Toxocara canis

Toxocara canis (also known as dog roundworm) is a worldwide-distributed helminth parasite of dogs and other canids. The name is derived from the Greek word 'toxon,' meaning bow or quiver, and the Latin word 'caro,' meaning flesh. They live in the small intestine of the definitive host. In adult dogs, the infection is usually asymptomatic but may be characterized by diarrhea. By contrast, massive infection with Toxocara canis can be fatal in puppies, causing diarrhea, vomiting, an enlarged abdomen, flatulence, and poor growth rate. As paratenic hosts, a number of vertebrates, including humans, and some invertebrates can become infected. Humans are infected, like other paratenic hosts, by ingestion of embryonated T. canis eggs. The disease (toxocariasis) caused by migrating T. canis larvae results in two syndromes: visceral larva migrans and ocular larva migrans. Owing to transmission of the infection from the mother to her puppies, preventive anthelmintic treatment of newborn puppies is strongly recommended. Several antihelmintic drugs are effective against adult worms, for example pyrantel, fenbendazole, and selamectin. The adult T. canis has a round body with spiky cranial and caudal parts, covered by yellow cuticula. Toxocara canis is gonochoristic. The cranial part of the body contains two lateral alae (length 2–3.5 mm, width 0.1 mm). Male worms measure 9–13 cm by 0.2–0.25 cm and female worms 10–18 cm by 0.25–0.3 cm. T. canis eggs have oval or spherical shapes with granulated surfaces, are thick-walled, and measure from 72 to 85 μm. Eggs are deposited in feces of dogs, becoming infectious after 2–4 weeks. Dogs ingest infectious eggs, allowing the eggs to hatch and the larval form of the parasite to penetrate through the gut wall. In dogs under 3 months of age, the larvae hatch in the small intestine, get into the bloodstream, migrate through the liver, and enter the lungs. Once in the lungs, the larvae crawl up the trachea. The larvae are then coughed up and swallowed, leading back down to the small intestine, where they mature to adulthood. This process is called tracheal migration. In dogs older than 3 months of age, the larvae hatch in the small intestine and enter the bloodstream, where they are carried to somatic sites throughout the body (muscles, kidney, mammary glands, etc.) where they become encysted second stage larvae. This process is called somatic migration. At the height of pregnancy, the encysted eggs in an infected female dog will migrate from the mother to the developing fetus, where they will reside in the liver. After parturition, the larvae migrate from the pup's liver to the lungs to undergo tracheal migration. Alternatively, the migrating larvae in the mother may encyst within the mammary glands, becoming active during lactation and passing directly to the nursing puppy via the milk. Larvae transmitted in this manner do not migrate once they are within the small intestine of the puppy; they will develop directly into the adult stage in the small intestine. Once infected, a female dog will usually harbor sufficient larvae to subsequently infect all of her litters, even if she never again encounters an infection. A certain amount of the female dog's dormant larvae penetrate into the intestinal lumen, where molting into adulthood takes place again, thus leading to a new release of eggs containing L1 larvae.