- Author: Sun Huh, MD, PhD; Chief Editor: Mark R Wallace, MD, FACP, FIDSA more...
Toxocariasis is an infection caused by the ingestion of larvae of the dog roundworm Toxocara canis or the cat roundworm Toxocara cati. The soil of parks and playgrounds is commonly contaminated with the eggs of T canis, and infection may cause human disease that involves the liver, heart, lung, muscle, eye, and brain.[1, 2]
Three syndromes of Toxocara infection are generally recognized, as follows:
Visceral larva migrans is caused by the migration of larvae through the internal organs of humans and the resulting inflammatory reaction. A constellation of symptoms develops, including fatigue, anorexia, weight loss, pneumonia, fever, cough, bronchospasm, abdominal pain, headaches, rashes, and, occasionally, seizures. Examination may reveal hepatomegaly, lymphadenitis, and/or wheezing. Occasionally, pleural effusions develop. Chronic urticaria has been described. Severe cases can lead to myocarditis or respiratory failure.
Ocular larva migrans, which is caused by migration of larva into the posterior segment of the eye, tends to occur in older children and young adults. Patients may present with decreased vision, red eye, or leukokoria (white appearance of the pupil). Granulomas and chorioretinitis can be observed in the retina, especially at the macula. Unilateral visual loss, retinal fibrosis, retinoblastoma, and retinal detachment occur. Serum antibodies to Toxocara are often absent or present in low titers.
Prevention of toxocariasis is obviously preferable, but eradicating T canis infection is difficult because of the complexity of its life cycle. Good hygiene practices, timely disposal of pet feces, and routine deworming of pets are strategies necessary to reduce ocular toxocariasis in humans.
Diagnosis of toxocariasis is difficult because confirmation of infection requires demonstration of larvae via biopsy. Therefore, clinicians use serologic testing (eg, enzyme-linked immunosorbent assay [ELISA], immunoblot) to infer diagnosis. Fortunately, toxocariasis usually carries a very good prognosis.
Adult worms of the Toxocara species live in the small intestine of dogs and puppies and range from 4-12 cm in length. Almost all puppies are infected at or soon after birth. During the summer, in wet conditions, Toxocara eggs are embryonated in 2-5 weeks and become infective. They survive for years in the environment, and humans typically ingest the eggs via oral contact with contaminated hands. Once introduced into the human intestine, the eggs decorticate, releasing the larvae. The larval form is visible only under a microscope because it is less than 0.5 mm in length and 0.02 mm wide. The larvae penetrate the bowel wall and migrate through vessels to the muscles, liver, and lung and sometimes to the eye and brain.
Disease severity depends not only on the number of larvae ingested but also on the degree of the allergic reaction. Patients with atopy may experience more severe toxocariasis. The pathologic manifestations result from inflammation caused by the immune response directed against the excretory-secretory antigens of larvae. These antigens are released from their outer epicuticle coat, which is readily sloughed off when bound by specific antibodies. These antigens are a mix of glycoproteins, including a potent allergenic component named TBA-1. The inflammatory reaction causes epithelioid cells to surround each larva, and, subsequently, a dense fibrous capsule invests each granuloma.
Although the main clinical manifestations vary depending on the organs infected, the most common characteristic is chronic eosinophilia. Other typical findings follow according to the involved organs. With liver involvement, hepatomegaly, fever, and abdominal pain are common. With lung involvement, pulmonary symptoms (eg, dyspnea, cough, chest tightness), bronchospasm, interstitial pneumonitis, and, possibly, pleural effusion can be present. Ocular toxocariasis can induce decreased visual acuity, uveitis, retinal granuloma, endophthalmitis, and other ocular lesions that often lead to sudden vision loss in the affected eye. If the brain is involved, neurologic manifestations may occur, including seizures.
Because the anti-Toxocara immunoglobulin-positive population is much higher than the prevalence of clinical toxocariasis, most patients are thought to have subclinical infection. In French adults, toxocariasis is termed common toxocariasis and is clinically characterized by the following:
Increased total serum immunoglobulin E (IgE) level
Elevated antibody titers to T canis
In Irish children with high anti-Toxocara titers, the condition is termed subclinical toxocariasis, and the most frequent clinical findings are as follows:
Toxocariasis should be strongly considered when the patient has eosinophilia, characteristic clinical symptoms, and a positive finding on Toxocara serologic test.
In the United States, about 10,000 cases of Toxocara infection are reported in humans each year. Almost 14% of the US population is infected with Toxocara, a parasite of dogs and cats that can be passed from animals to humans. In Cleveland, Ohio, the prevalence of T canis infection in the cohort ages 2, 3, and 4-10 years was 2%, 12%, and 12%, respectively.
The US Centers for Disease Control and Prevention (CDC) reported on human ocular toxocariasis from 2009 through 2010. They described the results of a Web-based survey distributed to uveitis, retinal, and pediatric ophthalmologic specialists from around the United States. Epidemiologic, demographic, and clinical information was collected on 68 patients. Among the 44 patients, the median patient age was 8.5 years (range, 1-60 y) and 25 patients (57%) lived in the South at the time of diagnosis.
Toxocariasis is a worldwide infection. Seroepidemiological surveys show a 2-5% positive rate in healthy adults from urban Western countries and 14.2-37% in rural areas. In tropical countries, surveys show a positive rate of 63.2% in Bali, 86% in Saint Lucia (West Indies), and 92.8% in La Reunion (French Overseas Territories, Indian Ocean).[5, 6] Research also suggests the prevalence of the disease in North America is significant.
Toxocariasis is almost always a benign, asymptomatic, and self-limiting disease, although brain involvement can cause severe morbidity. Brain involvement can evoke meningitis, encephalitis, or epilepsy.
Ocular involvement may cause loss of visual acuity or unilateral blindness.
Pulmonary and hepatic forms can cause protracted symptoms if the patient does not receive treatment.
No ethnic predilection for toxocariasis has been reported. However studies by the US Centers for Disease Control and Prevention (CDC) have found that, among all age groups, toxocariasis is more common in non-Hispanic blacks than in Mexican Americans and non-Hispanic whites.
Toxocariasis has no sexual predilection.
CDC studies have shown that transmission of Toxocara larvae is most common in young children and persons younger than 20 years.
Toxocariasis is predominantly a disease of children, typically those aged 2-7 years.
Ocular toxocariasis is most common in older children and young adults.
Moreira GM, Telmo PD, Mendonça M, Moreira AN, McBride AJ, Scaini CJ, et al. Human toxocariasis: current advances in diagnostics, treatment, and interventions. Trends Parasitol. 2014 Sep. 30(9):456-464. [Medline].
Centers for Disease Control and Prevention. Ocular toxocariasis--United States, 2009-2010. MMWR Morb Mortal Wkly Rep. 2011 Jun 10. 60(22):734-6. [Medline].
Nelson S, Greene T, Ernhart CB. Toxocara canis infection in preschool age children: risk factors and the cognitive development of preschool children. Neurotoxicol Teratol. 1996 Mar-Apr. 18(2):167-74. [Medline].
Chomel BB, Kasten R, Adams C, et al. Serosurvey of some major zoonotic infections in children and teenagers in Bali, Indonesia. Southeast Asian J Trop Med Public Health. 1993 Jun. 24(2):321-6. [Medline].
Magnaval JF, Michault A, Calon N, et al. Epidemiology of human toxocariasis in La Réunion. Trans R Soc Trop Med Hyg. 1994 Sep-Oct. 88(5):531-3. [Medline].
CDC. CDC Guidelines for Veterinarians: Prevention of Zoonotic Transmission of Ascarids and Hookworms of Dogs and Cats. Available at http://www.cdc.gov/NCIDOD/DPD/PARASITES/ascaris/prevention.htm.
CDC. Centers for Disease Control and Prevention (CDC). Available at http://www.cdc.gov/NCIDOD/DPD/PARASITES/toxocara/factsht_toxocara.htm.
Choi D, Lim JH, Choi DC, et al. Transmission of Toxocara canis via Ingestion of Raw Cow Liver: A Cross-Sectional Study in Healthy Adults. Korean J Parasitol. 2012 Mar. 50(1):23-7. [Medline]. [Full Text].
Congdon P, Lloyd P. Toxocara infection in the United States: the relevance of poverty, geography and demography as risk factors, and implications for estimating county prevalence. Int J Public Health. 2011 Feb. 56(1):15-24. [Medline].
Iddawela RD, Rajapakse RP, Perera NA, et al. Characterization of a Toxocara canis species-specific excretory-secretory antigen (TcES-57) and development of a double sandwich ELISA for diagnosis of visceral larva migrans. Korean J Parasitol. 2007 Mar. 45(1):19-26. [Medline].
Kwon SI, Lee JP, Park SP, Lee EK, Huh S, Park IW. Ocular toxocariasis in Korea. Jpn J Ophthalmol. 2011 Mar. 55(2):143-7. [Medline].
Magnaval JF. Comparative efficacy of diethylcarbamazine and mebendazole for the treatment of human toxocariasis. Parasitology. 1995 Jun. 110 (Pt 5):529-33. [Medline].
Magnaval JF, Fabre R, Maurieres P, et al. Application of the western blotting procedure for the immunodiagnosis of human toxocariasis. Parasitol Res. 1991. 77(8):697-702. [Medline].
Magnaval JF, Fabre R, Maurieres P, et al. Evaluation of an immunoenzymatic assay detecting specific anti-Toxocara immunoglobulin E for diagnosis and posttreatment follow-up of human toxocariasis. J Clin Microbiol. 1992 Sep. 30(9):2269-74. [Medline].
Magnaval JF, Glickman LT, Dorchies P, et al. Highlights of human toxocariasis. Korean J Parasitol. 2001 Mar. 39(1):1-11. [Medline].
Magnaval JF, Malard L, Morassin B, et al. Immunodiagnosis of ocular toxocariasis using Western-blot for the detection of specific anti-Toxocara IgG and CAP for the measurement of specific anti-Toxocara IgE. J Helminthol. 2002 Dec. 76(4):335-9. [Medline].
Moiyadi A, Mahadevan A, Anandh B, et al. Visceral larva migrans presenting as multiple intracranial and intraspinal abscesses. Neuropathology. 2007 Aug. 27(4):371-4. [Medline].
Muñoz-Guzmán MA, del Río-Navarro BE, Valdivia-Anda G, Alba-Hurtado F. The increase in seroprevalence to Toxocara canis in asthmatic children is related to cross-reaction with Ascaris suum antigens. Allergol Immunopathol (Madr). 2010 May-Jun. 38(3):115-21. [Medline].
Park HY, Lee SU, Huh S, et al. A seroepidemiological survey for toxocariasis in apparently healthy residents in Gangwon-do, Korea. Korean J Parasitol. 2002 Sep. 40(3):113-7. [Medline].
Park SP, Huh S, Magnaval JF, et al. A case of presumed ocular toxocariasis in a 28-year old woman. Korean J Ophthalmol. 1999 Dec. 13(2):115-9. [Medline].
Park SP, Park I, Park HY, et al. Five cases of ocular toxocariasis confirmed by serology. Korean J Parasitol. 2000 Dec. 38(4):267-73. [Medline].
Smith HV. Antibody reactivity in human toxocariasis. Lewis JW, Maizels RM, eds. Toxocara and Toxocariasis: Clinical, Epidemiological, and Molecular Perspectives. London: Institute of Biology and the British Society for Parasitology; 1993. 91-109.
Sun T, Bellosa ML, Lucio-Forster A, et al. A comparison of the protein constituents of the major body compartments of the dog roundworm, Toxocara canis. Vet Parasitol. 2007 Nov 30. 150(1-2):111-5. [Medline].
Won K, Kruszon-Moran D, Schantz P, et al. Abstract of the 56th American Society of Tropical Medicine and Hygiene. National seroprevalence and risk factors for zoonotic Toxocara spp. infection. Philadelphia, Pennsylvania; Nov 4-8, 2007.