Toxocariasis is one of the most common
human helminth infections in North America and Europe, with the
highest occurrence among disadvantaged minority populations.1-3 There
are 3 major forms of the disease: visceral larva migrans (VLM),
ocular larva migrans (OLM), and covert toxocariasis.
VLM and OLM are usually caused by helminth larvae of dogs and cats
that ordinarily cannot complete their life cycle in humans (eFig. 331.1).
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 syndrome of VLM and OLM are most commonly caused by
larvae of the dog ascarid Toxocara canis and, less
frequently, the cat ascarid T 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 and wheezing.2 Seroprevalence
surveys show presence of antibody in up to 50% of African
American and Hispanic children living in poverty in the United States.1-3 Infection
is common in both urban and rural areas, and some studies have linked
environmental exposure to Toxocara larvae as a
contributor to asthma.2,3
Life cycle of Toxocara canis.
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 infections have been reported in young children
1 to 4 years of age with a history of pica, especially geophagy.
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
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. The syndrome produced by granulomas
from toxocariasis in the eye is termed ocular ...