Toxocariasis is caused by helminth larvae of dogs and cats that ordinarily cannot complete their life cycle in humans (Fig. 326-1). Migrating larvae of zoonotic ascarids may be associated with significant pathology by wandering through extraintestinal viscera, causing tissue necrosis and provoking eosinophilic granulomatous inflammation. The clinical syndromes of visceral larva migrans (VLM), ocular larva migrans (OLM), or covert toxocariasis are most commonly caused by larvae of the dog ascarid Toxocara canis and, less frequently, the cat ascarid Toxocara cati. Covert toxocariasis refers to infection with either T canis or T cati that results in either asymptomatic infection or infection associated only with asthma, wheezing, or other pulmonary dysfunctions due to larval lung migrations, or possibly developmental delays due to larval migrations through the brain.
The life cycle of Toxocara canis.
PATHOGENESIS AND EPIDEMIOLOGY
Adult Toxocara live in the dog’s small intestine and are 8 to 12 cm long. The ova are deposited with the dog’s feces and become infective in approximately 2 weeks. If swallowed by young dogs, second-stage larvae hatch in the small intestine, penetrate the intestinal wall, and migrate through canine tissues where they can undergo arrested development. Some larvae return to the small intestine, where they mature, mate, and oviposit. Arrested larval development more often occurs in female dogs than males, and the dormant larvae in tissues can migrate transplacentally (or possibly enter the mammary tissues) and thereby serve as a source of perinatal and postnatal infection in puppies. In the United States, large numbers of newborn puppies are infected and pose a health risk to those who handle them.
In humans, most VLM cases have been reported in young children 1 to 4 years of age with a history of pica, especially geophagy (defined as the eating of earthy substances). However, OLM is more frequently seen in older children. After a human ingests the embryonated egg, a second-stage larva emerges in the small intestine, penetrates the intestinal wall, and initiates somatic migration that may last for many weeks or months.
The liver and lungs are most often involved with Toxocara, the former probably because of the mesenteric venous portal drainage. Tissue granulomas consist of many eosinophils and histiocytes, with an occasional multinucleated foreign body giant cell in an area of necrosis. A portion of a second-stage larva also may be evident. Granulomas can also be found in lung, kidney, lymph node, eyes, brain, heart, and skeletal muscle. OLM results from granulomas in the eye, typically without evidence of additional visceral involvement. The prevalence of OLM has been estimated to be approximately 1 case per 1000 persons or approximately 1% of people with vision loss.
Toxocariasis is one of the most common human helminth infections in North America ...