Toxocariasis

Toxocariasis is an illness of humans caused by larvae (immature worms) of either the dog roundworm (Toxocara canis), the cat roundworm (Toxocara cati) or the fox roundworm (Toxocara canis). Toxocariasis is often called visceral larva migrans (VLM). Depending on geographic location, degree of eosinophilia, eye and/or pulmonary signs, the terms ocular larva migrans (OLM), Weingarten's disease, Frimodt-Møller's syndrome, and eosinophilic pseudoleukemia are applied to toxocariasis. Other terms sometimes or rarely used include nematode ophthalmitis, toxocaral disease, toxocarose, and covert toxocariasis. This zoonotic, helminthic infection is a rare cause of blindness and may provoke rheumatic, neurologic, or asthmatic symptoms. Humans normally become infected by ingestion of embryonated eggs (each containing a fully developed second stage larva, L2) from contaminated sources (soil, undercooked meat, fresh or unwashed vegetables.)Species include: Toxocariasis is an illness of humans caused by larvae (immature worms) of either the dog roundworm (Toxocara canis), the cat roundworm (Toxocara cati) or the fox roundworm (Toxocara canis). Toxocariasis is often called visceral larva migrans (VLM). Depending on geographic location, degree of eosinophilia, eye and/or pulmonary signs, the terms ocular larva migrans (OLM), Weingarten's disease, Frimodt-Møller's syndrome, and eosinophilic pseudoleukemia are applied to toxocariasis. Other terms sometimes or rarely used include nematode ophthalmitis, toxocaral disease, toxocarose, and covert toxocariasis. This zoonotic, helminthic infection is a rare cause of blindness and may provoke rheumatic, neurologic, or asthmatic symptoms. Humans normally become infected by ingestion of embryonated eggs (each containing a fully developed second stage larva, L2) from contaminated sources (soil, undercooked meat, fresh or unwashed vegetables.) Toxocara canis and Toxocara cati are perhaps the most ubiquitous gastrointestinal worms (helminths) of domestic dogs, cats, and foxes. There are many 'accidental' or paratenic hosts including humans, birds, pigs, rodents, goats, monkeys, and rabbits. In paratenic hosts, the larvae never mature and remain at the L2 stage. There are three main syndromes: visceral larva migrans (VLM), which encompasses diseases associated with major organs; covert toxocariasis, which is a milder version of VLM; and ocular larva migrans (OLM), in which pathological effects on the host are restricted to the eye and the optic nerve. 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 organ(s) affected. Patients can present with pallor, fatigue, weight loss, anorexia, fever, headache, rash, cough, asthma, chest tightness, increased irritability, abdominal pain, nausea, and vomiting. Sometimes the subcutaneous migration tracks of the larvae can be seen. Patients are commonly diagnosed with pneumonia, bronchospasms, chronic pulmonary inflammation, hypereosinophilia, hepatomegaly, hypergammaglobulinaemia (IgM, IgG, and IgE classes), leukocytosis, and elevated anti-A and –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. A case study published in 2008 supported the hypothesis that eosinophilic cellulitis may also be caused by infection with Toxocara. In this study, the adult patient presented with eosinophilic cellulitis, hepatosplenomegaly, anemia, and a positive ELISA for T. canis. The incubation period for Toxocara canis and cati eggs depends on temperature and humidity. T. canis females, specifically, are capable of producing up to 200,000 eggs a day that require 2–6 weeks minimum up to a couple months before full development into the infectious stage. Under ideal summer conditions, eggs can mature to the infective stage after two weeks outside of a host. Provided sufficient oxygen and moisture availability, Toxocara eggs can remain infectious for years, as their resistant outer shell enables the protection from most environmental threats. However, as identified in a case study presented within the journal of helminthology, the second stage of larvae development poses strict vulnerabilities to certain environmental elements. High temperatures and low moisture levels will quickly degrade the larvae during this stage of growth.