Trichinosis - Cause and Pathogenesis

Trichinosis is caused by round worms of the family Trichinellidae, which includes two species - Trichinella spiralis with three varieties (T. Spiralis, T. S. Nativa, T. S. Nelsoni) and Trichinella pseudospiralis. In the pathology of the population of Ukraine, the most important are the T. S. Spiralis and G. S. Nativa. Trichinella s. Spiralis is ubiquitous, parasitic in domestic pigs, pathogenic to humans. Trichinella s. Nativa occurs in the northern hemisphere, parasitizes wild mammals, is extremely resistant to cold, pathogenic to humans. Trichinella s. Nelsoni lives in Equatorial Africa, parasitizes wild mammals. Slightly pathogenic for humans. Trichinella pseudospiralis is ubiquitous, parasitic in birds and wild mammals. Pathogenicity for humans is not proven.

Trichinella - small nematodes with a cylindrical colorless body, covered with a transparent cuticle of a ringed structure. The length of the unfertilized female is 1.5-1.8 mm, fertilized - up to 4.4 mm, the mature male is about 1.2-2 mm, the diameter of the helminths is less than 0.5 mm. Unlike other nematodes, trichinella are viviparous helminths. Their larvae, young Trichinella, have a rod-shaped shape, length up to OD mm; After 18-20 days of development, the larva lengthens to 0.7-1.0 mm.

The same organism of the warm-blooded animal for Trichinella serves first definitive (intestinal Trichinella), and then intermediate (encapsulated in larval muscles) by the host. In the body of the new host, the parasite is caught with animal meat, which contains live encapsulated larvae. Under the influence of gastric juice, the capsule dissolves, the larvae in the small intestine actively penetrate the mucous membrane in an hour. On the 4th-7th day, females begin to produce live larvae. Each female during the reproductive period, lasting 10-30 days, gives birth to 200 to 2000 larvae. From the intestine, the larvae are carried by the current of blood throughout the body. Further development of the parasite is possible only in the striated muscles. In the third week after infection, the larvae become invasive and assume a typical spiral shape. By the beginning of the second month after infection around them, a fibrous capsule is formed in the muscles, which begins calcification after 6 months. In capsules, the larvae retain their viability for 5-10 years or more. In human muscles, capsules of Trichinella larvae 0.3-0.6 mm in size always have a lemon-like shape.

Trichinella larvae in the muscles of animals are resistant to high and low temperatures. The heating of meat containing encapsulated Trichinella larvae in a microwave oven to 81 ° C does not ensure their inactivation. When cooking a piece of meat about 10 cm thick, the larvae die only after 2-2.5 hours. The larvae are resistant to such types of cooking as salting, smoking, and freezing. Thermally unprocessed meat products are the greatest danger: stroganina, fat (bacon), etc.

Pathogenesis of trichinosis

At the heart of the pathogenesis of trichinosis is the sensitization of the organism to helminth antigens, manifested in varying degrees in the intestinal, migratory and muscular stages of invasion. By the end of the first week after infection, predominantly in the small intestine, females of Trichinella, immersed in the mucosa, are found around which a local catarrhal-hemorrhagic inflammatory reaction develops. In severe invasion, ulcerative necrotic lesions of the intestinal mucosa are observed. Adult individuals of the helminth isolate immunosuppressive substances that suppress a violent inflammatory reaction, which facilitates the migration of larvae. In the jejunum the kinin system is activated. Other hormones that cause functional disorders, pain syndrome. Metabolites of migrating larvae, products released after their death, are antigens that have sensitizing, enzymatic and toxic properties. As a result, severe allergic reactions develop with blood vessel damage, coagulation disorders, tissue edema, increased secretory activity of the mucous membranes. In the second week, larvae are found not only in skeletal muscles, but also in the myocardium, lungs, kidneys, and the brain. In the parenchymal organs, the larvae perish. Developing immunopathological reactions lead to severe lesions: myocarditis, meningoencephalitis, pneumonia. Inflammatory processes eventually subside, but after 5-6 weeks can be replaced by dystrophic, the consequences of which disappear only after 6-12 months. Of the skeletal muscles most often affected by groups with abundant blood supply (intercostal, chewing, oculomotor muscles, diaphragm, neck muscles, tongue, upper and lower extremities). In patients with severe disease, 50-100 or more larvae of Trichinella are found in 1 g of muscle mass. By the end of the third week, the larvae acquire a spiral shape, an intense cellular infiltration is observed around them, whereupon a fibrous capsule is formed. The process of capsule formation is disturbed by excessive antigenic loading (with massive invasion), and also under the influence of substances with immunosuppressive properties (glucocorticoids, etc.). In parenchymal organs nodular infiltrates are encountered. In the myocardium, Trichinella larvae cause the appearance of multiple inflammatory foci in the interstitial tissue, but real capsules in the cardiac muscle are not formed. With intensive infestation in the myocardium, a focal-diffuse inflammatory reaction and dystrophic changes develop; Possible formation of granulomas and the development of vasculitis with the defeat of arterioles and capillaries of the brain and meninges.

Trichinosis is characterized by persistent non-sterile immunity, which is due to the presence of encapsulated larvae of the pathogen in the muscles of infected people. The high content of specific antibodies in the blood serum is observed from the end of the second week and reaches a maximum at the 4-7th week. The complex of reactions in the enteral stage prevents the penetration of a significant part of the larvae into the bloodstream, which limits their spread in the body.