GENERAL CHARACTERISTICS
Toxocara canis (intestinal ascarid of dogs) and Toxocara cati (intestinal ascarid of cats) are the cause of a human syndrome resulting from larval migration within the host.
EPIDEMIOLOGY
Toxocariasis is a zoonotic disease with worldwide distribution. Humans become infected with the accidental ingestion of eggs (Figure 1). The definitive hosts, dogs (T. canis) and cats (T. cati), pass the larvae transplacentally or lactogenically to their offspring and pass unembryonated eggs in the feces. The eggs mature in 10 to 20 days, and then become infective. Once the eggs are ingested, the larvae are released in the small intestine, penetrate the mucosa, and migrate to the liver, lungs, or other body sites. The larvae migrate up the respiratory tract and are swallowed, returning to the intestinal tract where they mature into adult worms. The adult worms are unable to mature in a human host and therefore wander throughout the body causing the migratory syndromes.
Fig1. Toxocara canis egg. Note the rough appearance on the outer surface of the egg. The egg also contains an infectious L2 larvae. (Courtesy Dr. Henry Travers, Sioux Falls, S.D.)
PATHOGENESIS AND SPECTRUM OF DISEASE
Typically the infections are mild but may be severe. Severe life-threatening infections occur when there is involvement in the heart, brain, or other vital organs.
Disease is frequently found in young children and may persist for long periods with minimal pathologic manifestations. Larvae that remain in the liver or lungs may become encapsulated in fibrous tissue. Visceral (tissue) larva migrans (VLM) may result in a high degree of eosinophilia; however, this may be absent in ocular larva migrans (OLM). Symptoms may include fever, hepatomegaly, hyperglobulinemia, pulmonary infiltration, cough, neurologic symptoms, and endophthalmitis. OLM may result in the development of a granulomatous reaction in the retina of the eye.
LABORATORY DIAGNOSIS
Toxocariasis must be differentiated from other migratory helmintic diseases including A. lumbricoides, S. stercoralis, and Trichinella spp. A history of exposure to dogs and cats is of importance when considering an infection with Toxocara spp. Because humans are an insufficient host for completion of the organism’s life cycle, eggs are not passed in the stool. Diagnosis typically requires biopsy of tissue.
Serologic diagnosis has proven effective, particularly in OLM. Aqueous humor–elevated antibody titer specific for Toxocara spp., in comparison to serum levels, is considered diagnostic. Although serologic testing has been useful, it is important to note that antibody titers may vary depending on the location of the infection. A serum titer of 1 : 8 is considered significant for OLM; 1 : 32 is significant for VLM.
THERAPY
Effective therapy depends on the location of infection but several anthelmintic medications have been used including thiabendazole, ivermectin, albendazole, and diethylcarbamazine. Antiinflammatory medications including corticosteroids may be used to reduce the pathology associated with inflammation. Photocoagulation has been used to treat OLM. The prognosis is good, even in cases of OLM, with prompt diagnosis and proper treatment.
PREVENTION
Small children should be kept out of sandboxes and playgrounds frequented by dogs and cats. Sandboxes should be covered when not in use. Encouraging regular hand washing and teaching children to keep dirt out of their mouths will decrease the potential for infection. In addition, regular deworming of dogs and cats will reduce the spread of infective eggs.
