Toxocariasis is one of the most commonly reported zoonotic helminth infections in the world that is caused by the parasitic roundworms Toxocara canis and Toxocara cati. Most human infections occur in small children who eat dirt that contains embryonated eggs. Humans can also be infected if they ingest eggs in contaminated food, water, or larva in raw or undercooked meat. Ocular and visceral larva migrans, as well as worsening of asthmatic allergies, are often associated with Toxocara spp. infection in humans.3 Dogs and other canids are the definitive hosts for T. canis. Cats and dogs allowed to hunt and eat rodents or other prey are more likely to be infected.
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. Immature larvae can also be found occasionally in the feces. When the parasites reach the intestines a second time, they develop into adults, mate, and release eggs. Mature worms shed large numbers of unembryonated eggs into the feces. The eggs become embryonated in the environment. 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.
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Clinical signs depend on the stage and severity of the infection, as well as the age of the dog. Young puppies usually have the most severe signs of toxocariasis. The typical symptoms include poor growth, loss of condition, and an enlarged abdomen ("potbelly"). Worms may be passed in the feces or vomited. Other possible symptoms are diarrhea, constipation, vomiting, flatulence, coughing, and 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, as well as 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 disease 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 older dogs, T. canis larvae can cause skin bruising (due to bleeding into tissues) and pneumonia. Severe inflammation and fatty degeneration of the liver have been reported. Diagnosis is based on fecal examination or ELISA test. Anthelmintics are effective for worms in the intestines, but larvae in the tissues are resistant to treatment. Parasites that renew their migration during pregnancy are susceptible to various drugs, but treatment of pregnant animals is controversial. Anthelmintics have not been tested in pregnant cats. Heartworm preventative programs help control toxocariasis.
Prevention of Toxocariasis
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. Puppies and kittens should be dewormed to eliminate the shedding of eggs. Adult animals may also need to be treated for hidden infections.Toxocara eggs are very resistant to chemical disinfectants and can survive both composting and sewage treatment. At the same time, they can be destroyed by aqueous iodine, ultraviolet light (direct sunlight), high temperatures, and prolonged drying. Removal of feces and thorough cleaning is important in kennels. In public areas contamination can be decreased by collection of feces by dog owners and prevention of animal access to public places such as children's playgrounds. Currently, the European Scientific Counsel Companion Animal Parasites (ESCCAP) recommends to deworm adult dogs (>6 months of age) at least four times a year to reduce the impact of patent infections on the environmental contamination with Toxocara eggs.3
- 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
- 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
- Environmental contamination with Toxocara eggs: a quantitative approach to estimate the relative contributions of dogs, cats and foxes, and to assess the efficacy of advised interventions in dogs