Causes and pathogenesis of plague

Causes of the plague

The causative agent of plague is a gram-negative small polymorphic non-motile rod Yersinia pestis of the Enterobacteriaceae family of the genus Yersinia. It has a mucous capsule and does not form spores. It is a facultative anaerobe. It is stained with bipolar aniline dyes (more intensely at the edges). There are rat, marmot, gopher, field and gerbil varieties of the plague bacterium. It grows on simple nutrient media with the addition of hemolyzed blood or sodium sulfate; the optimal temperature for growth is 28 °C. It occurs in the form of virulent (R-forms) and avirulent (S-forms) strains. Yersinia pestis has more than 20 antigens, including a heat-labile capsular antigen that protects the pathogen from phagocytosis by polymorphonuclear leukocytes, a heat-stable somatic antigen that includes V- and W-antigens that protect the microbe from lysis in the cytoplasm of mononuclear cells, ensuring intracellular reproduction, LPS, etc. The pathogenicity factors of the pathogen are exo- and endotoxin, as well as aggressive enzymes: coagulase, fibrinolysin and pesticins. The microbe is resistant in the environment: it survives in soil for up to 7 months; in corpses buried in the ground, up to a year; in bubo pus - up to 20-40 days; on household items, in water - up to 30-90 days: it tolerates freezing well. When heated (at 60 °C it dies within 30 seconds, at 100 °C - instantly), dried, exposed to direct sunlight and disinfectants (alcohol, chloramine, etc.), the pathogen quickly disintegrates. It is classified as group 1 pathogenicity.

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Pathogenesis of plague

The causative agent of plague penetrates the human body most often through the skin, less often through the mucous membranes of the respiratory tract, digestive tract. Changes in the skin at the site of pathogen penetration (primary focus - phlyctena) rarely develop. Lymphogenously from the site of penetration, the bacterium enters the regional lymph node, where it multiplies, which is accompanied by the development of serous-hemorrhagic inflammation spreading to the surrounding tissues, necrosis and suppuration with the formation of a plague bubo. When the lymphatic barrier is broken, hematogenous dissemination of the pathogen occurs. Penetration of the pathogen by the airborne route contributes to the development of an inflammatory process in the lungs with melting of the alveolar walls and concomitant mediastinal lymphadenitis. Intoxication syndrome is characteristic of all forms of the disease, caused by the complex action of the pathogen's toxins and characterized by neurotoxicosis, ISS and thrombohemorrhagic syndrome.

Epidemiology of plague

The leading role in preserving the pathogen in nature is played by rodents, the main ones being marmots (tarbagans), gophers, voles, gerbils, and also lagomorphs (hares, pikas). The main reservoir and source in anthropurgic foci is the gray and black rat, less often - house mice, camels, dogs and cats. A person with the pulmonary form of plague is especially dangerous. Among animals, the main distributor (carrier) of the plague is the flea, which can transmit the pathogen 3-5 days after infection and remains infectious for up to a year. The mechanisms of transmission are varied:

• transmissible - when bitten by an infected flea;
• contact - through damaged skin and mucous membranes when removing skins from sick animals: slaughter and cutting up carcasses of camels, hares, as well as rats, marmots, which are eaten in some countries: through contact with secretions of a sick person or with objects contaminated by them:
• feco-oral - when eating insufficiently cooked meat from infected animals:
• aspiration - from a person sick with pulmonary forms of plague.

Human morbidity is preceded by epizootics among rodents. The seasonality of the disease depends on the climate zone and in countries with a temperate climate is recorded from May to September. Human susceptibility is absolute in all age groups and with any mechanism of infection. A patient with the bubonic form of plague does not pose a danger to others before the bubo opens, but when it turns into a septic or pulmonary form, he becomes highly contagious, releasing the pathogen with sputum, bubo secretion, urine, and feces. Immunity is unstable, repeated cases of the disease have been described.

Natural foci of infection exist on all continents except Australia: in Asia, Afghanistan, Mongolia, China, Africa, South America, where about 2 thousand cases are registered annually.

Anti-plague specialists and epidemiologists monitor the epidemic situation in these regions. Over the past 30 years, no group outbreaks have been registered in the country, and the incidence rate has remained low - 12-15 episodes per year. Each case of human disease must be reported to the territorial center in the form of an emergency notification followed by the announcement of quarantine. International rules define a quarantine of 6 days, observation of persons in contact with the plague is 9 days.

Currently, plague is included in the list of diseases whose pathogen can be used as a means of bacteriological weapons (bioterrorism). Highly virulent strains resistant to common antibiotics have been obtained in laboratories.

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