Parasitology - Small Animal PGE

Question 1

What is the most important nematode of dogs in the UK that causes PGE?

What superfamily does this worm belong to?

Answer 1

Toxocara canis

It’s an ascarid, unfortunately.

Question 2

What is the name of the PGE condition caused by Toxocara canis?

Answer 2

Toxocarosis

Question 3

What are the main nematodes that affect dogs in the UK? What are their superfamilies & which part of the GIT do they target?

Answer 3

ASCARIDS targeting small intestine:

Toxocara canis

Toxascaris leonina

HOOKWORMS targeting small intestine:

**Uncinaria **

Ancylostoma

Question 4

Toxocara canis is a typical ascarid. What does this mean in terms of size, how it gets to the small intestine and eggs?

Answer 4

Size: big - up to 18 cm

Migration: hepato-tracheal

Eggs: dark brown & pitted

Question 5

The life cycle of * T. canis* is complicated because it changes from when the dog is a puppy to when it is older. What is the life cycle when it’s a puppy up to two months old?

Answer 5

It’s got a typical ascarid lifecycle at this point:

Puppy ingests embryonated egg or larvae → larvae undergoes hepato-tracheal migration to small intestine → moults into adult → passed in faeces

PPP = 4-5 weeks

After six weeks, there is a spontaneous expulsion of worms from the intestine

Question 6

What happens to the life cycle of T. canis in an older dog?

Answer 6

There’s a “somatic waiting phase” of larvae, which waits for host to become pregant:

Dog ingests embryonated eggs or larvae → larvae begin hepato-tracheal migration from intestine to liver via hepatic portal → to heart → lungs → BACK TO HEART → distributed via aorta to somatic tissues such as liver, kidney, musculature

In the somatic tissues, the T. canis larvae cause granulomatous reactions as the larvae remain arrested at about 0.5 mm

HOWEVER, they are metabolically very active, producing large amts excretory/secretory antigens spread over larval cuticle to evade immune system → wait for pregnancy of host to occur → after 42nd day of gestation: larvae activation → pre-natal infection → transmammary infection → subsequent- pregnancy activation

(male hosts = dead-end hosts)

Question 7

How does the life cycle of T. canis differ in other warm-blooded, non-canid animals?

Answer 7

Warm-blooded, non-canids (paratenic hosts):

Same life cycle as in older dogs but somatic larvae wait for their paratenic host to be eaten by a dog, fox, wolf or other canid instead of waiting for pregnancy of host → establish as adults or somatic larvae (depending on age of predator)

Question 8

GIven what you know about T. canis’s different life cycles, what are the different ways dogs and puppies can become infected by T. canis?

Answer 8

1. ingestion of embryonated egg
2. pre-natal infection via activated somatic larvae
3. transmammary infection via activated somatic larvae
4. ingestion of paratenic host

Question 9

How can newborn puppies become contaminants of T. canis?

Answer 9

At birth, prenatally derived larvae already migrating thru pups’ liver & lungs so adult worms reach intestine & start to lay eggs when pups only 2-3 weeks old

(even tho PPP = 4-5 weeks)

Question 10

How long can the embryonated Toxocara canis eggs survive in the environment?

Answer 10

4-5 years

Question 11

Why are foxes and playgrounds important sources of T. canis eggs?

Answer 11

45% of foxes have patent infection & leave a lot of eggs in urban areas

Eggs accumulate in playgrounds and sandpits where dogs are allowed to foul

Question 12

How do humans get T. canis? What are the clinical signs”?

Answer 12

Usually kids eating embryonated eggs in soil. Most infections are asymptomatic.

Question 13

What are the three syndromes associated with T. canis in HUMANS?

Answer 13

• Visceral Larval Migrans (VLM) - eosinophilia, hepatomegaly, fever, asthma
• Ocular Larval Migrans (OLM) - unilateral impairment of vision; about 55 cases/year in UK
• Covert Toxocarosis - non- specific clinical signs associated with high antibody titre

Question 14

What anthelminthic is effective against both adult and larval T. canis?

Answer 14

Fenbendazole (Panacur) active against larvae & adults

• treat puppies twice at 3 weeks and 6 weeks
• must treat nursing bitches
• treat adult dogs 2-4 times/year
• somatic larvae killed only by daily doses

Question 15

What is the other ascarid nematode besides Toxocara canis that affects dogs in the small intestine?

Answer 15

Toxascaris leonina

Question 16

What are the main similarities and differences between T. canis and Toxascaris leonina?

Answer 16

Very similar in morphology to T. canis but slightly smaller (up to 10 cm vs 18 cm for T. canis)

NO prenatal or transmammary transmission

NO hepatotracheal migration in final host

Zoonotic potential uncertain

Less hazardous than T. canis.

Eggs lighter in colour, smoother.

Affects both felids & canids

Question 17

To what superfamily do the hookworms of dogs - Uncinaria stenocephala & Ancylostoma caninum – belong?

Answer 17

Ancylostomatoidea
(Strongyles)

Question 18

Uncinaria stenocephala & Ancylostoma caninum as strongyles have typical life cycles (like Ostertagia, basically).

L3 is the infective stage. Is infection caused by ingestion by dogs in both nematodes?

Answer 18

No.

Ingestion of L3 Uncinaria

Skin penetration of L3 Ancylostoma

Question 19

What are the differences in morphology & pathogenicity between Uncinaria and Ancylostoma?

Answer 19

Uncinaria:

Plug-feeder w/ two cutting plates/shears (see photo) → results in “protein-losing enteropathy”

Heavy infections → intermittent diarrhoea, lower weight-gain

No vertical transmission

Ancylostoma:

More pathogenic: Avid blood-sucker with three-pronged teeth → severe anaemia in unweaned puppies with heavy infection

Transmammary infection

Question 20

Which hookworm are you more likely to see in a dog in the UK, Uncinaria stenocephala or Ancylostoma caninum?

Answer 20

Uncinaria stenocephala

This hookworm is common in greyhounds, hounds, sheepdogs in UK & in cooler climates.

Ancylostoma caninum, on the other hand, is more prevalent in warmer/hotter climates like Southern Europe. This is lucky since this is nastier, more pathogenic, and is an avid blood-sucker with teeth as an adult. It enters percutaneously and can be transmitted via milk from bitch to puppy.

Question 21

Which of the hookworms, Uncinaria or Acylostoma, cause pedal dermatitis?

Answer 21

Both

Question 22

What is the major difference between the Uncinaria & Ancylostoma life cycles?

Answer 22

Ancylostoma migrates from intestine via blood to lungs to small intestine.

Uncinaria doesn’t migrate. It just stays in the small intestine to moult to adulthood.

Question 23

Dogs are affected by the whipworm, Trichuris vulpis of the Trichinelloidea family.

(remember we covered Trichuris suis with pigs). It’s a “typical whipworm”. What does this mean for morphology and life cycle?

Answer 23

Whipworms have narrow heads and fat tails, and burrow their heads into the caecal mucosa, making little holes that leave the animal susceptible to secondary bacterial infection.

Trichuris eggs are bi-opercular (two opercula on each end) and they are infective at the embryonated stage.

It’s a direct life cycle: Dog eats embryonated egg, larvae moults into adult into lumen, eggs excreted in faeces.

Question 24

Trichuris vulpis invades the caecum. What does it cause in dogs?

Answer 24

Intermittent diarrhoea.

Question 25

Compare and contrast Toxocara cati & Toxocara canis morphologically.

Answer 25

Both T. cati & T. canis are nematodes of the Ascaroidea family. Morphologically, they both have three buccal lips.

T. cati, T. canis & T. leonina, an ascarid that also infects cats, are distinguished morphologically by their cervical alae. In T. cati, the cervical ale is arrow-shaped and quite wide, while T. canis’s alae have bumps and T. leonina has a more tapered head (see photo).

Question 26

How does Toxocara cati’s life cycle differ from that of Toxocara canis? Remember T. canis has different life cycles for puppies, older dogs and paratenic hosts.

Answer 26

NO pre-natal transmission from activation of somatic waiting phase larvae across placenta into foetus.

PPP = 8 weeks instead of 4-5 weeks in T. canis

Question 27

What are the other nematodes besides Toxocara cati can affect cats?

Answer 27

Pretty much the same as those that affect dogs:

Toxascaris leonida

* Ancylostoma spp* (hookworm)

Trichuris spp (whipworm)

Dirofilaria immitus (heartworm - not in the UK; cats are abnormal hosts for this nematode)