Introduction

Toxocara canis is a cosmopolitan parasite of dogs.  It is the causative organism of the visceral larva migrans syndrome in humans which was first described by Beaver in 1952.   Since then, numerous cases have been reported world wide thus making Toxocara infections a serious problem.

Morphology

The ova of Toxocara canis are dark brown in appearance and are pitted superficially.  They measure 85 by 75 microns.  The adult male worms are 4 to 6 cm. long and the females are 6.5 to 10 cm. long.  The adult worms have the characteristic three lips of the ascarids.  This species also has distinct cervical alae or wings which are much longer than broad and extend from the anterior extremity along the lateral margins.  The perianal papillae of the male worms are also characteristic. 

Life cycle and transmission

The ova of Toxocara canis are passed out into the soil when dogs defecate.  Humans become infected by the accidental ingestion of embryonated ova in contaminated soil.  Children and toddlers are at particular risk because they frequently play in soil, especially in public parks which are popular places for dog walkers.

Once ingested the eggs hatch in the human intestine and release the second stage larva which penetrates the intestinal wall and migrates to various tissues such as the liver, the central nervous system and the eye.  There they are deposited and remain without further development.

In its normal host, the dog, the larvae are deposited in tissues but the life cycle only continues in pregnant bitches.  The larvae then migrate from the tissues, cross the placenta and infect the fetal puppies where the life cycle is completed.  Puppies are therefore born with adult worms.  The Toxocara life cycle can also be completed in puppies that ingest the ova.

Clinical disease

Visceral larval migrans, which is caused by the migration of the larvae to the tissues, has a wide variety of clinical symptoms ranging from coughing, wheezing and fever, to liver enlargement and cerebral involvement.  Invasion of the eye is a serious complication resulting in granuloma formation in the retina.  Eosinophilia is feature of the disease.

Laboratory diagnosis

Confirmation of Toxocara infection is usually made by serodiagnosis and occasionally biopsy.  Enzyme linked immunosorbent assays (ELISA) are usually employed.

An embryonated ovum of Toxocara canis

An unembryonated ovum of Toxocara canis