Causes of anthrax

Causes of anthrax

Anthrax is caused by a large, gram-positive, non-motile rod Bacillus anthracis of the genus Bacillus of the family Bacillaceae, an aerobe or facultative anaerobe. It grows on simple nutrient media and forms spores when exposed to free oxygen. Under favorable conditions (entering a living organism), it forms a vegetative form. The pathogen contains two capsular polypeptide and one somatic polysaccharide antigens. It produces an exotoxin consisting of protein and lipoprotein, and includes a protective antigen. It interacts with cell membranes and mediates the action of other components: a lethal factor with a cytotoxic effect and causing pulmonary edema, and an edema factor causing an increase in cAMP concentration and the development of tissue edema. The components of the toxin cause a toxic effect only when acting together. The pathogenicity of B. anthracis depends on the presence of a capsule and toxin formation. Strains that do not have these abilities are avirulent. The capsule plays an important role at the beginning of the infectious process, preventing phagocytosis of the pathogen. The toxin mediates the main clinical manifestations of the disease. Vegetative forms of the microbe are unstable, they die instantly when boiled, disinfectant solutions kill them in a few minutes. In unopened corpses, the pathogen survives for 7 days. Spores are formed after the death of the host, they are extremely stable, withstand boiling for up to 30 minutes, and survive rapid drying and freezing. Disinfectants (1% formalin solution, 10% sodium hydroxide solution) kill them in 2 hours. In the soil, they can survive for several decades (up to 60 years) and germinate both when entering a living organism and in the soil under favorable conditions.

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Pathogenesis of anthrax

Infection occurs when both the vegetative form and the spores of the pathogen enter the human body, where they are protected from phagocytosis due to the capsule and produce an exotoxin that damages the vascular endothelium. Microcirculatory disorders and increased vascular permeability are an important link in the pathogenesis of anthrax. As a result of these changes, serous-hemorrhagic inflammation, perivascular hemorrhages, hemorrhagic infiltrates, and severe edema develop in the places where the pathogen reproduces (skin, lymph nodes, lungs, intestinal wall). When infected by aerosol and alimentary routes, the pathogen easily overcomes the lymphatic barrier and spreads hematogenously. The infection becomes generalized with massive seeding of organs (septicemia), accompanied by the development of infectious-toxic shock, thrombohemorrhagic syndrome, and multiple organ failure. In percutaneous infection, generalization of infection is rarely observed. Inflammation is limited to the skin and is local in nature, but toxins cause increased vascular permeability with the development of extensive edema and local anesthesia.

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Epidemiology of anthrax

The reservoir of infection is the soil, in which, due to the repetition of biological cycles (spore-vegetative cell), the pathogen is preserved and accumulated for a long time. This feature of B. anthracis contributes to the creation of long-term active soil foci ("cursed" fields) and potentially dangerous territories. This leads to periodic epizootics and cases of anthrax in humans. The disease develops when vegetative cells or spores of the pathogen enter the body. Sources of B. anthracis for humans are large (buffalo, cows) and small (goats, sheep) cattle, horses, camels, and also (in rare cases) wild animals (hares, wolves, bears, arctic foxes, etc.). Sick animals excrete the pathogen with urine, feces and other secretions. The incidence of human disease depends on the degree of spread of this infection among animals. The mechanisms of transmission of the pathogen to humans are contact (when personal hygiene rules are violated when caring for sick animals, slaughtering and cutting up carcasses, removing skins, contact with wool, skins and other livestock products contaminated with B. anthracis), aspiration (airborne dust when inhaling infected dust, bone meal), feco-oral (food route when eating meat from an infected animal) and transmission (through the bites of horseflies, stinging flies, mosquitoes). Transmission factors are soil, water, air, food, household items, animals and their waste products, ectoparasites.

There are three types of anthrax: professional-agricultural, professional-industrial and household. Professional-agricultural cases in countries with a temperate climate are recorded mainly in livestock regions from May to October. The probability of developing the disease depends on the dose of the pathogen, the method of infection and the resistance factors of the macroorganism. With a contact mechanism of transmission, a person is slightly susceptible to the pathogen and infection is possible only if the integrity of the skin and mucous membranes is compromised. With airborne dust and alimentary routes of infection, susceptibility is almost 100%. Sick people do not pose a danger to others. Immunity in those who have recovered is unstable, and cases of repeated illnesses are known.

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