(See also Approach to Parasitic Infections Approach to Parasitic Infections Human parasites are organisms that live on or in a person and derive nutrients from that person (its host). There are 3 types of parasites: Single-cell organisms (protozoa, microsporidia) Multicellular... read more .)
Pathophysiology of Toxocariasis
The eggs of Toxocara canis, T. cati, and other animal ascarid helminths mature in soil and infect dogs, cats, and other animals. Humans may accidentally ingest eggs in soil contaminated by stool from infected animals or may ingest undercooked infected transfer hosts (eg, rabbits). The eggs hatch in the human intestine. Larvae penetrate the bowel wall and may migrate through the liver, lungs, central nervous system (CNS), eyes, or other tissues. Tissue damage is caused by the parasite and the local immune response it elicits.
The larvae usually do not complete their development in the human body but can remain alive for many months.
Symptoms and Signs of Toxocariasis
Visceral larva migrans
Visceral larva migrans (VLM) consists of fever, anorexia, hepatosplenomegaly, rash, pneumonitis, and asthmatic symptoms, depending on the affected organs. Larvae of other helminths including Baylisascaris procyonis, Strongyloides species, and Paragonimus species can cause similar symptoms and signs when they migrate through tissue.
VLM occurs mostly in 2- to 5-year-old children with a history of geophagia or in adults who ingest clay.
The syndrome is self-limiting in 6 to 18 months if egg intake ceases. Deaths due to invasion of the brain or heart occur rarely.
Ocular larva migrans
Ocular larva migrans (OLM), also called ocular toxocariasis, is usually unilateral and has no or very mild systemic manifestations. OLM lesions consist mostly of granulomatous inflammatory reactions to a larva, resulting in uveitis Overview of Uveitis Uveitis is defined as inflammation of the uveal tract—the iris, ciliary body, and choroid. However, the retina and fluid within the anterior chamber and vitreous are often involved as well.... read more and/or chorioretinitis. As a result, vision can be impaired or lost.
OLM occurs in older children and less commonly in young adults. The lesion may be confused with retinoblastoma or other intraocular tumors.
Diagnosis of Toxocariasis
Enzyme immunoassay for antibodies against Toxocara plus clinical findings
Diagnosis of toxocariasis is based on clinical, epidemiologic, and serologic findings.
For visceral larva migrans (VLM), enzyme immunoassay (EIA) for antibodies against Toxocara is recommended to confirm the diagnosis. Isoagglutinins may be elevated, but the finding is nonspecific. CT or MRI can show multiple, ill-defined, 1.0- to 1.5-cm oval lesions scattered in the liver or poorly defined subpleural nodules in the chest.
Hypergammaglobulinemia, leukocytosis, and marked eosinophilia Eosinophilia Eosinophilia is defined as a peripheral blood eosinophil count > 500/mcL (> 0.5 × 109/L). Causes and associated disorders are myriad but often represent an allergic reaction or a parasitic infection... read more are common in VLM.
Biopsies of the liver or other affected organs may show eosinophilic granulomatous reactions, but larvae are difficult to find in tissue sections and biopsies are low yield. Stool examinations are worthless.
For ocular larva migrans (OLM), ophthalmologic expertise is essential for diagnosis. Granulomatous reactions appear as oval, white lesions in the posterior pole or periphery of the retina. Some patients present with endophthalmitis manifesting as a red, painful eye with diffuse intraocular inflammation.
The presence of anti-Toxocara antibodies and characteristic ophthalmologic findings are helpful in differentiating OLM from retinoblastoma Retinoblastoma Retinoblastoma is a cancer arising from the immature retina. Symptoms and signs commonly include leukocoria (a white reflex in the pupil), strabismus, and, less often, inflammation and impaired... read more and preventing unnecessary surgical enucleation of the eye. Unfortunately anti-Toxocara antibody titers can be low or undetectable in patients with OLM.
Treatment of Toxocariasis
Albendazole or mebendazole
Asymptomatic patients and patients with mild visceral larva migrans (VLM) symptoms do not require anthelmintic therapy because infection is usually self-limited.
For patients with moderate to severe symptoms, albendazole 400 mg orally twice a day for 5 days or mebendazole 100 to 200 mg orally twice a day for 5 days is used, but the optimal duration of therapy has not been determined.
Antihistamines may suffice for mild symptoms. Corticosteroids (prednisone 20 to 40 mg orally once a day) are indicated for patients with severe symptoms.
Ophthalmologic expertise is essential in the care of ocular larva migrans (OLM). Corticosteroids, both local and oral, are indicated to reduce inflammation within the eye. The role of anthelmintic therapy is uncertain. Albendazole used with corticosteroids may reduce recurrences, but comparative data are not available on the optimal dose and duration of therapy, and there is no evidence that albendazole improves visual outcome. Unfortunately, almost all patients have visual impairment.
Laser photocoagulation has been used to kill larvae in the retina. Cryosurgery or surgical vitrectomy have been used in some circumstances.
Prevention of Toxocariasis
Infection with T. canis in puppies is common in the United States; infection with T. cati in cats is less common. Both animals should be dewormed regularly. Contact with dirt or sand contaminated with animal feces should be minimized. Sandboxes should be covered.
The Toxocara canis life cycle normally involves dogs; humans are infected only accidentally, when they ingest eggs in soil contaminated by stool from infected animals or ingest undercooked infected transfer hosts (eg, rabbits).
In humans, toxocariasis causes 2 main syndromes: visceral larva migrans (which causes various symptoms depending on the organ infected) and ocular larva migrans (which usually causes no or mild symptoms but can result in impaired or lost vision).
Diagnose based on clinical evaluation and enzyme immunoassay for Toxocara antigens.
Most cases of visceral larva migrans are self-limited and do not require treatment, but if needed, the following can be used: albendazole or mebendazole for moderate to severe symptoms, possibly antihistamines for mild symptoms, and corticosteroids for severe symptoms.
For ocular larva migrans, systemic and local steroids, sometimes albendazole, and laser therapy, cryotherapy, or surgical procedures depending on the circumstances are used.
Deworming dogs and cats can help prevent toxocariasis.