Toxocariasis

Toxocariasis is a zoonotic (animal to human) infection caused by the parasitic roundworms commonly found in the intestine of dogs (Toxocara canis) and cats (T. cati). Toxocara canis and T. cati are found worldwide in the soil. Toxocariasis is one of the most commonly reported zoonotic helminth infections in the world.

Dogs and other canids are the definitive hosts for T. canis. Mature worms, found in the intestines, shed large numbers of unembryonated eggs into the feces. The eggs become embryonated in the environment, in approximately 9-15 days at the optimum humidity and temperature (25-30ºC), and 35 days at 16.5ºC. Larvae do not develop at temperatures less than 10°C and die below -15°C. Cold temperatures can delay development for months or years. Only embryonated eggs are infectious.

When a dog ingests embryonated eggs, the larvae hatch in the intestines. In puppies less than 4 to 5 weeks old, the larvae penetrate the intestinal wall and are carried in the bloodstream to the lungs, where they enter the alveoli and migrate up the bronchioles, bronchi and trachea. Larvae in the throat are swallowed. When the parasites reach the intestines a second time, they develop into adults, mate and release eggs. Immature larvae can also be found occasionally in the feces. Dogs of any age can develop patent infections if they eat tissues containing dormant (hypobiotic) larvae – for instance, larvae found in prey. These larvae can mature in the dog's intestines without further migration.

Most human infections occur in small children who eat dirt that contains embryonated eggs. Humans can also be infected if they ingest eggs on unwashed hands or in contaminated food or water, or larvae in raw or undercooked tissues (particularly the liver).

Signs of Toxocariasis

Young puppies usually have the most severe signs of toxocariasis. The typical symptoms include poor growth, loss of condition and sometimes an enlarged abdomen (“potbelly”). Worms may be passed in the feces or vomited. Other possible symptoms are diarrhea, constipation, vomiting, flatulence, coughing or nasal discharge. Chronic enteritis can result in thickening of the intestinal walls or intussusception. In severe cases, puppies may die from obstruction of the gall bladder, bile duct or pancreatic duct, or rupture of the intestine and peritonitis. Intestinal infections with small numbers of parasites tend to have no signs.

The passage of the larvae through the liver and lungs can result in inflammation and respiratory distress of varying severity. Pneumonia can be seen soon after birth if the puppy was infected in utero; affected puppies may die within 2 or 3 days of birth. Severe infections can also cause ascites, fatty degeneration of the liver, secondary bacterial pneumonia or chronic stunting. Myocarditis is a rare complication.

In dogs, T. canis larvae can cause petechial hemorrhages and pneumonia during their migration through the lungs, and larvae may be found in the pleural cavity and diaphragm. Severe inflammation may be seen in the liver, and ascites and fatty degeneration of the liver have been reported.

Diagnosis of Toxocariasis

Diagnosis is based on fecal examination or ELISA test.

Treatment of Toxocariasis

Anthelmintics are effective for worms in the intestines, but larvae in the tissues are resistant to treatment. In dogs, parasites that renew their migration during pregnancy are susceptible to various drugs but treatment of pregnant animals is controversial. These drugs have not been tested in pregnant cats. Heartworm preventative programs help to control Toxocara infections.

Prevention of Toxocariasis

Puppies and kittens should be dewormed to eliminate the shedding of eggs. Adult animals may also need to be treated for patent infections. Removal of feces and thorough cleaning is important in kennels. Cats and dogs allowed to hunt and eat rodents or other prey are more likely to be infected. Contamination can be decreased in public areas by restrictions on uncontrolled dogs and cats, collection of feces by dog owners and prevention of animal access to public places such as children’s playgrounds. Puppies from 3 weeks to 3 months old excrete large numbers of T. canis eggs and are the greatest hazard. Cats shed T. cati particularly between the ages of 2 and 6 months. Dogs excrete large numbers of Toxocara eggs; even a mildly infected dog will shed 10,000 eggs in each gram of feces.

There are four forms of the infection: visceral larva migrans, ocular larva migrans, and the more recently recognized "common" (in adults) and "covert" (in children). Poor personal hygiene as well as consumption of raw vegetables grown in contaminated kitchen gardens may result in chronic low-dose infections.

Ocular larva migrans (OLM) typically occurs unilaterally in children and young adults. The most common symptom is visual loss, with onset over a period of days to weeks. In some individuals these signs may wax and wane over a period of years. Ocular infection may also be subclinical and only detected during a routine eye examination. OLM apparently is an endemic disease in some areas with an estimated incidence of 1 per 100,000 persons in Alabama.

Toxocara eggs are very resistant to chemical disinfectants but can be destroyed by aqueous iodine, ultraviolet light (direct sunlight), high temperatures and prolonged drying. Toxocara eggs can survive both composting and sewage treatment.

References:
1. Toxocariasis, Institute for International Cooperation in Animal Biologics An OIE Collaborating Center, Iowa State University College of Veterinary Medicine, The Center for Food security and Public Health.
2. Highlights of human toxocariasis. Jean-François Magnaval, Lawrence T. Glickman, Philippe Dorchies, and Bruno Morassini. In: The Korean Journal of Parasitology, Vol. 39, No. 1, 1-11, March 2001