Toxocara canis

Overview

Toxacara canis is a typical ascarid nematode that infects dogs where its predilection site is the small intestines. These worms can be found throughout the world with varying prevalence. Control of this ascarid is typically difficult due to its extended persistence in the environment. T. canis is also important in humans medicine as the species most responsible for visceral larval migrans (VLM). The human is a non-permissive host of T. canis meaning it cannot complete its life cycle and reproduce, however the larval stages do migrate through the human body causing pathology.

Identification

Toxacara canis has the typical gross morphology of an ascarid, it is a large, fleshy white worm and can be up to 18cm long. The females are longer than the males who can normally reach 10cm in length. Microscopically T. canis

• Causes eosinophilic enteritis in the dog.

Recognition

T. canis is a typical ascarid. The fleshy adult worm may be up to 18cm long and lives in the small intestine of the dog, fox and other canidae. It is slightly bigger than the other canine ascarid species (Toxoscaris leonina - up to 10cm). Female T. canis produce characteristic dark eggs with a pitted surface

Significance

• Almost all puppies are born with T. canis infection.
• Small numbers of worms are usually asymptomatic.
• Larger numbers cause pot-belly, diarrhoea, vomiting, weight-loss and other clinical signs in young puppies; intestinal obstruction may occur in severe cases.
• It is also an important zoonosis.

Life-Cycle

• T. canis has a complicated life-cycle.
• The behaviour of the parasite changes as the dog becomes older.

In pups under approximately 2-3months of age:

• After ingestion of embryonated eggs or larvae:
  • T. canis larvae migrate via the hepato-tracheal route to the small intestine, where they become adults, and eggs are then passed in faeces
  • the prepatent period is about 4-5weeks
  • after approximately 6weeks of age there is a spontaneous expulsion of worms from the intestine.

In older dogs:

• Larvae migrate to the liver, via the heart, and then on to the lungs (hepato-tracheal migration), where they stay in the blood stream.
• They then migrate back to the heart and then on to somatic tissues (such as the liver, kidney, musculature etc.), where granulomatous reactions may occur. This is called the 'waiting phase'.
• Somatic ('waiting') larvae:
  • do not develop or grow (size = approximately 0.5mm)
  • but metabolically, they are highly active:

→ large quantities of excretory/secretory (ES) antigens

→ spread over the cuticle

→ immune evasion: rapid turnover

→ sloughing of antibody and immune cells (also mimic some host proteins)

• • somatic larvae wait for pregnancy to occur.
• After approximately 42nd day of gestation:
  • a proportion of the somatic larvae are activated
  • the majority of activated larvae migrate to the placenta, and gain access to the foetuses - this is prenatal infection
  • the remainder of the activated larvae migrate to the mammary gland, and get passed to the puppies via colostrum and milk - this is transmammary infection
  • larvae not activated in the first pregnancy may be activated in subsequent pregnancies.

In warm-blooded non-canid animals:

• Events occur just as in the older dog, i.e. larvae migrate → liver → lungs → heart → somatic tissues → granulomatous reactions → 'waiting phase'; but in this case, the somatic larvae are waiting for the animal that they are in (which is acting as a paratenic host) to be eaten by a dog, fox, wolf or other canid, where they will establish as adults or somatic larvae (depending on the age of the predator).
• This explains how humans (as warm-blooded non-canid animals) enter into the epidemiological picture.

Epidemiology

• Infection of dogs is by ingestion of the L2 larvae, which can occur in four ways:

1) ingestion of the embryonated egg

2) prenatal infection

3) transmammary infection

4) ingestion of a paratenic host.

• Infection of a paratenic host can occur by:

1) ingestion of the embryonated egg

2) ingestion of larvae in the tissues of another paratenic host.

• Each female T. canis can lay up to 250,000 eggs per day:
  • the eggs are not infective until the L2 is fully developed
  • this process takes a few weeks in summer, but many weeks in the winter
  • the embryonated egg is tough and can survive for 4-5years
  • eggs therefore accumulate in the environment, and can easily be demonstrated in soil scrapings from, for example, breeding kennels or city parks
  • when eggs from the environment are swallowed by a bitch, larvae accumulate in her somatic tissues - to be activated during pregnancy
  • at birth, prenatally derived larvae are already migrating through the pups' liver and lungs
  • adult worms reach the intestine and start to lay eggs when the pups are 2-3weeks old
  • pups are therefore a potent source of environmental contamination (particularly in breeding kennels) until spontaneous expulsion occurs after approximately 6weeks of age.
  • in general, only approximately 15% of adult dogs have patent infection - an exception is nursing bitches, who often pass large numbers of eggs
  • up to 45% of foxes have patent infection, and are therefore a potent source of eggs in urban areas.

Human Infection

• Humans are infected by swallowing embryonated eggs from the environmental reservoir.
• This is most likely to happen in young children.
• Most infections are asymptomatic.
• Approsimately 2.5% of the British population are seropositive.

Efficacy of Anthelmintics Against Life-Cycle Stages of T. canis

Compound	Trade-Name	Intestinal Worms	Migrating Larvae	Somatic Larvae
Piperazine Pyrantel Pyrantel + Febantel Fenbendazole Mebendazole Flubendazole Nitroscanate Selamectin Milbemycin Moxidectin	various Strongid Drontal Panacur Telmin Flubenol Lopatol Stronghold Milbemax Advocate	+ ++ +++ +++ +++ +++ +++ +++ +++ +++	- - - +++ ? ? ? ? ? ?	- - - - - - - ? - ?

Control of T. canis

• The only satisfactory way of breaking the life-cycle in breeding kennels and reducing zoonotic risk is to eliminate T. canis eggs from the environment.
• Hygiene is important (but note that the eggs stick to surfaces and that few disinfectants will kill them).
• To prevent dogs excreting eggs, pups must be dosed regularly from 2weeks of age.
• Most anthelmintics are only active against adult worms in the intestine.
• These adult worms are quickly replaced by developing larvae that survived treatment.
• Therefore, pups should be dosed at 2, 4, 6, 8 and 12weeks of age.
• Fenbendazole is active against both adults and larvae.
• So, an equivalent result can be obtained with just two treatments: one in the third week of life, and again 3weeks later.
• Nursing bitches should also be treated.
• Otherwise, adult dogs should be dosed 2-4times a year.
• Current anthelmintics at normal dose-rates will not kill somatic larvae.
• This can be done, however, with daily high doses of fenbendazole.
• Pregnant bitches are given daily doses (25mg/kg) from the 42nd day of pregnancy.

T. canis in Veterinary Public Health

T. canis is associated with at least three disease syndromes in humans:

1) visceral larval migrans (VLM) (→ eosinophilia, hepatomegaly, fever, asthma)

2) ocular larval migrans (OLM) (→ unilateral partial impairment of vision)

3) covert toxocarosis (non-specific clinical signs associated with high antibody titre)

Around 55cases, mostly OLM, are diagnosed in the UK each year.