Causes and pathogenesis of plague
Last reviewed: 23.04.2024
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Causes of plague
The causative agent of plague is a small, polymorphic, fixed immobilized rod Yersinia pestis of the Enterobacteriaceae family of the genus Yersinia. Has a mucous capsule, does not form a spore. Facultative anaerobic. It is colored by bipolar aniline dyes (more intensively around the edges). Isolate rat, groundworm, gopher, flight and sandy varieties of plague bacteria. It grows on simple nutrient media with the addition of hemolyzed blood or sodium sulfate, the optimum temperature for growth is 28 ° C. Occurs in the form of virulent (R-forms) and avirulent (S-form) strains. Yersinia pestis has more than 20 antigens, including thermolabile capsular, which protects the pathogen from phagocytosis by polymorphonuclear leukocytes, a thermostable somatic, to which the V and W antigens belong, which protect the microbe from lysis in the cytoplasm of mononuclears. Providing intracellular reproduction, LPS, etc. Pathogenicity factors of the pathogen are exo- and endotoxin, as well as enzymes of aggression: coagulase, fibrinolysin and pesticides. The microbe is stable in the environment: in the soil it persists up to 7 months; in the corpses buried in the ground, up to a year: in the pus of bubo - up to 20-40 days; on household items, in water - up to 30-90 days: well tolerates freezing. When heated (at 60 ° C perishes in 30 seconds, at 100 ° C - instantly), drying, exposure to direct sunlight and disinfectants (alcohol, chloramine, etc.), the causative agent quickly breaks down. He is referred to the 1 st group of pathogenicity.
[Pathogenesis of plague
Pathogen plague penetrates the human body more often through the skin, less often through the mucous membranes of the respiratory tract, digestive tract. Changes on the skin at the site of the pathogen (primary focus - flicten) are rare. Lymphogenically from the site of introduction, the bacterium enters the regional lymph node, where it multiplies, which is accompanied by the development of serous-hemorrhagic inflammation, which spreads to surrounding tissues, necrosis and suppuration with the formation of plague bubo. With the breakthrough of the lymphatic barrier, hematogenous dissemination of the pathogen occurs. The entry of the pathogen by the aerogenic route promotes the development of the inflammatory process in the lungs with the melting of the walls of the alveoli and concomitant mediastinal lymphadenitis. Intoxication syndrome is common to all forms of the disease, is due to the complex effect of toxins of the pathogen and is characterized by neurotoxicosis, ITH and thrombohemorrhagic syndrome.
Epidemiology of the plague
Rodents play the leading role in preserving the causative agent in nature, the main ones are marmots (tarbagans), ground squirrels, voles, gerbils, and also lagiformes (rabbits, pikas). The main reservoir and source in anthropurgic foci are gray and black rats, less often house mice, camels, dogs and cats. Of particular danger is a person who has a pulmonary form of the plague. Among animals, the main spreader (carrier) of the plague is the flea, which can transmit the pathogen 3-5 days after infection and retains the infectiousness up to a year. The mechanisms of transmission are diverse:
- transmissible - with a bite of an infected flea;
- contact - through damaged skin and mucous membranes when removing skins from sick animals: slaughter and cutting of carcass, hare, as well as rats, tarbagans, which in some countries consume: when in contact with the discharge of a sick person or with the infected objects:
- fecal-oral - when eating not enough heat-treated meat of infected animals:
- aspirating - from a person sick with pulmonary forms of plague.
The morbidity of humans precedes an epizootic among rodents. The seasonality of the disease depends on the climatic zone and in countries with a temperate climate it is registered from May to September. The susceptibility of a person is absolute in all age groups and under any mechanism of infection. The sick bubonic form of the plague before the opening of the bubo does not pose a danger to others, but when it goes into the septic or pulmonary form it becomes highly infectious, highlighting the pathogen with sputum, the secret of bubo, urine, feces. Immunity is unstable, repeated cases of the disease are described.
Natural foci of infection exist on all continents, except for Australia: in Asia. Afghanistan, Mongolia. China, Africa, South America, where about 2,000 people are registered each year.
Specialists of anti-plague institutions and epidemiologists monitor the epidemic situation in these regions. Over the past 30 years, group outbreaks have not been recorded in the country, and the incidence rate has remained low - 12-15 episodes a year. Every case of human disease should be reported to the territorial center in the form of an emergency notification followed by a quarantine announcement. International rules define a quarantine for 6 days, observing contact with plague of persons is 9 days.
At present, the plague is included in the list of diseases, the causative agent of which can be used as a means of bacteriological weapons (bioterrorism). Highly virulent strains resistant to common antibiotics were obtained in laboratories.