Toxocara canis

Sidney MillironAaron Bettenhausen

Taxonomy

• Kingdom: Animalia • Phylum: Nematoda • Class: Secernentea • Order: Ascaridida • Family: Toxocaridae • Genus: Toxocara • Species: Toxocara canis

Geographic range

• World wide distribution• Found commonly in all domestic dogs and

other canids• Possible to have prenatal infections of puppies

Hosts• Primary host is dogs• Adults are infected upon

infestation of embryonated eggs, or by eating rodents infected with visceral larva migrans

• Worms can penetrate the placenta and infect unborn puppies

• Puppies can be born infected• Puppies can also become

infected via the transmammary route

Other hosts• If a rodent eats the embryonated

eggs the worms will hatch and begin migration before going dormant

• If the rodent is then eaten by a dog the dog can become infected

• Rodent acts as paratenic host• Rodents develop visceral larva

migrans• Humans who ingest the eggs also

develop visceral larva migrans

Egg

• Eggs– Brownish– Almost spherical – Surficial pits– Unembryonated

when laid

Larvated Eggs – Infective Stage

Larvae

• Found in Intestines, Circulatory, Lungs and Esophagus of Hosts

Adults

• Adults– Large (M=4-

6cm; F=6.5-15+cm)

– Have 3 lips– Prominent

cervical alae in both sexes

3 Prominent Lips

Life Cycle

• Adult worms live in small intestine of their host– Produces prodigious numbers of eggs

• Eggs are passed in feces• Develop into L3 in 5-6 days under optimal

conditions

Canine Oral ingestion • In Puppies:– Worms hatch and migrate through the portal system

and lungs – Get swallowed and find their way back to intestine– Can be fatal to puppies due to malnutrition

• In older dogs (secondary immune response)– Juveniles do not complete the lung migration – Wander through the body, eventually entering a

developmental arrest for a long periods– Most adult dogs show no symptoms when infected and

gain an increased immunity to reinfection as they age

Life Cycle

Infection during canine pregnancy

• Dormant juveniles are activated by host hormones late in pregnancy

• Reenter the circulatory system– Carried to placentas

• Penetrate through to the fetal bloodstream– Complete lung migration en route to the intestine

• Juveniles can also be passed by the trasmammary route, in mother’s milk

Other routes

• If a rodent or other mammal eats embryonated eggs– Juveniles begin to migrate but then become

dormant and continues it’s developmental arrest• If rodent is eaten by a dog– Worms promptly migrate through the lungs to the

intestine or into tissues to continue their wait, depending on age.

Visceral Larva Migrans

• Occurs when nematode gains entry into paratenic host– They do not complete the normal migration but

undergo developmental arrest– Begin an extended, random wandering through

various organs and tissues of the body

Pathogenesis

• Juveniles provoke a delayed-type hypersensitivity reaction in paratenic hosts– Eventually they will find their way to the brain– In other tissues, juveniles will form a granuloma – Can cause chronic ocular inflammation, as well as

retinal granulomas – Rarely can cause eosinophilic meningoencephalitis

Visceral Larva Migrans

• Occurs when a mammal other than the intended host (Dogs) ingest the eggs

• Worms hatch and begin migration but eventually undergo developmental arrest

• Worms begin a randomly wandering through the body• Can be caused by a number of different species

however Toxocara canis is the most common species that causes this disease in humans

• In experimental hosts juvenile worms seem to prefer residing in the brain.

Symptoms in Humans

• Fever, pulmonary symptoms, hepatomegaly, and eosinophilia

• Worms migrate indiscriminately to and from any organ including but seem to prefer the liver

• Can cause blindness if they migrate to the eye• Can cause neurological symptoms• Extent of tissue damage is proportional to the

numbers of juvenile worms in the body

Prevalence of infection

• In the united states 4.6-7.3% of children ages 1 to 11 years old have Visceral Larva migrans

• Up to 30% infection has been found in African American children of low socioeconomic status

• Up to 34% infection has been found in Irish school children

• In some developing tropical countries the rates of infection among children can be around 50 to 80%

Diagnosis

• An ELISA using secretory-excretory antigens• A liver biopsy might reveal a juvenile

surrounded by a granuloma• Infected patients may present with high

eosinophilia

Treatment and Control

• Mebendazole• Periodic deworming of household pets• Proper disposable of animal feces• Don’t let your dog eat rodents• Covering sandpits in public parks when not in

use