the gut wall, penetrate blood vessels and migrate to the liver via the hepatic portal vein. Molting to L4s occurs in two weeks (range from 11 to 18 days), after which larvae wander throughout the liver before they leave beginning about 42 days after infection.  Larvae escape from the liver by migrating under the liver capsule,  through the hepatic ligament and underneath the parietal peritoneum to the right flank. Here they molt to immature adults between 13 and 15 weeks after infection and finally return to the large intestine via the mesentery bertween 3 and 5 months after infection. They migrate through the wall of the cecum and colon via formation of nodules. Maturation takes place in the large intestine and the prepatent period is generally acknowledged to be approximately 11 months although it is claimed by some authors that this may be as short as 6 months.

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Strongylus edentatus - immature adults under the peritoneum of the right flank.
Image courtesy of Dr. Harold Drudge
Strongylus edentatus - an immature adult dissected out of a subperitoneal cyst on the right flank.
Image courtesy of Merial Inc.
Strongylus edentatus - an immature adult (arrow) having returned to the large intestine via the nodule (N).
Image courtesty of Dr. Harold Drudge

 

    

 

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