Toxocara canis Thanh T. and Dhool M.

Taxonomic Consideration Kingdom:Animalia Phylum: Nematoda Class: Rhabditea Order: Ascaridida Family: Ascarididae Genus: Toxocara Species: canis

Characteristics The Ascarids are typically very large, stout intestinal worms. They frequently have three prominent lips surrounding their oral opening. The life cycle is usually direct Referred to as "roundworm” video

Introduction Geographic region: Worldwide Definitive host: Dogs and other canids Paratenic host: Rodents Accidental host- Human Toxocara canis is a worldwide distributed helminth parasite of the dogs and other canids. T. canis is gonochorist (which means the sex of the parasite is present on different worms—opposed to hermaphroditism A puppy can be born infected with T. canis. Dormant juveniles in the mother are activated by the host hormones in late pregnancy and reenter the circulatory system where they are carried to the placenta, there they penetrate through the fetal bloodstream, where they do a complete lung migration.

Introduction continue.. If the host is a young puppy and has no prior infection, the worms hatch and migrate though the portal system and lungs and back into the intestines. If the host is an older dog the juveniles do not complete the lung migration, they wander the body and eventually enter a developmental arrest. T. canis can infect its host via a transmammory route (which is through the mother’s milk), however this is less common. If a female dog is lactating and ingests infective juvenile eggs then they can complete the migration and produce a patent infection.

Morphology Adult T. canis have a round body with spiky cranial and caudal parts, covered by yellow cuticula. Cranial part of the body contains two lateral alae (length 2–2.5 mm, width 0.2 mm). Male worms measure 9–13 × 0.2–0.25 cm and female worms 10–18 × 0.25–0.3 cm. T. canis eggs have oval or spherical shape with granulated surface, thick-walled, and measures from 72 to 85 μm

Life cycle of Toxocara canis

The Life Cycle of T. canis

Life cycle continued… Unembroyonated eggs are released in the feces and forms embryo in the external environment. Once ingested by dogs the infective eggs hatch and release larvae that penetrates the gut wall and migrate to various tissues where they encyst (if the dogs is older than 5 weeks). Older dogs are often asymptomatic. In younger dogs, the larvae migrate through the lungs, bronchial tree, and esophagus; adult worms develop and oviposit in the small intestine. In the older female dogs, the encysted stages are reactivated  during pregnancy, and can infect puppies via the transplacental and transmammary routes in which small intestine adult worms become established.  Thus, infective eggs are excreted both by lactating bitches(female dogs) and puppies. However in older male dogs the parasite meets a dead end, because the adult male will not shed eggs in the feces and are not affected if they ingest the eggs.

Life cycle continued.. Humans are accidental hosts who become infected by ingesting infective eggs in contaminated soil.  After ingestion, the eggs hatch and larvae penetrate the intestinal wall and are carried by the circulation to a wide variety of tissues (liver, heart, lungs, brain, muscle, eyes).  While the larvae do not undergo any further development in these sites, they can cause severe local reactions (such as inflammation) that are the basis of toxocariasis.  The two main clinical presentations of toxocariasis are visceral larva migrans (VLM) and ocular larva migrans (OLM)*.

Visceral Larva Migrans Occurs when a juvenile of several species of nematodes gain entry to an improper host and begin a typical tissue migration They do not complete the normal migration but undergo developmental arrest and begin an extended, random wandering through various organs and tissues of the body, affected organs can include the liver, heart (causing myocarditis), brain, and the CNS (causing dysfunction, seizures, and coma). The VLM remains unapparent in most cases. Symptomatic cases are most frequently observed in children aged two to five years.

Visceral Larva Migrans Symptoms Physical symptoms: includes fatigue, anorexia, weight loss, pneumonia, fever, cough, bronchospasm, abdominal pain, headaches, rashes, and, occasionally seizures The clinical symptoms :depend on the localization and degree of pathological changes and include non-specific and varied conditions such as eosinophilia, leukocytosis, hepatomegaly, mild gastrointestinal disorders, asthmatic attacks, pneumonic symptoms, urticarial skin changes, central nervous disorders with paralyses

Ocular larva Migrans Ocular larva migrans, is caused by migration of larva into the posterior segment of the eye, tends to occur in children and young adults. Patients may present with decreased vision, red eye, or leukokoria (white appearance of the pupil). Granulomas and choriorentinitis (inflammatation of the the uveal tract of the eye), Unilateral visual loss, retinal fibrosis, retinoblastoma (common primary ocular malignancy/ eye cancer of children) and retinal detachment occur. Clinically, serum antibodies to Toxocara are often absent or present in low titers. .

Diagnosis and Treatment ELISA using secretory excretory antigen Finding Toxocara larvae within a patient is the only definitive diagnosis for toxocariasis, however biopsies to look for second stage larvae in humans are generally not very effective Liver biopsy might demonstrate the characteristic granuloma surrounding a juvenile High levels of eosinophils are present, especially if they had previous parasitic infection Mebendazole is effective on patient with severe symptoms

Prevention and Control Clean your pet’s living area at least once a week. Feces should be either buried or bagged and disposed of in the trash. Do not allow children to play in areas that are soiled with pet or other animal stool and cover sandboxes when not in use to make sure that animals do not get inside and contaminate them. Wash your hands with soap and warm water after playing with your pets or other animals, after outdoor activities, and before handling food. Teach children the importance of washing hands to prevent infection and that it is dangerous to eat dirt or soil. Dirt is not a good way to incorporate minerals into your diet!

Quick Review What are the Definitive host: Dogs and other canids What kind of Host are rodents: Paratenic host How do humans get infected by T.canis: Ingesting infecting soils containing eggs How do puppies get infected: ingesting infected eggs, through transplacental route, and transmammory route(RARE!) Toxocara free = happy puppies and happy babies 

References http://en.wikipedia.org/wiki/Toxocara_canis http://en.wikipedia.org/wiki/Toxocariasis http://emedicine.medscape.com/article/229855-overview http://emedicine.medscape.com/article/1222849-overview http://www.novatec-id.com/products/infectious-diseases/worms/toxocara-canis/ http://animal.discovery.com/invertebrates/monsters-inside-me/toxocariasis-toxocara-roundworm/