Trichinosis - Cause and Pathogenesis

Trichinellosis is caused by roundworms of the Trichinellidae family, which includes two species - Trichinella spiralis with three varieties (T. s. spiralis, T. s. nativa, T. s. nelsoni) and Trichinella pseudospiralis. In the pathology of the population of Ukraine, T. s. spiralis and G. s. nativa are of greatest importance. Trichinella s. spiralis is widespread, parasitizes domestic pigs, and is pathogenic for humans. Trichinella s. nativa is found in the northern hemisphere, parasitizes wild mammals, is extremely resistant to cold, and is pathogenic for humans. Trichinella s. nelsoni lives in Equatorial Africa, parasitizes wild mammals. is low pathogenic for humans. Trichinella pseudospiralis is widespread, parasitizes birds and wild mammals. Pathogenicity for humans has not been proven.

Trichinella are small nematodes with a cylindrical colorless body covered with a transparent ringed cuticle. The length of an unfertilized female is 1.5-1.8 mm, fertilized - up to 4.4 mm, sexually mature male - 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 form, up to 10 mm in length; after 18-20 days of development, the larva lengthens to 0.7-1.0 mm.

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

Trichinella larvae found in animal muscles are resistant to high and low temperatures. Heating meat containing encapsulated Trichinella larvae in a microwave oven to 81 °C does not inactivate them. When boiling 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, freezing. The greatest danger is posed by thermally untreated meat products: stroganina, lard (fat), etc.

Pathogenesis of trichinosis

The pathogenesis of trichinellosis is based on the sensitization of the organism to helminth antigens, which manifests itself to varying degrees in the intestinal, migratory and muscular stages of invasion. By the end of the first week after infection, female trichinella are found mainly in the small intestine, immersed in the mucous membrane, around which a local catarrhal-hemorrhagic inflammatory reaction develops. In severe invasion, ulcerative-necrotic damage to the intestinal mucosa is observed. Adult helminths secrete immunosuppressive substances that suppress the violent inflammatory reaction, which promotes 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 with sensitizing, enzymatic and toxic properties. As a result, severe allergic reactions develop with damage to blood vessels, coagulation disorders, tissue edema, and 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 brain. In parenchymatous organs, the larvae die. Developing immunopathological reactions lead to severe lesions: myocarditis, meningoencephalitis, pneumonia. Inflammatory processes subside over time, but after 5-6 weeks they can be replaced by dystrophic ones, the consequences of which disappear only after 6-12 months. Of the skeletal muscles, groups with abundant blood supply are most often affected (intercostal, chewing, oculomotor muscles, diaphragm, muscles of the neck, tongue, upper and lower extremities). In patients with a severe form of the disease, 50-100 or more Trichinella larvae are found in 1 g of muscle mass. By the end of the third week, the larvae acquire a spiral shape, intensive cellular infiltration is observed around them, in place of which a fibrous capsule is then formed. The process of capsule formation is disrupted by excessive antigen load (with massive invasion), as well as under the influence of substances with immunosuppressive properties (glucocorticoids, etc.). Nodular infiltrates are found in parenchymatous organs. In the myocardium, Trichinella larvae cause the appearance of multiple inflammatory foci in the interstitial tissue, but real capsules are not formed in the heart muscle. With intensive invasion, a focal-diffuse inflammatory reaction and dystrophic changes develop in the myocardium; granulomas and vasculitis with damage to the arterioles and capillaries of the brain and meninges are possible.

Trichinellosis is characterized by persistent non-sterile immunity, which is caused by the presence of encapsulated larvae of the pathogen in the muscles of infected people. High levels of specific antibodies in the blood serum are observed from the end of the second week and reach a maximum in the 4th-7th week. A complex of reactions in the enteral stage prevents the penetration of a significant portion of the larvae into the bloodstream, which limits their spread in the body.