Toxocarosis - Causes and Pathogenesis

Toxocariasis is caused by the dog roundworm, which belongs to the Nemathelminthes type, Nematodes class, Ascaridata suborder, Toxocara genus. 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.

The first type of assisted cycle is characterized by transplacental transmission of Toxocara larvae to the fetus according to the scheme "definitive host (dog) - placenta - definitive host (puppy)". Thus, the newborn puppy already contains helminths. In addition, puppies can receive larvae during lactation.

The second type of auxiliary cycle occurs with the participation of paratenic (reservoir) hosts. These can be rodents, pigs, sheep, birds, earthworms. In their bodies, migrating larvae are not able to turn into adults. However, when a reservoir host is eaten by a dog or other animal of the canine family, the larvae, getting into the intestines of the obligate host, develop into adult helminths.

Thus, the widespread distribution of toxocariasis among animals is facilitated by a perfect mechanism of pathogen transmission, which combines direct (infection with eggs from the environment), vertical (infection of the fetus with larvae through the placenta), transmammary (transmission of larvae with milk) transmission routes and infection through paratenic hosts. The lifespan of sexually mature individuals in the intestines of the main hosts is 4-6 months. The female T. canis lays more than 200 thousand eggs per day. The maturation period of eggs in the soil (from 5 days to 1 month) depends on the ambient temperature and humidity. In central Russia, toxocara eggs can remain viable in the soil throughout the year.

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, which is 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-term 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. In case of intensive invasion, granulomatous tissue damage is observed in the lungs, pancreas, myocardium, lymph nodes, brain and other organs. The larva can survive in the human body for up to 10 years. Such viability is associated with the secretion of a masking substance that can protect the larva from the aggression of eosinophils and host antibodies. Helminth larvae in tissues periodically resume migration under the influence of various factors, which causes relapses of the disease. During migration, the larvae injure blood vessels and tissues, causing hemorrhages, necrosis, and inflammatory changes. Excretory-secretory antigens of living and somatic antigens of dead larvae have a strong sensitizing effect with the development of GNT and DTH reactions, which are manifested by edema, skin erythema, and obstruction of the respiratory tract. Immune complexes "antigen-antibody" play a certain role in pathogenesis. Factors determining the occurrence of eye toxocariasis have not been sufficiently studied. There is a hypothesis about selective eye damage in individuals with low-intensity invasion, in which a sufficiently pronounced immune response of the body does not develop. Compared with roundworms and some other helminths, T. canis has the strongest polyvalent immunosuppressive effect. It has been established that in children with toxocariasis, the effectiveness of vaccination and revaccination against measles, diphtheria and tetanus decreases.