Anthrax is classified as a zoonotic disease, meaning it is naturally transmissible from animals to humans. The infection is caused by the bacterium Bacillus anthracis, a rod-shaped, spore-forming organism found in the environment worldwide. Anthrax has been recognized for centuries, with historical accounts of its devastating impact on livestock and human populations. Its high fatality rate makes it a disease of significant public health concern.
Understanding Bacillus anthracis and Spore Formation
The disease is caused by Bacillus anthracis, which possesses a unique trait enabling its long-term survival outside of a host. When conditions are unfavorable, the vegetative bacteria transform into highly durable spores. This process, known as sporulation, is triggered by the presence of oxygen and is a defensive mechanism.
These bacterial spores are exceptionally resistant to extreme temperatures, desiccation, chemical disinfectants, and ultraviolet radiation. The spores can persist in the soil for years, acting as an environmental reservoir for the disease. This resilience explains why anthrax outbreaks are often linked to contaminated soil in endemic regions. When a susceptible host comes into contact with these dormant spores, they germinate inside the body, revert to their active vegetative form, and rapidly multiply to cause acute disease.
The Natural Cycle: Anthrax in Animal Populations
Anthrax naturally cycles through the environment, primarily affecting grazing animals such as cattle, sheep, goats, and wild herbivores. These animals become infected when they ingest or inhale the long-lived Bacillus anthracis spores from contaminated soil, vegetation, or water. The disease in animals is often acute, leading to sudden death with little to no prior clinical signs.
The animal carcass plays a central role in perpetuating the cycle. When an animal dies from anthrax, the vegetative bacteria, abundant in the bloodstream, sporulate upon contact with oxygen. The shedding of these bacteria and subsequent sporulation around the carcass creates a highly infectious zone that contaminates the soil for years. Environmental factors like alkaline soil, high organic matter content, and specific weather events such as flooding or drought influence the distribution and re-emergence of spores, leading to recurrent outbreaks in grazing populations.
How Humans Contract Anthrax
Humans are considered accidental or “dead-end” hosts, as anthrax is not transmitted from person to person. Infection occurs through contact with infected animals, contaminated animal products, or environmental exposure to the spores. The route of entry determines the specific clinical form of the disease.
The most common form worldwide is cutaneous anthrax, which results from spores entering a cut or abrasion on the skin. This usually happens when people handle infected livestock, carcasses, hides, wool, or hair. The site of infection develops a characteristic painless sore that progresses into a black, depressed ulcer, called an eschar.
Inhalation anthrax, the most severe form, occurs when a person breathes in a large number of airborne spores. This exposure is usually occupational, affecting people working with contaminated animal products like wool, or in the event of an intentional release. The initial symptoms resemble a common cold, but this rapidly progresses to severe respiratory distress and shock. Outcomes are often fatal if not treated quickly.
Gastrointestinal anthrax can develop after consuming raw or undercooked meat from an infected animal. This form causes severe abdominal pain, vomiting, and bloody diarrhea, and its fatality rate can be substantial. A less common form, injection anthrax, has been linked to injecting contaminated heroin.
Prevention and Control Strategies
Controlling anthrax relies on a unified “One Health” approach, integrating efforts across human, animal, and environmental health sectors. The most effective strategy for breaking the zoonotic cycle is the systematic vaccination of livestock in endemic areas. The Sterne spore vaccine, which is widely used, helps protect domestic animals like cattle and sheep and reduces the pool of infection available to humans.
Proper management of animal carcasses is also important for preventing environmental contamination. Animals suspected of dying from anthrax should not be necropsied or moved. Their carcasses must be disposed of safely, typically by deep burial with lime or incineration, to prevent spores from forming and spreading.
Humans exposed to spores, either occupationally or during an outbreak, are given a course of antibiotics, often for 60 days, to prevent the dormant spores from activating and causing illness. A human vaccine is available but is primarily reserved for individuals at high occupational risk, such as laboratory workers and certain military personnel.