Toxocariasis

Description, Causes and Risk Factors:

ICD-10: B83.0†

Toxocariasis is an infection transmitted from animals to humans (zoonosis) caused by the parasitic roundworms (nematode larvae) commonly found in the intestine of dogs (T. canis) and cats (T. cati). About 20% of dogs release T. canis in their stool.

Puppies usually contract Toxocara canis from the mother before birth or from her milk. The larvae mature rapidly in the puppy's intestines and when the pup is three or four weeks old, they begin to produce large numbers of eggs that contaminate the environment through the animal's stool. The eggs soon develop into infective larvae. Contraction occurs from ingesting contaminated soil or feces.

In the United States, about 10-thousand cases of Toxocara are reported each year in people. In most cases, the infections are not serious, and many people, especially adults may not notice any symptoms. The most severe cases are rare, but are more likely to occur in young children who often play in dirt or sand contaminated by dog or cat feces. Human can become infected after accidentally swallowing infective Toxocara eggs from larvae in soil or other contaminated surfaces.

Once ingested, the eggs hatch and larvae penetrate the intestinal wall and are then carried by the circulation to various tissues--liver, heart, lungs, brain, muscle, eyes--via blood and lymph. Ingested eggs may remain viable for years. While the larvae do not develop further in these sites, they can cause severe local inflammatory reactions.

With the ocular form, one or more larvae become trapped in the eye, causing a granuloma in the retina. The nematode may reside beneath or within the retina, or it may extend into the choroid or vitreous. It may remain viable for several years in the eye. The nematode may enter the eye via the central retinal artery, and will likely manifest as a peripheral granuloma. If the nematode enters via the short posterior ciliary arteries, the granuloma will likely be at the disc, macula or elsewhere in the posterior pole.

In most cases, Toxocara infections are not serious, and many people, especially adults infected by a small number of larvae (immature worms), may not notice any symptoms. The most severe cases are rare, but are more likely to occur in young children, who often play in dirt, or eat dirt (pica) contaminated by dog or cat stool.

Symptoms:

Toxocariasis can cause abnormal blood test results, enlargement of the liver, and sometimes enlargement of the spleen and lymph nodes. Other symptoms are loss of appetite, fever, muscle and joint pains, abdominal pain, skin rashes and eye problems. These symptoms are due to migration of the larvae through the organs and tissues. Eye infection may result in loss of vision in the affected eye. Only rarely does toxocariasis cause severe disease or death. Symptoms may persist for a year or even longer.

Diagnosis:

If an isolated, elevated granuloma in the optic disc or retina presents with vision loss, the diagnosis can be rather straightforward.

A blood test is available that looks for evidence of infection with Toxocara larvae. In addition to the blood test, diagnosis of toxocariasis includes identifying the presence of typical clinical signs of OLM (ocular larvae migrans) or VLM (visceral larvae migrans) and a history of exposure to cats and dogs.Optical coherence tomography can identify the larvae in the epidermis.

Order the ELISA (enzyme-linked immunoassay) test to check for Toxocara titers.

Treatment:

Visceral toxocariasis is treated with antiparasitic drugs, usually in combination with anti-inflammatory medications. Treatment of ocular toxocariasis is more difficult and usually consists of measures to prevent progressive damage to the eye. Clinicians typically use oral steroids as the organism initiates inflammatory reactions in the eye. For systemic infections, practitioners use the antiparasitic drug mebendazole; it is not clear whether this is an effective ocular treatment. More aggressive treatments include laser photocoagulation and cryoretinopexy.

Preventive Measures:

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.
  • Teach children that it is dangerous to eat dirt or soil.
  • Do not allow children to play in areas that are soiled with pet or other animal stool.
  • Have your veterinarian treat your dogs and cats, especially young animals, regularly for worms.
  • Clean your pet's living area at least once a week. Feces should be either buried or bagged and disposed of in the trash.

NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.

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