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.
|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