Pathogen of tularemia

Tularemia is the primary disease of animals (rodents), in man it occurs in the form of an acute infectious disease with a diverse clinical picture and a slow restoration of work capacity. The causative agent of tularemia, Francisella tularensis, was discovered by G. McCoy and S. Chepin in 1912 during an epizootic among earth squirrels in the area with Lake Tulare (California), studied in detail by E. Francis, after whom the genus was named.

These are very small coccoid or ellipsoidal polymorphic rods, 0.2-0.7 μm in size, which, with special staining methods, often give a bipolar color; They are immovable, gram-negative, do not form a spore; catalase negative, form H2S, strict aerobes, temperature optimum for growth 37 ° C, pH 6.7-7.2. Virulent strains have a capsule, form an acid without gas during fermentation of some carbohydrates (glucose, maltose, mannose, fructose, dextrin), the degree of fermentation varies in different strains, the content of G + C in DNA is 33-36 mol%. F. Tularensis does not grow on ordinary media. G. McCoy and S. Chepin used a folded yolk medium. It tularemia bacillus grows in the form of tender small colonies, reminiscent of droplets of dew, then the culture acquires the character of gentle shagreen plaque with mildly expressed mucous consistency. E. Francis proposed nutrient agar containing 0.05-0.1% cystine, 1% glucose and 5-10% blood for growing a tularemia bacillus. On such an environment, growth is more magnificent and coarse: colonies are round with a smooth surface, milky, moist, with a mucous consistence, surrounded by a characteristic green halo. The growth is slow, the maximum size of the colony is reached on the 3-5th day (1 - 4 mm). Tularemia bacteria multiply well in the yolk sac of the chick embryo, causing its death on the 3-4th day.

For the growth of F. Tularensis, the following amino acids are required: arginine, leucine, isoleucine, lysine, methionine, proline, threonine, histidine, valine, cystine, for some subspecies - tyrosine, aspartic acid; In addition, for Rost, they also need pantothenic acid, thiamine, and Mg2 ions. Given these features, synthetic media can be used to cultivate F. Tularensis.

The genus Francisella is classed as Gammaproteobacteria, a type of Proteobacteria. To the same genus belongs F. Novicida, the pathogenicity of which is not established for humans.

The causative agent of tularemia is an intracellular parasite. Its virulence is due to a capsule that inhibits phagocytosis; neuraminidase, which promotes adhesion; endotoxin; allergenic properties of the cell wall, as well as the ability to multiply in phagocytes and suppress their killer effect. The mechanisms of virulence have not yet been deciphered. In addition, the tularemia rod has receptors that can interact with Fc fragments of IgG immunoglobulins. As a result of this binding, the activity of complement and macrophage systems is disrupted.

F. Tularensis in S-form (virulent) has two antigens - O and Vi (capsular antigen). O-antigen reveals kinship with brucella antigens. Dissociation of S-> SR-> R leads to loss of capsule, virulence and immunogenicity. The species F tularensis is divided into three geographical races (subspecies):

• holarctic (weakly pathogenic for domestic rabbits, does not ferment glycerol and does not have the enzyme citrullinureidase, is found in the countries of the northern hemisphere);
• Central Asian (slightly pathogenic for rabbits, has citrullinureidase and ferments glycerin);
• nonarctic (American), more pathogenic for rabbits, ferments glycerol, has citrullineuridase.

In addition, strains of the American and Central Asian subspecies have phosphatase activity, which is absent in strains of the Holarctic subspecies.

[1], [2], [3], [4], [5], [6]

Resistance of the pathogen of tularemia

F. Tularensis is quite stable in the external environment, especially if it is contained in a pathological material. In forage, grain, contaminated by secretions of rodents, survives up to 4 months; in water - up to 3 months; in the ice - more than 1 month. It is sensitive to direct sunlight (dies in 30 minutes), high temperature (at 60 ° C perishes in 10 minutes), dies after 5-10 minutes with 3% solution of lysol, 50% alcohol, formalin and other antiseptics.

Epidemiology of tularemia

The main reservoir of tularemia in nature are rodents, among which, under natural conditions, epizootics are observed. A person becomes infected only from animals, the pathogen is not transmitted from person to person. The causative agent was found in 82 species of rodents and lagiformes, most often found in representatives of four families: Muridae, Hare (Leporidae), squirrel (Sciuridae) and Tush pench (Dipodidae). In Russia, the main carriers are mouse rodents: water rats, ordinary voles, house mice and muskrats.

By sensitivity to tularemia, animals can be divided into four groups:

• 1st group - the most susceptible (voles, water rats, house mice, white mice, guinea pigs and some others). The minimum lethal dose is one microbial cell;
• 2 nd group - less sensitive (gray rats, ground squirrels, etc.). The minimal lethal dose is 1 billion microbial cells, however, one microbial cell is sufficient to infect some of them;
• 3rd group (predators - cats, foxes, ferrets). Resistant to high infectious doses, the disease occurs without visible manifestations;
• 4 th group - are immune to tularemia (ungulate animals, cold-blooded, birds).

For a person, the minimal infectious dose is one microbial cell. Infection of a person occurs in all possible ways: direct and indirect contact with sick rodents, their corpses or with objects infected with rodents; alimentary (with the use of food and water, infected with rodents), airborne and transmissible. Infection with tularemia bacteria was established in 77 species of bloodsucking arthropods. Especially important are ixodid mites, in which the causative agent persists throughout life and is even transmitted transovarially to offspring. These circumstances contribute to the rooting of the disease in nature. Infection of a person with ticks occurs not by biting, but as a result of the causative agent on the skin along with the tick feces.

On the territory of Russia, there are 7 main landscape types of natural foci of tularemia: name-bog, meadow-field, steppe, forest, foothill-creek, tundra and tugai (name-desert).

[7], [8], [9], [10], [11],

Symptoms of tularemia

The causative agent of tularemia penetrates the body through the outer covers (damaged and undamaged skin and mucous membranes). Often, ulcers are formed at the site of implantation. Through the lymphatic vessels, bacteria enter the regional lymph node and multiply freely in it; The inflammatory process leads to the formation of bubo. Hence the causative agent penetrates into the blood, bacteremia causes the generalization of the process, various organs and tissues are involved in it, the multiplication in which bacteria leads to the formation of granulomas and necrotic ulcers. With bacteremia and generalization associated allergic alteration of the body. The incubation period of tularemia varies from 2 to 8 days. The disease begins acutely: there is a fever, a headache, pain in the muscles, hyperemia of the face. The further course depends on the location of the entrance gate, according to which the following clinical forms of tularemia are distinguished: ulcerous-glandular (bubonic), glandular, anginal-glandular, abdominal and pulmonary. Mortality with tularemia does not exceed 1-2%.

Post-infectious immunity is strong, resistant, in most cases lifelong, has a cellular nature, mainly due to T-lymphocytes and macrophages, to a lesser extent - antibodies. Phagocytosis in persons with immunity has a completed character.

Laboratory diagnosis of tularemia

For the diagnosis of tularemia, all microbiological methods are used. The research is carried out in the laboratories. Material for the study - blood, punctate from the bubo, scraping from the ulcer, detachable conjunctiva, plaque from the pharynx, sputum, etc. - is determined by the clinical form of the disease. In addition, the study can take water and food. In natural foci of tularemia, systematic systematic studies are carried out to isolate the causative agent of tularemia from rodents.

The bacteriological method of diagnosing tularemia in humans rarely gives positive results. Pure culture, as a rule, is isolated after its accumulation on susceptible laboratory animals. For white blood test, white mice and guinea pigs are used. Mice are infected subcutaneously, guinea pigs - intraperitoneally; animals die on the 3rd-6th day, sometimes on the frost. Infected animals are kept in special conditions (as in the diagnosis of plague) and observed for 6-14 days. Gels experimental animals for 7-15 days do not die, they are killed on the 15-20th day and the corpses are opened. In the presence of tularemia, pathological and anatomical changes are revealed in the form of a productive process with necrosis. Pure culture is isolated from the internal organs on the vitelline medium, glucosocysteine blood center, etc. Identification is based on the morphology and tinctorial properties of the pathogen, the absence of growth on MPA, agglutination by homologous serum. Pathogenicity for white mice and guinea pigs. A pure culture can be identified by infecting 12-day-old chick embryos and yolk sac. To isolate the pure culture of the pathogen from the water, it is centrifuged or filtered through bacterial filters and sediment infested with laboratory animals. When researching food products, they are washed with MP B, centrifuged and sediment infested with laboratory animals.

Simultaneously with the bacteriological study of the test material, smear prints are prepared and stained by Romanovsky-Giemsa. In smears from the organs, small coccoid and rod-shaped bacteria can be found that are located intracellularly and in the form of clusters, forming a tender capsule.

For diagnostics use the developed agglutination reaction, RPGA, RIF.

Allergic tests are used for the early diagnosis of tularemia (from the 5th day of the onset of the disease). Use two forks of tulareth and, accordingly, two methods of their introduction: dermal and intradermal. Since the concentration of the allergen in both species of tularin is different, it is unacceptable to use the cutaneous tularin for the intradermal test and vice versa. Results of an allergic reaction are considered in dynamics through 24. 36, 48 hours. For a positive result, infiltrate with a diameter of at least 5 mm is taken. In persons vaccinated with or having recovered from tularemia, allergic tests remain positive for a number of years (anamnestic reaction).

Specific prophylaxis of tularemia

For specific prevention, a vaccination against tularemia, received in 1930 by domestic military doctors B.Ya. Elbert and N.A. Gaisky from the Mae strain 15. The vaccine provides lasting immunity for 5-6 years when infected with European and Holarctic subspecies and is effective against the American variety of pathogens. Vaccination is carried out according to epidemiological indications, as well as persons belonging to risk groups. Simultaneous vaccination against tularemia and brucellosis is allowed; tularemia and plague, as well as against tularemia and some other infections.

Nonspecific prophylaxis of tularemia is the same as with other zoonoses, and is aimed primarily at fighting rodents.