Pathogen plague

Plague (pestis) - an acute infectious disease, proceeding according to the type of hemorrhagic septicemia. In the past, the plague was a formidable scourge for humanity. Three pandemic plague are known, which claimed millions of human lives.

The first pandemic was in the VI. N. E. From it died from 531 to 580, about 100 million people - half the population of the Eastern Roman Empire ("Justinian" plague).

The second pandemic erupted in the XIV century. It began in China and struck many countries in Asia and Europe. In Asia, 40 million people died from it, and in Europe from 100 million people 25 million died. Here is how NM Karamzin describes this pandemic in the "History of the Russian State": "The disease was detected by glands in the soft depressions of the body, the person spit blood and on the second or third day he died. It is impossible, say the chroniclers, to imagine spectacles more terrible ... From the Pekin to the shores of the Euphrates and Ladoga, the earth's interior was filled with millions of corpses, and the state was emptied ... There were not a single resident left in Glukhov and Belozersk ... This cruel ulcer came several times and was returning. In Smolensk, it raged three times, and finally, in 1387, only 5 people remained in it, which, according to the chronicle, left and closed the city filled with corpses. "

The third pandemic of the plague began in 1894 and ended in 1938, killing 13-15 million people.

The causative agent of the plague was discovered in 1894 by the French scientist A. Iersen, after whom he was called Yersinia pestis. The genus Yersinia belongs to the family Enterobacteriaceae and includes 11 species, of which three are pathogenic to humans: Yersinia pestis, Yersinia pseudotuberculosis and Yersinia enterocolitica; the pathogenicity of the others is not yet clear.

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

Morphology of the causative agent of the plague

Yersinia pestis has a length of 1-2 μm and a thickness of 0.3-0.7 μm. In smears from the body of the patient and from the corpses of people and rodents who died from the plague, it looks like a short ovoid (egg-shaped) stick with a bipolar color. In smears from broth culture stick is a chain, in smears from agar cultures - randomly. Bipolar coloration in both cases is preserved, but in smears from agar cultures somewhat weaker. The causative agent of the plague on Gram is colored negatively, it is better painted with alkaline and carbolic dyes (blue Loeffler), does not form a spore, does not have flagella. The content of G + C in DNA is 45.8-46.0 mol% (for the whole genus). At a temperature of 37 ° C forms a gentle capsule of protein nature, which is detected on moist and slightly acidic nutrient media.

Biochemical properties of the causative agent of plague

Yersinia pestis - aerobic, gives good growth on normal nutrient media. The optimum temperature for growth is 27-28 ° С (range - from 0 to 45 ° С), РН = 6,9-7,1. The plaque wand gives characteristic growth on liquid and dense nutrient media: on broth it is manifested by the formation of a loose film, from which the filaments descend in the form of icicles resembling stalactites, at the bottom - a loose sediment, the broth remains transparent. Development of colonies on dense media passes through three stages: in 10-12 hours under a microscope growth in the form of colorless plates (the stage of "broken glass"); after 18-24 hours - the stage of "lacy handkerchiefs", when microscopically visible light lacy zone, located around the protruding central part, a yellowish or slightly brownish color. After 40-48 hours, the "adult colony" stage begins - a brownish-delineated center with a pronounced peripheral zone. Yersinia pseudotuberculosis and Yersinia enterocolitica do not have the stage of "broken glass". On media with blood, the colonies of Yersinia pestis are granular with a weakly expressed peripheral zone. In order to obtain the fastest growing growth in media on Yersinia pestis, it is recommended to add growth stimulants to them: sodium sulfite, blood (or its preparations) or lysate of the culture of sarcin. The plaque has a pronounced polymorphism, especially on media with a high concentration of NaCl, in old cultures, in the organs of decayed plague corpses.

The plague rod has no oxidase, does not form indole and H2S, has catalase activity and ferments glucose, maltose, galactose, mannitol to form an acid without gas.

[6], [7], [8], [9], [10]

Antigenic composition of the causative agent of plague

Yersinia pestis, Yersinia pseudotuberculosis and Yersinia enterocolitica detected up to 18 similar somatic antigens. Yersinia pestis is characterized by the presence of capsular antigen (fraction I), T, VW antigens, plasmacoagulase proteins, fibrinolysin, outer membrane proteins and pHb antigen. However, in contrast to Yersinia pseudotuberculosis and Yersinia enterocolitica, Yersinia pestis is antigenically more homogeneous; There is no serological classification of this species.

[11], [12], [13], [14], [15]

Resistantness of the causative agent of plague

In sputum, a plaque can persist for up to 10 days; on clothes and clothes, soiled with discharge of the patient, remains for weeks (protein and mucus protect it from the harmful effect of drying). In the bodies of people and animals that perished from the plague survives from the beginning of autumn to winter; low temperature, freezing and thawing do not kill it. Sun, drying, high temperature is fatal for Yersinia pestis. Heating to 60 ° C kills after 1 hour, at a temperature of 100 ° C perishes in a few minutes; 70% alcohol, 5% phenol solution, 5% lysol solution and some other chemical disinfectants kill for 5-10-20 minutes.

Factors of pathogenicity of the causative agent of plague

Yersinia pestis is the most pathogenic and aggressive among bacteria, and therefore causes the most serious disease. In all animals sensitive to it and in humans, the causative agent of the plague suppresses the protective function of the phagocytic system. It penetrates into phagocytes, suppresses in them an "oxidative explosion" and multiplies unhindered. The inability of phagocytes to carry out their killer function against Yersinia pestis is the main cause of susceptibility to plague. High invasiveness, aggressiveness, toxicity, toxicity, allergenicity and the ability to suppress phagocytosis are due to the presence of a whole arsenal of pathogenicity factors in U. Pestis, which are listed below.

The ability of cells to absorb exogenous dyes and hemin. It is associated with the function of the iron transport system and provides Yersinia pestis the ability to multiply in the tissues of the body.

• Dependence of growth at a temperature of 37 ° C on the presence of Ca ions in the medium.
• Synthesis of VW-antigens. Antigen W is located in the outer membrane, and V - in the cytoplasm. These antigens ensure the reproduction of U. Pestis within macrophages.
• Synthesis of "mouse" toxin. The toxin blocks the process of electron transfer in the mitochondria of the heart and liver of sensitive animals, affects platelets and vessels (thrombocytopenia) and disrupts their functions.
• Synthesis of the capsule (fractions I - Fral). The capsule inhibits the activity of macrophages.
• Synthesis of pesticides is a specific feature of Yersinia pestis.
• Synthesis of fibrinolysin.
• Synthesis of plasmocoagulase. Both these proteins are localized in the outer membrane and provide high invasive properties of Yersinia pestis.
• Synthesis of endogenous purines.
• Synthesis of thermoinduced proteins of the outer membrane - Yop-proteins (English Yersinia outer proteins). Proteins YopA, YopD, YopE, YopH, YopK, YopM, YopN suppress the activity of phagocytes.
• Synthesis of neuraminidase. It promotes adhesion (releases receptors for Yersinia pestis).
• Synthesis of adenylate cyclase. It is supposed that it suppresses the "oxidative explosion", ie it blocks the killer effect of macrophages.
• Synthesis of adhesion piles. They inhibit phagocytosis and ensure the introduction of Yersinia pestis, as an intracellular parasite, into macrophages.
• Synthesis of aminopeptidases with a wide spectrum of action.
• Endotoxin (LPS) and other components of the cell wall, which have a toxic and allergenic effect.
• pHb antigen. It is synthesized at a temperature of 37 ° C and low pH, suppresses phagocytosis and has a cytotoxic effect on macrophages.

A significant part of the pathogenicity factors of Yersinia pestis is controlled by genes, the carriers of which are the following 3 classes of plasmids, usually found together in all pathogenic strains:

• pYP (9.5 mp) is the pathogenicity plasmid. Carries 3 genes:
  • pst - encodes the synthesis of pesticine;
  • pim - determines the immunity to pesticide;
  • pla - determines fibrinolytic (plasminogen activator) and plasma-coagulase activity.
• pYT (65 MD) is a toxigenicity plasmid. Carries the genes that determine the synthesis of "mouse" toxin (a complex protein consisting of two fragments A and B, with 240 and 120 kD, respectively), and genes that control the protein and lipoprotein components of the capsule. Its third component controls the genes of the chromosome. Previously, the plasmid was called pFra.
• pYV (110 bp) is a virulence plasmid.

It determines the dependence of growth of Y. Pestis at 37 ° C on the presence of Ca2 + ions in the medium, therefore it has another name - LCR-plasmid (English low calcium response). The genes of this, especially important, plasmid also encode the synthesis of antigens V and W and thermo-inducible proteins Yop. Their synthesis is carried out under complex genetic control at a temperature of 37 ° C and in the absence of Ca2 + in the medium. All types of Yop proteins, other than YopM and YopN, are hydrolyzed by the activity of the plasminogen activator (pla gene of plasmid pYP). The Yop proteins largely determine the virulence of Yersinia pestis. YopE-protein has an antifagocytic and cytotoxic effect. YopD provides the penetration of YopE into the target cell; YopH has antifagocytic and protein-tyrosine-phosphatase activity; protein YopN - the properties of the calcium sensor; YopM binds to human atrombin.

[16], [17], [18]

Postinfectious immunity

Postinfectious immunity is strong, lifelong. Repeated plague diseases are extremely rare. The nature of immunity is cellular. Although antibodies appear and play a role in acquired immunity, it is mainly mediated by T lymphocytes and macrophages. In persons who have been infected with the plague or vaccinated, phagocytosis has a completed character. It determines the acquired immunity.

Epidemiology of the plague

The circle of warm-blooded carriers of the plague microbe is extremely extensive and includes more than 200 species of 8 orders of mammals. The main source of plague in nature are rodents and lagiformes. Natural contamination is established in more than 180 of their species, more than 40 of them are part of the Fauna of Russia and adjacent territories (within the former USSR). Of the 60 species of fleas for which the possibility of transferring a plague pathogen has been established in experimental conditions, 36 live on this territory.

The plague germ replicates in the lumen of the digestive tube of fleas. In its anterior part a cork ("plague block") is formed, containing a large number of microbes. When bitten by mammals with a reverse blood flow into the wound from the plug, some of the microbes are washed away, which leads to infection. In addition, the excreta released by the flea when fed to the wound can also lead to infection.

The main (main) carriers of Y. Pestis in the territory of Russia and Central Asia are ground squirrels, gerbils and marmots, in some foci there are also pits and voles. The existence of the following foci of plague is associated with them.

• 5 foci in which the main carrier of the plague microbe is a small suslik (North-Western Caspian, Tersko-Sunzhensky interfluve, Prielbruskiy outbreak, Volga-Ural and Zauralsky semi-desert foci).
• 5 foci in which carriers are ground squirrels and groundhogs (in Altai - pikas): Zabaikalsky, Gorno-Altai, Tuva and high-mountainous Tien-Shan and Pamir-Alai foci.
• Volga-Ural, Transcaucasian and Central Asian desert foci, where the main carriers are gerbils.
• High-mountainous Transcaucasian and Gissar foci with the main carriers - voles.

Different classifications of Yersinia pestis are based on different groups of traits - biochemical features (glycerin-positive and glycerin-negative variants), areas of distribution (oceanic and continental variants), species of major carriers (rat and gopher varieties). According to one of the most common classifications proposed in 1951 by the French plague researcher R. Devignat, depending on the geographical distribution of the pathogen and its biochemical properties, three intraspecific forms (biovar) of Yersinia pestis are distinguished.

According to the classification of domestic scientists (Saratov, 1985), the species Yersinia pestis is divided into 5 subspecies: Yersinia pestis subsp. Pestis (the main subspecies, it includes all three biovars of the classification of R. Devigny), Y. Pestis subsp. Altaica (Altaic subspecies), Yersinia pestis subsp. Caucasica (Caucasian subspecies), Y. Pestis subsp. Hissarica (Hissarian subspecies) and Yersinia pestis subsp. Ulegeica (Udege subspecies).

Infection of a person occurs through the bite of fleas, with direct contact with infectious material, airborne, rarely by alimentary route (for example, when eating meat from camels, plague patients). In 1998-1999 years. Plague in the world, there were 30,534 people, of whom 2,234 died.

[19], [20], [21], [22],

Symptoms of the plague

Depending on the mode of infection, the bubonic, pulmonary, intestinal form of the plague is distinguished; rarely septic and cutaneous (purulent vesicles at the site of the flea bite). The incubation period for plague varies from a few hours to 9 days. (in persons subjected to seroprevention, up to 12 days.). Pathogen plague penetrates through the smallest damage to the skin (a flea bite), sometimes through the mucous membrane or airborne droplets, reaches the regional lymph nodes, in which it begins to multiply rapidly. The disease begins suddenly: severe headache, fever with chills, face hyperemic, then it becomes dark, under the eyes dark circles ("black death"). Bubon (enlarged inflamed lymph node) appears on the second day. Sometimes the plague develops so rapidly that the patient dies earlier than the bubo will appear. Especially hard is the pneumonic plague. It can arise as a result of the complication of the bubonic plague, and during infection by airborne droplets. The disease also develops very roughly: chills, high fever and already in the first hours pains in the side, cough, at first dry, and then with bloody sputum, join; there is delirium, cyanosis, collapse, and death sets in. A patient with a pulmonary plague poses an exceptional danger to others, as he gives off a large number of pathogens with sputum. In the development of the disease, the main role is played by suppression of the activity of phagocytes: neutrophilic leukocytes and macrophages. Unrestrained reproduction and propagation of the pathogen through the blood throughout the body completely suppresses the immune system and leads (in the absence of effective treatment) to the death of the patient.

Laboratory diagnosis of plague

Bacterioscopic, bacteriological, serological and biological methods are used, as well as an allergic test with a pestin (for retrospective diagnosis). The material for the study is: punctate from the bubo (or its detachable), sputum, blood, with intestinal form - feces. Yersinia pestis is identified on the basis of morphology, cultural, biochemical features, samples with plague phage and using a biological test.

A simple and reliable method for determining plague antigens in the test material is the use of RPGA, especially with the use of erythrocyte diagnosticum, sensitized with monoclonal antibodies to the capsule antigen, and IPM. These same reactions can be used to detect antibodies in the serum of patients.

The biological method of diagnosis consists in contamination with the test material (when it is very contaminated with the accompanying microflora) of guinea pig skin, subcutaneously or, rarely, intraperitoneally.

When working with material containing the causative agent of the plague, strict compliance is required, therefore all studies are conducted only by well-trained personnel in special anti-plague facilities.

Prophylaxis of plague

Constant control over the natural foci of the plague and the organization of measures to prevent diseases of people in the country is carried out by a special anti-plague service. It includes five anti-plague Institutes and dozens of anti-plague stations and offices.

Despite the presence of natural foci, since 1930 there have not been a single case of plague in Russia on the territory of Russia. For specific prevention of plague, vaccination against plague is used - a live attenuated vaccine from the strain EV. It is injected dermally, intradermally or subcutaneously. In addition, a dry tablet vaccine for oral administration has been proposed. Postvaccinal immunity is formed by the 5th-6th day after vaccination and persists for 11-12 months. For its evaluation and retrospective diagnosis of plague, an intradermal allergic test with a pestin was suggested. The reaction is considered positive if a seal at least 10 mm in diameter is formed at the injection site of the pestin after 24-48 h and redness appears. An allergic test is positive in people who have postinfectional immunity.

A great contribution to the study of the plague and the organization of the fight against it was made by Russian scientists: DS Samoilovich (the first not only in Russia but also in Europe "hunter" for the plague microbial as early as in the 18th century, he also suggested vaccination against the plague ), DK Zabolotny, NP Klodnitsky, IA Deminsky (study of natural foci of the plague, carriers of its causative agent in foci, etc.), etc.