Parasitology - Cestodes & Horse Bots

Question 1

Cestodes are tapeworms and they affect cats, dogs, ruminants and poultry to cause gastro-intestinal diseases.

Both dogs and cats can be infected by Dipylidium, which attacks the small intestine in both animals.

What is the morphology of this common tapeworm in terms of how it can be identified and its size?

Answer 1

These cestodes can grow up to 50 cm long. They are identified by their gravid segments:

OVAL (c.f. RECTANGULAR for Taenia)
- two lateral genital pores
- eggs inside in packets that are not easily
broken up by water

Question 2

Should owners of dogs and cats be very worried if they see the motile proglottids of Dipylidium in their pets’ faeces?

Answer 2

No, because Dipylidium caninum are very common parasites of dogs & cats

- Minor clinical significance

• Causes occasional anal irritation
• Diarrhoea may result if tapeworms present in high numbers

Question 3

What is the life cycle of Dipylidium? How does knowledge of its life cycle contribute to how you would treat it or prevent it?

Answer 3

It’s an indirect life cycle that uses the flea (Ctenocephalides spp) as an intermediate host. The final host is the dog or cat.

Egg packets dropped along along with eggs of flea (the IH) on dog/cat fur → flea eggs fall off & hatch OFF final host → flea larva eat flea dirt in carpet/ environment along with Dipylidium eggs → Dipylidium oncosphere (egg) inside flea larva transforms to cysticercoid (ie., larval metacestaode stage) → becomes infective → flea pupates → adult flea emerges carrying Dipylidium cysticercoid → flea jumps onto & infests final host → final host over-grooms, ingesting infected flea

Question 4

How does the Dipylidium gravid segments differ morphologically from Taenia gravid segments?

Answer 4

Taenia, which are musculo-skeletal tapeworms, have rectangular-shaped gravid segments with only one genital pores (see photo) while Dipylidium have oval-shaped gravid segments with two lateral genital pores.

Question 5

What are the differences between the life cycles of Dipylidum caninum and Taenia? How does this affect treatment?

Answer 5

Dipylidium is a tapeworm that affects the small intestine of dogs and cats. Its intermediate host is the flea (Ctenocephalides spp).

Taenia is a tapeworm that affects the muscles of dogs, cats and humans. Its intermediate hosts are ruminants and pigs.

Dipylidium requires FLEA CONTROL.

Question 6

What is the final host of the tapeworm, Anoplocephala perfoliata? What is its target area in the GIT?

Answer 6

Horses

• mainly in caecum clustered around ileo-caecal junction

Question 7

What is special about the morphology of Anoplocephala perfoliata?

Answer 7

Looks more like trematode (flat, leaf-shaped) than tapeworm (a strobila of proglottids) - see photo

• Many wide, thin segments
• ~ 5 cm or longer

Question 8

What is the life cycle of Anoplocephala perfoliata? What is its intermediate host and final host?

Answer 8

Intermediate host = PASTURE MITES (free-living, ie., don’t live on horse)

Final host = Horses

Pasture mites eat A. perfoliata eggs → oncosphere becomes infective as cysticercoid (larval metacestode stage) inside mite → horse ingests mites while grazing

Question 9

Anoplocephala perfoliata is found mostly in the caecum around the ileo-caecal junction. What clinical signs does it cause and how pathogenic is it?

Answer 9

• causes superficial ulceration & mild inflammation
• little clinical significance but heavy infections of >20 tapeworms are a risk factor in some forms of colic

Question 10

What is the morphology of the Anoplochephalid egg? What is the “pyriform apparatus” of the Anoplocephala perfoliata egg?

Answer 10

Egg has “rounded triangular shape”; oncosphere contained in pyriform apparatus, which is a chitinous ring with two projections. See photo.

Question 11

What is the tapeworm that affects sheep?

Answer 11

Moniezia

Question 12

Moniezia & Anoplocephala perfoliata are both tapeworms that affect animals in the UK. What are the animals that they affect, and what are the similarities and differences in the tapeworms’ size and morphology as adults and as eggs?

Answer 12

Moniezia spp affect sheep and cattle, but mostly sheep here in the UK. Anoplocephala perfoliata is found in horses.

Moniezia are typical tapeworms in that they are a strobila of proglottids with a scolex head. However, Monieza are ENORMOUS and can grow UP TO 2 METRES! They can sometimes be seen hanging out of the anus of a sheep.

Anoplocephala are small and grow to about 5 cm (can be longer but not two meters).

The eggs contained in the gravid segments (proglottids) are similar in both: rounded triangular with ovum inside chitinous pyriform apparatus. See drawing.

Question 13

What is the life cycle of the Moniezia tapeworm that affects sheep? What is its intermediate host?

Answer 13

LIfe cycle is like that of Anoplocephala spp. Like A. perfoliata, the intermediate host is a free-living pasture mite.

Pasture mites eat Moniezia eggs → oncosphere in the pyriform apparatus becomes infective as cysticercoid (larval metacestode stage) inside mite → sheep ingests mites while grazing (see diagram)

Question 14

Are any of the mites that are intermediate hosts of Anoplocephala and Moniezia tapeworms likely to be any of the mites we have studied?

Answer 14

No, because the intermediate hosts of these tapeworms tend to be free-living pasture mites. We have been studying parasitic mites that are burrowing or surface mites (eg., Sarcoptes, Chorioptes, Demodex, Knemidocoptes, Psoroptes, Cheyletiella).

Question 15

What is the clinical significance of a Moniezia tapeworm infection in sheep?

Answer 15

Not much. Even though it looks very dramatic at two metres long, possibly protruding from the anus of a sheep, it has little clinical significance.

Question 16

Sheep farmers become aware of Moniezia infestations in late summer. Why?

Answer 16

Thiat is when spontaneous expulsion of womrns often occurs. That is when you see lengths of strobila hanging from the sheep’s anus.

Question 17

What is the main tapeworm (cestode) of interest that affects poultry?

Answer 17

Davainea spp

Question 18

Davainea spp, the tapeworm that affects poultry & pidgeons, is highly pathogenic. What condition of the GIT does the tapeworm cause?

Answer 18

Haemorrhagic enteritis

Question 19

As with all cestodes, Davainea spp have intermediate hosts. What is it?

Answer 19

Molluscs.

Final host are poultry & pidgeons.

Molluscs eat eggs. Oncosphere reaches infective cysiceroid stage (metacestode) inside mollusc. Bird eats mollusc.

Question 20

Where is Davainea spp tapeworm found in the poultry’s GIT?

Answer 20

Small intestine.

Question 21

Davainea spp, the tapeworm that affects poultry & pidgeons, is “armed”. What does this mean? What else about its morphology differs from other cestodes such as Moniezia or Dipylidium?

Answer 21

“Armed” means they have hooks around suckers as well as on rostellum (see drawing)

Davainea spp are very tiny tapeworms (remember that they affect birds, not sheep) of about ~3 mm long

Instead of long strobila, they only have 4-9 segments

The look like Echinococcus (which has only 3-4 segments)

Question 22

In general, how many circular suckers do cestodes have located around the scolex?

Answer 22

Four. They can be armed or unarmed (with hooks).

Question 23

Each gravid segment at the end of a cestode strobila contains more than 100K eggs. These eggs are IMMEDIATELY INFECTIVE (unlike nematodes, they don’t have to undergo various larval stages).

What is inside each egg?

Answer 23

Each cestode egg (oncosphere) contains a larva with six hooks. The oncosphere/egg is surrounded by a shell made of numerous blocks that give the shell a striated appearance under a microscope.

Question 24

The life cycle of all cestodes (tapeworms) is indirect, going through one intermediate host before reaching its final host.

While inside the intermediate host, which has eaten the tapeworm egg/oncosphere, the larva inside develops via several stages.

What is the name of the larva when it is inside the intermediate host?

Answer 24

Metacestode

Question 25

There are six metacestode stages of the cestode in increasing complexity (more scolexes/adults inside). What are they?

Answer 25

Cysticercus - fluid-filled bladder iwth one inverted scolex

Cysticercoid - pinhead size, only found in invertebrates, instead of bladder, there’s a potential space and the scolex is not inverted

Coenurus - like cysticercus but has multiple inverted scolices

Strobilocercus - only the cat tapeworm Taenia taeniaformis; again like cysticercus but single scolex is attached to bladder by chain of segments (strobila)

Hydatid cyst - metacestode of Echinococcus granulosus; can grow to size of football; lined with germinal epithelium that buds off brood capsules internally; inverted scolices form inside these brood capsules = “hydatid sand”; host walls off cyst with fibrous tissue; between germinal membrane and fibrous tissue is amorphous layer

Alveolar cycst - metacestode of Echinococcus multilocolaris; like hydatid cyst but daughter cysts bud off external & internal surface so it expands by infiltrating through tissue like a tumour. Seen in the liver.

Question 26

What is the proper name of the horse bot?

Answer 26

Gasterophilus spp

Question 27

What are the three species of horse bot in the UK? What is the most common?

Answer 27

Gasterophilus intestinalis (common in UK)

G. nasalis (moderately common)

G. haemorrhoidalis (rare)

Question 28

What is the pathogenic significance of horse bots?

Answer 28

Not much.

Larvae parasitize while in the stomach of the horse. Attachment to stomach wall provokes marked inflammatory reaction (ring-like thickening around the base of each larva).

Large numbers may interfere with passage of food or action of sphincters

Adult doesn’t parasitize but cause annoyance as they hover around head laying eggs.

Question 29

What is the life cycle of the Gasterophilus intestinalis?

Answer 29

Females lay creamy-white eggs (1-2 mm long) on hair of horse’s forelegs & shoulders → hatch spontaneously or stimulated by warmth & moisture of horse’s self-grooming → larvae crawl into mouth or are taken up during licking → penetrate tongue or buccal mucosa → wander around for several weeks in oral mucosa → emerge → swallowed → stomach → attach to gastric mucosa (prefers cardiac region of stomach) → BOTS with spines

→ 10-12 months later → bots detach → pass in faeces → larvae pupate on ground → adult emerges after one to two months → die after a few days to weeks

Only one generation per year in temperate regions

Question 30

What is a “bot”? What do they look like?

Answer 30

Bots are the larval stage of the Gasterophilus fly. After migrating around the oral mucosa, it is swallowed and passed into the stomach, where it grows 10-20 mm long and develops SPINES, which are used for identification. See photo.

Question 31

How are the adult horse-bot flies identified?

Answer 31

By their wings. They have only one set of wings with brown patches but they look like bees otherwise (bees have two sets of wings). See photo.

Question 32

How are horse-bot flies controlled?

Answer 32

Insecticide treatment of horses over winter (AFTER first frost when all adult flies should be dead) breaks life cycle in temperate parts of world because the entire population should be present as BOTS in the stomach.

Sponge horses’ coat with warm water containing insecticide if eggs are seen in late summer (stimulates hatching & kills larvae)