Toxocariasis

Toxocariasis is an illness of humans caused by the dog roundworm (Toxocara canis) and, less frequently, the cat roundworm (Toxocara cati). These are the most common intestinal roundworms of dogs, coyotes, wolves and foxes and domestic cats, respectively. Humans are among the many "accidental" or paratenic hosts of these roundworms.

While this zoonotic infection is usually asymptomatic, it may cause severe disease. There are three distinct syndromes of toxocariasis: covert toxocariasis is a relatively mild illness very similar to Löffler's syndrome. It is characterized by fever, eosinophilia, urticaria, enlarged lymph nodes, cough, bronchospasm, wheezing, abdominal pain, headaches, and/or hepatosplenomegaly. Visceral larva migrans (VLM) is a more severe form of the disease; signs and symptoms depend on the specific organ system(s) involved. Lung involvement may manifest as shortness of breath, interstitial lung disease, pleural effusion, and even respiratory failure. Brain involvement may manifest as meningitis, encephalitis, or epileptic seizures. Cardiac involvement may manifest as myocarditis. Ocular larva migrans (OLM) is the third syndrome, manifesting as uveitis, endophthalmitis, visual impairment or even blindness in the affected eye.

Physiological reactions to Toxocara infection depend on the host's immune response and the parasitic load. Most cases of Toxocara infection are asymptomatic, especially in adults. When symptoms do occur, they are the result of migration of second-stage Toxocara larvae through the body.

Covert toxocariasis is the least serious of the three syndromes and is believed to be due to chronic exposure. Signs and symptoms of covert toxocariasis are coughing, fever, abdominal pain, headaches, and changes in behavior and ability to sleep. Upon medical examination, wheezing, hepatomegaly, and lymphadenitis are often noted.

High parasitic loads or repeated infection can lead to visceral larva migrans (VLM). VLM is primarily diagnosed in young children because they are more prone to exposure and ingestion of infective eggs. Toxocara infection commonly resolves itself within weeks, but chronic eosinophilia may result. In VLM, larvae migration incites inflammation of internal organs and sometimes the central nervous system. Symptoms depend on the organs affected. Children can present with pallor, fatigue, weight loss, anorexia, fever, headache, urticaria skin rash, cough, asthma, chest tightness, increased irritability, abdominal pain, nausea, and vomiting. Sometimes the subcutaneous migration tracks of the larvae can be seen. Children are commonly diagnosed with pneumonia, bronchospasms, chronic pulmonary inflammation, hypereosinophilia, hepatomegaly, hypergammaglobulinaemia (IgM, IgG, and IgE classes), leukocytosis, and elevated anti-A and anti-B isohaemagglutinins. Severe cases have occurred in people who are hypersensitive to allergens; in rare cases, epilepsy, inflammation of the heart, pleural effusion, respiratory failure, and death have resulted from VLM.

Ocular larva migrans (OLM) is rare compared with VLM. A light Toxocara burden is thought to induce a low immune response, allowing a larva to enter the host's eye. Although there have been cases of concurrent OLM and VLM, these are extremely exceptional. OLM often occurs in just one eye and from a single larva migrating into and encysting within the orbit. Loss of vision occurs over days or weeks. Other signs and symptoms are red eye, white pupil, fixed pupil, retinal fibrosis, retinal detachment, inflammation of the eye tissues, retinal granulomas, and strabismus. Ocular granulomas resulting from OLM are frequently misdiagnosed as retinoblastomas. Toxocara damage in the eye is permanent and can result in blindness.

Skin manifestations commonly include chronic urticaria, chronic pruritus, and miscellaneous forms of eczema. A case study published in 2008 supported the hypothesis that eosinophilic cellulitis may also be caused by infection with Toxocara: the adult patient presented with eosinophilic cellulitis, hepatosplenomegaly, anemia, and a positive ELISA for T. canis.

Toxocara is usually transmitted to humans through ingestion of infective eggs. T. canis can lay around 200,000 eggs per day. These eggs are passed in cat or dog feces, but the defecation habits of dogs cause T. canis transmission to be more common than that of T. cati. Both Toxocara canis and Toxocara cati eggs require a several week incubation period in moist, humid weather outside a host before becoming infective, so fresh eggs cannot cause toxocariasis.