Anthrax is a serious infectious disease caused by a spore-forming bacterium called Bacillus anthracis. It primarily affects livestock and wild animals but can infect humans who come into contact with contaminated animals, animal products, or spores. The disease takes different forms depending on how the spores enter the body, and severity ranges from a treatable skin infection to a life-threatening illness with mortality rates above 85% if left untreated.
What Causes Anthrax
Anthrax is caused by Bacillus anthracis, a rod-shaped bacterium that forms spores when conditions become hostile. These spores are the actual infectious agent for all forms of the disease. What makes them so dangerous is their durability: spores can survive in soil for decades, possibly centuries, resisting heat, UV radiation, dehydration, and chemical exposure that would kill virtually any other bacterium.
This resilience comes from the spore’s physical structure. The inner core is extremely dehydrated and metabolically dormant, wrapped in protective layers that block water and chemicals from getting in. The outermost shell resists enzymatic and chemical breakdown. But once spores land in a hospitable environment, like the warm, nutrient-rich tissue inside a human body, they can germinate and begin multiplying within hours.
Soil is the natural reservoir for anthrax spores. Grazing animals pick them up while feeding, and humans typically become infected through contact with sick animals, contaminated hides or wool, or undercooked meat. Anthrax does not spread from person to person.
How Anthrax Toxins Damage the Body
Once inside the body, the bacteria produce two main toxins that work together to overwhelm the immune system and damage tissues. The first, called lethal toxin, is an enzyme that disables key signaling pathways inside cells. By cutting and inactivating proteins that cells rely on to coordinate their immune response, lethal toxin essentially blinds the body’s defenses at a molecular level.
The second, edema toxin, floods cells with a signaling molecule called cAMP by converting it from the cell’s energy supply (ATP) at a rate of 1,000 to 2,000 molecules per second. This causes cells to dump water and electrolytes into surrounding tissues, producing the swelling that gives the toxin its name. Because it also depletes the cell’s energy reserves, it disrupts a range of normal cellular functions beyond just fluid balance. Together, these toxins are what make systemic anthrax so rapidly fatal.
Cutaneous Anthrax: The Most Common Form
About 95% of naturally occurring anthrax cases are cutaneous, meaning the spores enter through a cut or scrape in the skin. This form most often appears on the head, neck, forearms, and hands. Symptoms typically develop one to seven days after exposure.
The hallmark sign is a skin sore that starts as an itchy bump or blister, then progresses into a painless ulcer with a distinctive black center (the word “anthrax” comes from the Greek word for coal). The surrounding area often swells. Cutaneous anthrax is the least dangerous form of the disease and responds well to antibiotics.
Inhalation Anthrax: The Most Dangerous Form
Inhalation anthrax occurs when spores are breathed into the lungs. It follows a characteristic two-phase pattern. The first phase feels like a common cold or flu: fever, body aches, fatigue, and a dry cough. This initial stage can last hours to a few days, and because the symptoms are so generic, it often goes unrecognized.
The second phase arrives abruptly. Severe shortness of breath, low oxygen levels, heavy sweating, and shock develop rapidly. Without treatment, inhalation anthrax kills 85% to 90% of those infected. Even with aggressive treatment using multiple antibiotics and antitoxin therapy, mortality drops by only 40% to 50%. The narrow window between the mild first phase and the catastrophic second phase is what makes this form so lethal. Symptoms typically begin one to seven days after inhaling spores.
Gastrointestinal and Injection Anthrax
Gastrointestinal anthrax develops after eating raw or undercooked meat from an infected animal. Symptoms include nausea, bloody vomiting, stomach pain, swelling of the abdomen, and fainting. This form is rare in developed countries but occurs in regions where livestock anthrax is common and meat inspection is limited.
Injection anthrax is the newest recognized form, first identified in heroin users in northern Europe. It occurs when spores are introduced directly into the body through contaminated drugs injected under the skin or into muscle. It can resemble cutaneous anthrax but tends to cause deeper tissue infection and more severe swelling, making it harder to diagnose and more dangerous. Unlike cutaneous anthrax, it does not produce the classic black sore on the skin’s surface.
How Anthrax Is Diagnosed
Growing the bacteria from a patient’s blood, skin, or other tissue sample remains the gold standard for confirming anthrax. Doctors also use PCR testing, which detects the bacterium’s genetic material and can return results faster than a traditional culture. For skin infections, a small tissue sample (punch biopsy) is taken from the sore for both culture and microscopic examination.
If a patient has already started antibiotics, live bacteria may be harder to grow, so doctors rely more heavily on tissue staining and a technique called immunohistochemistry that identifies bacterial proteins in the sample. Blood tests can also detect anthrax toxins directly. Because anthrax is rare and its early symptoms mimic common illnesses, diagnosis often depends on a doctor considering the possibility based on a patient’s exposure history.
Treatment
Antibiotics are the backbone of anthrax treatment. For cutaneous anthrax without complications, a single oral antibiotic is typically enough. The preferred options are doxycycline, ciprofloxacin, or levofloxacin, taken for a course lasting weeks depending on the circumstances of exposure.
Systemic anthrax, meaning any form that has spread beyond a local skin infection, requires a more aggressive approach. Patients receive multiple intravenous antibiotics simultaneously, combining a bactericidal drug (one that kills the bacteria directly) with a protein synthesis inhibitor (one that stops the bacteria from producing toxins). For systemic cases, antitoxin therapy is added on top of antibiotics. Two antitoxin drugs are available, both given as a single intravenous dose, and they work by neutralizing the toxins already circulating in the bloodstream.
The critical factor in survival is timing. Once inhalation anthrax progresses to its second phase, even the best available treatment fails in a significant number of cases. Early recognition and immediate antibiotic therapy are what save lives.
Who Gets Vaccinated
An anthrax vaccine exists and is used for people at elevated risk of exposure. This includes military personnel deployed to regions where anthrax could be used as a weapon, laboratory workers who handle the bacterium, and some veterinarians or livestock handlers in high-risk areas. The vaccine requires multiple doses over a period of months to build full protection, followed by annual boosters.
For the general public, vaccination is not recommended or necessary. After a known exposure event, such as a bioterrorism attack, people who may have inhaled spores are given antibiotics as post-exposure prophylaxis, sometimes combined with the vaccine to extend protection beyond the antibiotic course.
Why Anthrax Is a Bioterrorism Concern
The 2001 anthrax letter attacks in the United States, which killed five people and infected 17 others, demonstrated why anthrax ranks among the top bioterrorism threats. Spores are small enough to be inhaled deeply into the lungs, stable enough to survive being mailed in an envelope, and can be produced in quantities sufficient to cause mass casualties. The bacteria’s natural ability to form nearly indestructible spores means no special engineering is needed to create a durable weapon.
Anthrax is classified as a Category A bioterrorism agent by the CDC, the highest threat tier. This classification drives ongoing investment in stockpiling antibiotics and antitoxins, maintaining vaccine production capacity, and training public health systems to detect and respond to an anthrax release quickly enough to get people treated before symptoms progress.