Toxocariasis is an important parasitic disease of humans caused by infection with the second stage larvae of two main species of parasitic nematodes, Toxocara canis and T.cati. The role of toxocaras in human disease was disclosed in the 1950th by Wilder and Beaver et al. who first identified toxocara larvae in ocular and visceral tissues respectively.
Adult worms of T. canis and T.cati live within the lumen of the small intestine of dogs and cats respectively which serve as definitive hosts for these helminths. Unembryonated eggs produced by adult worms are shed in the feces of the definitive hosts. Eggs embryonate in the environment, and could be ingested by definitive hosts as well as accidental, paratenic hosts. After ingestion by the definitive hosts, microscopic (300 µm long and 20 µm in diameter) larvae hatch in the intestine, penetrate the intestinal wall, reach lungs via bloodstream, penetrate alveolar walls, ascend the bronchial tree to the throat where they are swallowed, and mature into adult worms. In humans and other paratenic hosts, the larvae are unable to undergo the full development cycle described above; instead, they are carried by the circulation to a wide variety of organs and tissues (liver, heart, lungs, brain, muscle, eyes), and cause severe local reactions that are the basis of toxocariasis.
The degree of host damage, and the concomitant signs and symptoms, varies with regard to which tissue has been invaded; the liver, lungs, and CNS, including the eyes, appear to be most sensitive. In the eye, migrating larvae can damage the retina, inducing granulomatous reactions leading to impaired sight or even loss of sight. The number of migrating larvae and the age of the host are two additional factors defining the presence and severity of symptoms.
Lifespan of the larvae could be as long as several years, and clinical disease could present in any time during this period, or even later, representing pathological immune response to dying or dead larvae.
For most people, an infection with these helminths causes no symptoms. Possible symptoms symptoms are nonspecific and include fever, fatigue, anorexia, or lymphadenopathy. Pulmonary symptoms and abdominal symptoms are similar to those under many other diseases; they are present when larvae migrate to the lungs or abdominal organs, respectively. Neurologic findings are diverse and also nonspecific.
Although no single ophthalmologic sign or symptom is specific, the presence of certain granulomas is highly suggestive of toxocara infection.
Humans or animals can be infected by accidentally ingesting toxocara eggs shed by dogs and cats. Although a less common route of transmission, humans also can become infected with toxocara by eating undercooked meat from an animal infected with toxocara larvae.
Humans, mainly children accidentally come into contact with toxocara eggs when they play in contaminated sandboxes and on playgrounds because of indiscriminate defecation on these sites by cats and dogs that harbor the adult worms.
Eggs have a resistant outer shell composed of ascarosides, which enables them to withstand various harsh chemicals (e.g., high concentrations of formalin and various inorganic acids), extreme temperature changes, and various degrees of moisture.
Risk factors for toxocara infection include contact with puppies <3 months of age, and a history of geophagia. Toxocara is more prevalent in hot, humid regions where eggs are kept viable in the soil all year round. Visceral toxocariasis (VT) is mainly a disease of small children. On the other hand, ocular toxocariasis (OT) occurs in children older than 8 years of age.
Typical levels of seropositivity in developed countries are 5-15%, but can reach as high as 40% or more in such countries as Indonesia and Brazil. Toxocariasis is associated with low socio-economic status.
Toxocara larvae secrete and excrete compounds that are highly immunogenic. At presentation, the great majority of patients have high-avidity anti-toxocara IgG antibodies and eosinophilia. The seroprevalence among patients with clinical symptoms for IgA and IgE was 47.8% and 78.3%. Specific IgM antibodies occur in both acute and chronic phases, differing from most infections in which they are transient.
After successful treatment, IgA and IgE antibodies disappear, but IgG remains elevated for many years. An increase in IgG titers is often observed soon after treatment, which could be explained by destruction of larvae and subsequent antigen release.
Investigations into the immunogenic proteins of toxocara have mainly focused on the secreted proteins of the migrating juvenile stages (toxocara excretory-secretory (TES) proteins). TES proteins constitute a family of at least six highly antigenic proteins associated with the cuticular surface. The predominant bands are TES 120 and TES 32. These proteins are highly glycosylated and contain both protein and carbohydrate epitopes.
Native TES antigen might cause false positives to blood group-like antigens. Therefore, both complete TES antigen as well as its separate more specific components are currently used for immunodetection purposes.
The highly specific direct method for definitive diagnosis of toxocariasis can be provided by patho-histological examination for toxocara larvae in biopsy materials. However, it is very difficult to detect larvae in tissues, and therefore biopsy is not usually recommended. Contrary to many other parasitic infections, stool examination for ova and parasites is not suitable to diagnose toxocara infection because larvae do not develop into adult worms and eggs are not excreted in human feces. Therefore, diagnosis of toxocariasis is based on the presence of signs and symptoms (i.e. pediatric patient with unexplained febrile illness and eosinophilia), history of exposure to a potential source of infectious eggs, and positive results by serological testing. Western blot analyses to detect antibodies directed against the TES is commonly used for confirmatory immunodiagnosis.
OT is diagnosed primarily on the basis of clinical criteria during an ophthalmologic examination. The immunodiagnostic tests used for VT are not as reliable for OT.
Besides their use for diagnostic purposes, ELISA tests could be utilized for seroepidemiologic surveys and for treatment evaluation of toxocariasis patients.
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