Toxocariasis 

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.

Three syndromes of Toxocara infection are generally recognized.

• In children, covert toxocariasis is a mild, subclinical, febrile illness. Symptoms can include cough, difficulty sleeping, abdominal pain, headaches, and behavioral problems. Examination may reveal hepatomegaly, lymphadenitis, and/or wheezing.
• 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.

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:

• Weakness
• Pruritus
• Rash
• Difficulty breathing
• Abdominal pain
• Eosinophilia
• 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:

• Abdominal pain
• Hepatomegaly
• Anorexia
• Nausea
• Vomiting
• Lethargy
• Sleep
• Behavior disorders
• Pneumonia
• Cough
• Wheeze
• Pharyngitis
• Cervical adenitis
• Headache
• Limb pain
• Fever

Toxocariasis should be strongly considered when the patient has eosinophilia, characteristic clinical symptoms, and a positive finding on Toxocara serologic test.

Frequency

United States

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.^[1]

International

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).^{[2, 3]}

Mortality/Morbidity

• 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.

Race

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.

Sex

Toxocariasis has no sexual predilection.

Age

• 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.