Toxocariasis: causes and pathogenesis

The cause of Toxocarosis is the Ascarid dog, which belongs to the type Nemathelminthes, the class Nematodes, the suborder Ascaridata, the genus Toxocara. T. Canis - dioecious nematodes, sexually mature specimens of which reach relatively large sizes (female length 9-18 cm, male 5-10 cm). Eggs of Toksokara are spherical in shape, 65-75 microns in size. T. Canis parasitizes in dogs and other representatives of the canine family.

In the life cycle of this helminth, the development cycles-the main and the two auxiliary ones-are singled out. The main cycle of development of toxocar corresponds to the "dog-soil-dog" scheme. After infection of the dog by alimentary tract, larvae leave the eggs in its small intestine and then migrate. The migration of ascarids in the human body is similar. After maturing females toxocar in the small intestine, the dog with feces begins to secrete the eggs of the parasite. This type of helminth development occurs in puppies under 2 months of age. In adult animals, larvae of the helminth migrate to various organs and tissues. Where granules form around them. In them, larvae for a long time remain viable, do not develop, but periodically can resume migration.

The first type of ancillary cycle is characterized by transplacental transmission of larvae toxocar to the fetus according to the "final host (dog) -placenta-final host (puppy)" scheme. Thus, the born puppy already contains helminths. In addition, puppies can get larvae in lactation.

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

Thus, the perfect mechanism of transmission of the causative agent among the animals is promoted by the perfect mechanism of transmission of the pathogen, in which the direct (contamination by eggs from the environment), vertical (infection of the fetus by larvae through the placenta), transmammar (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 a day. The maturation of eggs in the soil (from 5 days to 1 month) depends on the temperature of the environment and humidity. In the middle zone of Russia, toxocar eggs can survive in the soil throughout the year.

Pathogenesis of toxocariasis

T. Canis is an alien helminthic pathogen, the larvae of which never turn into adult individuals. It is the causative agent of helminthiasis of animals, capable of parasitizing a person in the migratory (larval) stage and causing a disease called "Visceral pаrvа migrans" syndrome . The syndrome is characterized by a long-term recurrent course and multiple-organ lesions of an allergic nature. In the human body, as in other parthenic hosts, the cycles of development and migration are carried out in the following way: from eggs of toxocar caught in the mouth, and then into the stomach and small intestine, larvae emerge, which through the mucous oolochku penetrate into the blood vessels and through the portal system veins migrate to the liver, somewhere some of them settle; they are surrounded by an inflammatory infiltrate, and granulomas are formed. With intensive invasion, granulomatous tissue damage is observed in the lungs, pancreas, myocardium, lymph nodes, brain and other organs. A larva in the human body can survive up to 10 years. This vitality is associated with the isolation of a masking substance capable of protecting the larva from the aggression of eosinophils and host antibodies. Larvae of the helminth in the tissues, under the influence of various factors, periodically resume migration, which causes a relapse of the disease. During migration, larvae injure blood vessels and tissues, causing hemorrhages, necrosis, inflammatory changes. Excretory-secretory antigens of living and somatic antigens of dead larvae have a strong sensitizing effect with the development of reactions of GNT and HRT, which are manifested by edema, cutaneous erythema, impaired airway patency. A certain value in pathogenesis has immune complexes "antigen-antibody". Insufficiently studied factors determining the occurrence of toxocariasis of the eyes. There is a hypothesis about selective lesion of the eyes in persons with low-intensity invasion, in which a sufficiently pronounced immune reaction of the organism does not develop. In comparison with ascarids and some other helminths, T. Canis has the strongest polyvalent immunosuppressive effect. It was found that with toxocarosis in children, the effectiveness of vaccination and revaccination against measles, diphtheria and tetanus decreases.