Series of Presentations on Nematodes from Veterinary Parasitology By Urquhart, Jenning

1. Strongyloidea

2. Strongyloidea
• There are several important parasites of domestic
mammals and birds in this superfamily of bursate
nematodes.
• Most are characterized by a large buccal capsule
which often contains teeths or cutting plates and in
some there are prominent leaf crowns surrounding the
mouth opening.
• The adults occur on mucosal surfaces of the
gastrointestinal and respiratory tracts and feeding is
generally by ingestion of plugs of mucosa.

3. Strongyloidea
With the excetion of three genera, syngamus and
mammonogamus, which are parasitic in the
trachea and bronchi, and stephanurus found in
the perirenal area, all other genera of veterinary
importance in this superfamily are found in the
intestine and conveniently can be divided into
two groups
• Strongyles
• Hook worms

4. Strongyloidea
• The strongyles are parasitic in the large
intestine and the important genera are
strongylus, Triodontophorous, Trichonema
(cyathostomes), Chabertia and
oesophagostomum.
• Hook worms are parasites of small
intestine and the three genera of
veterinary importance are Ancylostoma,
Uncinaria and Bunostomum.

5. Strongyles of Horses
Strongylus
• Members of this genus live in the large
intestine of horses and donkeys and, with
Triodontophorous, are commonly known as
large strongyles.
Hosts
• Horses and donkeys
Site
• Caecum and colon

6. Species
• S. vulgaris
• S. edentatus
• S. equines
Distribution
• Worldwide

7. Identification
Gross
– Dark red worms which are easily seen against
the intestinal mucosa.
– The well developed buccal capsule of the adult
parasite is prominent as is the bursa of the male.
Microscopic
– Species differentiation is based on the size and
the presence and the shape of the teeth in the
base of the buccal capsule.

8. Identification
S. vulgaris
1.5-2 cm, two ear shaped rounded teeth are present
S. edentatus
2.5-4.5 cm, No teeth
S. equines
2.5-5 cm, three conical teeth are present. One is
situated dorsally and I larger than the others.

10. Life cycle
• The adult parasites live in the caecum and
colon.
• Eggs are passed in the feces and
development from egg to the L3 requires
about two weeks (moist and temperate).
• Infection is by ingestion of L3.
• Parasitic larval development of the three
species differs with each other.

11. S. vulgaris
• The L3 penetrate the intestinal mucosa and
moult to L4 in the submucosa.
• These then enter small arteries and migrate
on the endothelium to their predilection site in
the cranial mesenteric artery and its main
branches.
• After a period of several months the larvae
moult to L5 and return to the intestinal wall via
the arterial lumina.

12. S. vulgaris
• Nodules are formed around the larvae mainly
in the wall of caecum and colon, when, due to
their size, they can travel no further within the
arteries and subsequent rupture of these
nodules releases the young adult parasites
into the lumen of intestine.
• The prepatent period is 6-7 months.

13. Strongylus equinus - life cycle
• The preparasitic phase of this life cycle is virtually
identical to that of Strongylus vulgaris.
• 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.

14. life cycle
• 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.

15. life cycle
• 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.

16. life cycle
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.

17. S. edentatus- Life cycle
• Freeliving stages similar to S. vulgaris
• L 3 exsheaths in small intestine, penetrates gut wall
to subserosa, and molts to L 4 ; remain here
(usually in nodules) for 3 months
• L 4 migrate to root of mesentery, some going to liver
and lungs and then to wall of cecum and right
ventral colon
• Immature adults then appear in lumen of these
portions of the large intestine
• Prepatent period - 11 months

18. Pathogenesis
Pathogenesis of migrating stages
• Migrating larvae of Strongylus equinus and edentatus
do not appear to cause pathological changes of
enough severity to translate into recognizable clinical
signs.
• However, migrations of larvae through the liver may
produce nodules and formation of fibrous tissue that
are readily seen at necropsies of infected horses.
• The early migrations of Strongylus equinus larvae into
the intestinal mucosa produce hemorrhagic nodules
that are easily recognized at necropsy or during
abdominal surgery in horses with colic.

19. Pathogenesis of migrating
stages
• Whether or not these nodules may
themselves produce colic in horses is
debatable.
• However, their presence certainly
indicates recent infections and it is safe to
assume that such horses will also be
infected with other large and small
strongyles and these may be primary
contributors to the observed signs of colic.

20. Pathogenesis of migrating
stages
• The life cycle of Strongylus edentatus includes
migration to the flanks where immature adults
and L4s may be found at necropsy enclosed in
subperitoneal cysts.
• Although these lesions have no clinical
significance, they do indicate the presence of
strongyle infections and a poor parasite control
program in the farm of origin.

21. Pathogenesis of adult worms
• 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.

22. Pathogenesis of adult worms
• 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.

23. Pathogenesis of adult worms
• 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 have shown that 30ml of
blood may be lost per day by infection with 75-100
Strongylus vulgaris adults. These ulcers appear to
heal readily after the feeding worms move on.

24. Pathogenesis of adult worms
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.