Parasitology - Equine PGE

Question 1

What is the most important nematode that causes equine parasitic gastro-enteris (PGE) compex?

Answer 1

Strongylus species - ie., Strongyles also known as RED WORMS

Question 2

What are parasites that invade the horse’s large intestine (the large ascending colon)?

Answer 2

Large strongyles aka large redworms:

• Strongylus vulgaris*
• S. edentatus*
• S. equinus*
• Triodontophorus spp.*

Small strongyles aka small redworms:

• Cyathosomins* (aka trichonemes) of which there are > 40 species;
• Oxyuris equi* aka “pinworms” - invades caecum, large intestine & rectum

Question 3

What is the name of the PGE caused by large red worms / large strongyles?

What parts of the pathogenesis do the different species of Strongylus (large red worms) share?

Answer 3

* Strongylosis*

Common Aspects of Pathogenesis:

Horse eats grass infested with L3 stage of Strongylus - can be S. vulgaris, S. edentatus, S. equinus or Triodontophorus.

Larvae pass down the intestinal tract & penetrate intestinal mucosa. At this larval stage, S. vulgaris is the most pathogenic, potentially damaging the cranial mesenteric artery & resulting in thrombosis & colitis.

Then, depending on the species, will go through different migrations & pre-patent periods.

Question 4

What is the condition caused by small red worms / small strongyles in the large intestine?

Answer 4

Cyathosominosis

Type I & more-serious Type II

Question 5

What is the morphology of the large red worms/large strongyles,

Strongylus vulgaris

S. edentatus

S. equinus

?

Answer 5

• *Gross**:
• Stout, 1.5-5 cm long
• Large buccal capsule
• Bursa visible to naked
  eye in male

Micro (buccal capsule):

• Double row leaf
  crowns(see photo of S. vulgaris)
  - Teeth (0,2,3 or more) -
  not in S. edentatus
• Dorsal gutter (channel
  for enzymatic secretions)

Question 6

All the large red worms (large strongyles) that cause Strongylosis in the large intestine (colon) of the horse share the common initial pathogenic pathway of being ingested at L3 stage by horses eating grass.

The larvae enter the intestinal tract, penetrating the intestinal mucosa, before they take different paths.

Describe the path of Strongylus vulgaris.

Answer 6

S. vulgaris, unlike S. edentatus or S. equinus, is potentially highly pathogenic at the larval stage, not just the adult stage. From the ascending colon, the larvae:

→ migrate into anterior (cranial) mesenteric artery over 2 weeks → burrow into intersection of artery & aorta to form verminous endarteritis lesion over 3-4 months → migrate back down via anterior mesenteric artery & moult → bore hole into descending colon & exit as adults over 6-8 weeks

As adults, they become plug feeders with their large buccal capsules, penetrating down into the muscularis layer and blood vessels. They leave circular bleeding ulcers that can lead to anaemia if there are many.

PPP = 6-7 months

NB: endarteritis is the inflammation of the intima of one or more arteries

Question 7

All the large red worms (large strongyles) that cause Strongylosis in the large intestine (colon) of the horse share the common initial pathogenic pathway of being ingested at L3 stage by horses eating grass.

The larvae enter the intestinal tract, penetrating the intestinal mucosa, before they take different paths.

Describe the path of Strongylus edentatus.

Answer 7

S. edentatus (without teeth) are relatively **non-pathogenicas larvae(unlike S. vulgaris). Their larval migration takes them to roomy tissue like theliver & mesentary** that can withstand more damage than the cranial mesentery artery as with S. vulgaris.

After entering the large intestine (ascending colon), S. edentatus larvae:

→ migrate via hepatic portal system to liver → form nodules in liver → moult → exit liver via peritoneum → migrate via periotoneum back to large intestine → exit as adults

PPP = 11-12 months

Question 8

All the large red worms (large strongyles) that cause Strongylosis in the large intestine (colon) of the horse share the common initial pathogenic pathway of being ingested at L3 stage by horses eating grass.

The larvae enter the intestinal tract, penetrating the intestinal mucosa, before they take different paths.

Describe the path of Strongylus equinus.

Answer 8

The larvae of S. equinus is relatively non-pathogenic, unlike S. vulgaris. Like S. edentatus, the larval migration of S. equinus is through roomier tissue such as the liver that is able to withstand damage better than the cranial mesenteric artery as in the case of S. vulgaris. After entering the horse’s ascending colon, the larvae:

→ crosses peritoneal cavity from intestine to liver → “wanders around” liver before returning to large intestine via peritoneal cavity again → exits as adult

PPP = 9 months

Question 9

Recap where the larvae of the large redworms go to migrate and moult after they enter the horse’s large intestine?

Strongylus vulgaris

S. edentatus

S. equinus

Answer 9

Strongylus vulgaris - cranial mesenteric artery to junction of artery & aorta, then back to colon via mesenteric artery

S. edentatus - hepatic portal system to liver then back to colon via peritoneum

S. equinus - peritoneal cavity to liver then back to colon via pernitoneal cavity

Question 10

All the large red worms (large strongyles) that cause Strongylosis in the large intestine (colon) of the horse share the common initial pathogenic pathway of being ingested at L3 stage by horses eating grass.

The larvae enter the intestinal tract, penetrating the intestinal mucosa, before they take different paths.

Describe the path of Triodontophorus.

Answer 10

Unlike the other large red worms, Strongylus vulgaris, S. edentatus & S. equinus, Triodontophorus is non-migratory. Instead, it just stays in the large intestine and moults there into an adult, where it feeds in “herds”.

After entering the large intestine of the horse, the larvae:

→ moves around wall of large intestine → moults to L4 → exits as adults

As adults, it causes superficial damage with its small buccal cavity, but feeds in herds, leaving large large, superficial ulcers several cm across that leave scars after they heal.

PPP = 2-3 months

Question 11

Aside from large redworms (large strongyles), what other nematode types cause PGE in the horse’s large intestine?

Answer 11

Small redworms (Cyathosomins, aka Trichonemes)

Pinworms (Oxyuris equi)

Question 12

What is the morphology of the small red worms, also known as Cyathostomins or Trichonemes? How do they differ in appearance from the large strongyles?

Answer 12

They look a lot like the large strongyles/red worms, but they are smaller, less than 1.5 cm (large redworms are 1.5-5 cm long).

They have small, shallow buccal capsules with double row of leaf crown and possibly teeth.

See photo.

Question 13

What happens to the small redworms, Cyathostomins, after they enter the large intestine of the horse as L3 with grass?

Answer 13

Cyathstomins are confined to the gut wall, but can take two life-cycle routes:

1. L3 can become arrested in gut wall for several months → moults to L4 → emerges as adult in gut lumen

OR

2. L3 moults to L4 → emerges as adult in gut lumen

PPP = 8-12 weeks

Question 14

Why would a small redworm, Cyathostomins aka Trichoneme, become **arrested at L3 for several months or years **in the large intestine of the horse rather than moult from L3 to L4 to adult in its normal 8-12-week PPP?

Answer 14

Similar to ostertagia in calves, the larvae become arrested in the gut wall (potentially for years) if they were ingested in late summer or early autumn. They become hypobiotic throughout the winter and emerge in late winter or early spring, and this is quite harsh for the horse.

Question 15

What is different about the hypobiotic larval stage of Cyathostomins compared with Ostertagia?

Answer 15

Cyathostomins, aka small red worms, become arrested as L3 and they can stay hypobiotic for years.

Ostertagia become arrested at L4 and typically are hypobiotic until the next grazing season.

Question 16

What is the name of the condition caused by Cyathostomins aka Trichonemes aka small red worms, and what are the different types?

Answer 16

Cyathostominosis.

Type I Cyathostominosis:
- Initial infection (L3) → local inflammation
- larval emergence in summer/autumn
+ plug-feeding adults = wormy horse

• *Larval / Type 2 Cyathostominosis:**
• larval emergence (late winter/early spring) → severe diarrhoea → severe weight loss

NB this is like ostertagia in calves & sheep, which can accumulate L3 that become hypobiotic & then attack harshly as adults

Question 17

When do small redworms become plug feeders?

Answer 17

Small redworms, Cyathostomins, are plug feeders as larvae & adults.

The large redworms Strongylus vulgaris and Triodontophorus are only a plug feeders as adults.

Question 18

What are the clinical signs of Cyathostominosis Type I, aka “wormy horse”? How many adult and larval worms are present?

Answer 18

Unthriftiness, poor coat, anaemia, diarrhoea

10s or 100s of thousands of adults and millions of mucosal larvae present.

Question 19

What are the clinical signs of Cyathostominosis Type II, aka “wormy horse”? How many adult and larval worms are present?

Answer 19

Massive inflammatory infiltration, serious diease characterised by severe diarrhoea &/or weight loss

Question 20

In what age of horse does Strongylosis typically occur? In what type of environment?

Answer 20

Young horses on permanent pasture

Adult horses especially if there is overcrowding or poor hygiene

Question 21

What are the sources of Strongylosis infection?

Answer 21

Like ostertagia, L3 on pasture, spring rise, overwintered L3

Question 22

In what age of horse does cyathostominosis typically occur?

Answer 22

Young horses less than 5 years old.

Question 23

The pinworm - aka Oxyuris equi - also attacks the large intestine in the horse, causing PGE. What is the morphology of the pinworm?

Answer 23

Females large: <10 cm long, white, with thin, pointed “pin-like” tails (see photo)

Males small: <1 cm long, difficult to see

Question 24

Where are the preferred targets of the pinworm in the horse GIT?

Answer 24

Caecum

Large intestine (colon)

Rectum

Question 25

What is the life cycle of the pinworm, Oxyuris equi, after its L3 is ingested by a horse?

Answer 25

L3 invades colonic mucosa → moults inside egg → hatches as L4 → L4 feed on colonic mucosa → develop to adulthood → adults migrate to rectum → females lay eggs on perianal hair → operculate eggs, 80-90 μm flattened on one side, fall to ground

PPP = 5 months

Question 26

What is the pathogenic stage of the pinworm, Oxyuris equi – larval or adult?

Answer 26

Larval:

L4 feed on colonic mucosa by nipping off epithelium, causing erosion; NOT true plug- feeders

Adults are non-pathogenic as they feed on gut contents.

However, egg-laying causes pruritus ani aka “seat itch”: broken hair & bare patches at rump & tail head

Question 27

What is the largest type of nematode found in the horse to cause PGE?

How big does it grow to and what are its other distinguishing morphological features?

Answer 27

Parascaris equorum, an ascarid

• Really huge: up to 50 cm (18 inches) long
• Three large lips around mouth (seen in all ascarid worms)

Question 28

What is the target organ of the Parascaris equorum larvae?

Answer 28

Small intestine via liver, heart & lungs

(hepato-tracheal migration)

Question 29

Which age of horse is most susceptible to Parascaris equorum infestation? How is it transmitted?

Answer 29

Mostly young foals < 4-6 months old

(like Nematodirus in sheep)

Transmission is from one year’s foals to next; usually first month of life; adult horses might be carriers as they carry only a few worms (like Nematodirus in sheep)

Question 30

What is the life cycle of the Parascaris equorum L2 embryonated egg after it is ingested and enters the foal’s intestines?

Answer 30

→ hepato- tracheal migration: intestine to hepatic portal system to liver → right side of heart → punches through lungs → up to trachea where it is coughed up and swallowed into small intestine → adult lays unembryonated egg with thick, sticky shell → in a couple of weeks it becomes L2 with larva inside

PPP = 10-12 weeks

Question 31

What is so different about the infectious stage of the Parascaris equorum worm?

Answer 31

It is ingested during its embryonated egg stage (L2). It reaches L4 inside the egg.

Question 32

What is special about the Parascaris equorum egg?

Answer 32

In the unembryonated stage in faeces, it is very strong with a thick, pitted shell that is sticky and can last for up to a year. Its stickiness means it can facilitate passive spread on fomites.

Question 33

The Parascaris equorum worm is very pathogenic. Describe the fecundity of the female as well as the damage caused by the larvae and the adults.

Answer 33

High fecundity of female - lay millions of eggs per day in infected foal.

LARVAE: leave eosiniphilic tracts & haemhorrages in lungs → nasal discharge

ADULTS: cause unthriftiness, weight loss, intestinal impaction & performation (rare)

Question 34

What is the first parasite to which foals are exposed and causes PGE?

Answer 34

Strongyloides westeri

This is a Rhabditoid - NOT A STRONGYLE

Question 35

What is the morphology of Strongyloides westeri?

Answer 35

• Very small, < 6 mm long
• similar to small trichostrongyles like Ostertagia
• long oesophagus (1/3 of body length)
• only female is parasitic

Question 36

What organ(s) does the Strongyloides westeri worm target in the foal?

Answer 36

The small intestine via the liver, right side of heart & lungs (hepato-tracheal migration).

Question 37

Why is the Strongyloides westeri considered facultatively parasitic?

What is its life cycle?

Answer 37

It can choose parasitic OR non- parasitic life cycle, depending on previous generation’s cycle:

Foal ingests L3 in mare’s milk and/or L3 with grass and/or skin penetration → hepato- tracheal migration → female lays larvated ie., embryonated eggs (50-60 μm), which are passed in faeces
PPP= 1 week

Question 38

Why is Strongyloides westeri considered the first parasite to which foals are exposed?

Answer 38

Because the L3 can be transmitted in mare’s milk.

Question 39

What is the immunity of mares?

Answer 39

Adults are normally immune, may act as carriers