Toxocarosis - Overview

Toxocariasis (Latin: toxocarosis) is a chronic tissue helminthiasis caused by the migration of larvae of the dog helminth Toxocara canis in the human body. It is characterized by a recurrent course with damage to internal organs and eyes.

ICD-10 code

B83.0. Visceral larva migrans.

Epidemiology of toxocariasis

Toxocariasis is a zoonosis with an oral transmission mechanism. The source of invasion in the synanthropic focus for people is dogs that pollute the soil with feces containing toxocara eggs. People cannot be a source of infection, since adult parasites do not form from larvae in the human body and eggs are not released. Man serves as a reservoir, or paratenic, host of toxocara, but in fact he is an "ecological dead end".

The incidence of toxocara in dogs varies depending on their gender, age, and the way they are kept, and is very high in almost all areas - up to 40-50% and higher, and in rural areas it can reach 100%. The highest incidence is observed in puppies aged 1-3 months. Direct contact with dogs does not play a significant role in the infection of people. The main prerequisites for the transmission of the pathogen are soil contamination with helminth eggs and human contact with it. The importance of geophagy in the incidence of toxocariasis in children has now been proven. Geophagy is an example of direct infection with helminthiasis pathogens without the participation of any other transmission factors, and in these cases a person receives a massive invasion, which usually predetermines a severe course of the disease. A high incidence of toxocariasis is noted among owners of garden plots, summer cottages, vegetable gardens, as well as people living in yards where dogs are walked, which confirms the role of household contact with soil in infection with toxocara eggs. Toxocara eggs can be transmitted with vegetables and table greens. The factors of toxocara transmission are contaminated animal hair, water, hands. The role of cockroaches in the spread of helminthiasis has been established: they eat a significant number of toxocara eggs and release up to 25% of the eggs into the environment in a viable state.

Toxocariasis is widespread. Children are most often affected. A relatively high incidence rate has been established for some professional groups: veterinarians, utility workers, and amateur gardeners. People become infected with toxocariasis throughout the year, but infection most often occurs in the summer-autumn period, when the number of eggs in the soil and contact with it are at their maximum.

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What causes toxocariasis?

Toxocariasis is caused by the dog roundworm, which belongs to the type Nemathelminthes, class Nematodes, suborder Ascaridata, genus Toxocara. T. canis is a dioecious nematode, sexually mature individuals of which reach relatively large sizes (the length of the female is 9-18 cm, the male - 5-10 cm). Toxocara eggs are spherical, 65-75 µm in size. T. canis parasitizes dogs and other representatives of the canine family.

In the life cycle of this helminth, there are development cycles - the main and two auxiliary ones. The main cycle of development of toxocara corresponds to the scheme "dog-soil-dog". After infection of the dog by the alimentary route, larvae emerge from the eggs in its small intestine, which then migrate. similar to the migration of roundworms in the human body. After the maturation of the female toxocara in the small intestine, the dog begins to excrete the parasite eggs with feces. This type of helminth development occurs in puppies up to 2 months of age. In adult animals, the helminth larvae migrate to various organs and tissues. where granulomas form around them. In them, the larvae remain viable for a long time, do not develop, but can periodically resume migration.

Pathogenesis of toxocariasis

T. canis is a causative agent of helminthiasis that is not typical for humans, the larvae of which never turn into adults. It is a causative agent of helminthiasis in animals, capable of parasitizing humans in the migratory (larval) stage and causing a disease called the "Visceral parva migrans" syndrome. The syndrome is characterized by a long relapsing course and polyorgan lesions of an allergic nature. In the human body, as in other parthenic hosts, the development and migration cycles are carried out as follows: from the eggs of toxocara that get into the mouth, and then into the stomach and small intestine, larvae emerge, which penetrate the blood vessels through the mucous membrane and migrate to the liver through the portal vein system, where some of them settle; they are surrounded by an inflammatory infiltrate, and granulomas are formed.

What are the symptoms of toxocariasis?

According to the severity of clinical manifestations, toxocariasis is divided into manifest and asymptomatic, and according to the duration of the course, acute and chronic.

Visceral toxocariasis affects both children and adults, but this form is much more common in children, especially at the age of 1.5 to 6 years. The clinical picture of toxocariasis is not very specific and is similar to the clinical symptoms of the acute phase of other helminthiases. The main clinical manifestations of acute toxocariasis are recurrent fever, pulmonary syndrome, enlarged liver, polyadenopathy. skin manifestations, blood eosinophilia. hypergammaglobulinemia. In children, toxocariasis often develops suddenly or after a short prodromal period. Body temperature is often subfebrile (in severe cases of invasion - febrile), more pronounced during the period of pulmonary manifestations. Various types of recurrent skin rashes (erythematous, urticarial) are noted. development of Quincke's edema, Muscle-Wells syndrome, etc. is possible. Skin syndrome can persist for a long time, sometimes it is the main clinical manifestation of the disease. Examination of children with a diagnosis of "eczema" for toxocariasis, conducted in the Netherlands, showed that 13.2% of them have high titers of specific antibodies to toxocara. Most of the infected, especially children, have moderately enlarged peripheral lymph nodes.

How is toxocariasis diagnosed?

A lifetime parasitological diagnosis of "toxocariasis" is extremely rare and only possible when examining biopsy material, when toxocara larvae can be detected and verified in the tissues. The diagnosis is established based on the epidemiological history and clinical symptoms. The presence of persistent long-term eosinophilia is taken into account, although it is not always found in ocular toxocariasis. An indication of keeping a dog in the family or close contact with dogs, geophagy indicates a relatively high risk of contracting toxocariasis.

How is toxocariasis treated?

Toxocariasis does not have a single etiotropic treatment regimen. Antinematode drugs are used: albendazole, mebendazole, diethylcarbamazine. All of the listed anthelmintic drugs are effective against migrating larvae and are not effective enough against tissue forms located in granulomas of internal organs.

How to prevent toxocariasis?

Toxocariasis can be prevented by maintaining personal hygiene and teaching children sanitary skills. Timely examination and deworming of dogs are important. Preimaginal treatment of puppies aged 4-5 weeks, as well as pregnant females, is most effective. It is necessary to limit the number of stray dogs and equip special areas for walking. Systematic sanitary and educational work should be carried out among the population, providing information on possible sources of invasion and transmission routes. Particular attention should be paid to persons whose work brings them into contact with sources of invasion (veterinary workers, pet owners, staff of service dog kennels, excavators, etc.).