Epidemiology of anthrax

Pathogen characteristics and forms

The infection is caused by the Gram-positive, non-motile bacillus Bacillus anthracis. It measures 6–10 µm in length with characteristically truncated ends. Depending on environmental conditions, the microbe exists in two forms:

• Vegetative cell: formed inside a susceptible host organism (strictly in an anaerobic environment). In this phase, the microbe synthesizes toxins and develops a capsule.
• Spore: formed in the external environment when free oxygen is available and temperatures range from 15 to 42 °C. Spores are located centrally within the bacterial rod.

Reservoir and sources of infection

The primary and main reservoir for anthrax is the soil. Spores of the microorganism can persist there for decades. The classical ecological chain of transmission proceeds as follows: the pathogen passes from the soil into mammals, and infected animals then become the source of threat to humans.

Main sources of infection:

• Livestock (most common): cows, buffaloes, deer, sheep, and goats. Horses and pigs are significantly less common carriers.
• Wildlife: hares, foxes, arctic foxes, minks, and sables.

It is important to remember that an infected human is a dead-end host—they are epidemiologically completely safe.

Transmission routes and clinical manifestations

The clinical form of the disease that develops in a patient depends directly on the mechanism of microbe entry into the body.

1. Contact route (primary): occurs via microtraumas on the skin upon contact with infected animal secretions (saliva, urine, feces) or when handling raw materials (hides, bones, wool, bristles). This route leads to the cutaneous form. Susceptibility is about 20%.
2. Alimentary route: associated with the consumption of contaminated meat and dairy products. Leads to the gastrointestinal (intestinal) form. Susceptibility in this case reaches 100%.
3. Aerogenic route: inhalation of dust containing spores causes the pulmonary (inhalation) form of anthrax. Susceptibility is also 100%.
4. Vector-borne route: transmission of the bacterium via bites from blood-sucking insects (mosquitoes or stable flies) is possible.

The incidence is often occupational, affecting people who work with animals. The seasonal peak occurs in summer and autumn, coinciding with animal outbreaks (epizootics). In occupational settings, infection is possible year-round.

Toxins and virulence factors

The virulence of Bacillus anthracis is driven by two plasmids: pX 02 encodes the formation of the anti-phagocytic capsule, and pX 01 is responsible for producing a complex protein exotoxin. The exotoxin itself consists of three components that produce pathological effects only in combination with one another.

The protective antigen acts as a carrier molecule, forming channels in target cell membranes to deliver other factors inside:

• Edema toxin (edema factor + protective antigen): disrupts cellular permeability, causes massive tissue edema, and suppresses phagocytosis by increasing cAMP production.
• Lethal toxin (lethal factor + protective antigen): the primary virulence factor. It triggers cytokine release from macrophages, impairs blood clotting, and leads to septic shock.

The complete three-component complex inevitably leads to necrosis, edema, and mortality.

Microorganism survival in the environment

The bacterium demonstrates vastly different survival rates depending on its form.

• Vegetative forms are extremely vulnerable. Heating to 60 °C kills them within 15 minutes, and boiling destroys them instantly. Standard disinfectant solutions inactivate them within minutes. In the carcasses of dead animals, bacteria can persist for up to 7 days.
• Spores exhibit extreme resistance. They can survive boiling for 10–15 minutes while retaining germination capacity. Guaranteed destruction requires autoclaving at 110 °C (for at least 40 minutes) or rigorous chemical treatment (e.g., 1% formalin or 10% sodium hydroxide solution for 2 hours). Spore survival also depends on the temperature at which they formed: spores generated at 18–20 °C possess higher resistance.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Antigenic structure of Bacillus anthracis (somatic and capsular antigens)
• Detailed mechanism of edema and lethal factors at the cellular level
• Environmental conditions stimulating toxin production (bicarbonates, CO2)
• Influence of sporulation temperature on overall spore resistance

Other topics in the section "Zoonotic infections — 2024 study guide"

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