Merck Manual

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Larry M. Bush

, MD, FACP, Charles E. Schmidt College of Medicine, Florida Atlantic University

Last full review/revision Jun 2019| Content last modified Jun 2019
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Anthrax is a potentially fatal infection with Bacillus anthracis, a gram-positive, rod-shaped bacteria (see figure How Bacteria Shape Up). Anthrax may affect the skin, the lungs, or, rarely, the digestive (gastrointestinal) tract.

  • Infection in people usually results from skin contact but can result from inhaling anthrax spores or eating contaminated meat.

  • Anthrax spores are a potential biological weapon.

  • Anthrax bacteria produce several toxins, which cause many of the symptoms.

  • Symptoms include bumps and blisters (after skin contact), difficulty breathing and chest pain (after inhaling spores), and abdominal pain and bloody diarrhea (after eating contaminated meat).

  • Symptoms suggest the infection, and identifying the bacteria in samples taken from infected tissue confirms the diagnosis.

  • People at high risk of being exposed to anthrax are vaccinated.

  • Antibiotics must be given soon after exposure to reduce the risk of dying.

(See also Overview of Bacteria.)

Anthrax can occur in wild and domestic animals that graze, such as cattle, sheep, and goats. Anthrax bacteria produce spores that can live for years in soil. Grazing animals become infected when they have contact with or consume the spores. Usually, anthrax is transmitted to people when they have contact with infected animals or animal products (such as wool, hides, and hair). Spores may remain in animal products for decades and are not easily killed by cold or heat. Even minimal contact is likely to result in infection. Although infection in people usually occurs through the skin, it can also result from inhaling spores or eating contaminated, undercooked meat.

Skin anthrax may be spread from person to person by direct contact with an infected person or an object contaminated by an infected person. However, anthrax due to inhaling spores (inhalation anthrax) or due to eating contaminated meat (gastrointestinal anthrax) is not spread from person to person.

Anthrax is a potential biological weapon because anthrax spores can be spread through the air and inhaled. In the U.S. anthrax bioattacks of 2001, spores were spread in envelopes mailed via the United States Postal Service.

Anthrax bacteria produce several toxins, which cause many of the symptoms.


Anthrax symptoms vary depending on how the infection is acquired:

  • Through the skin (most cases)

  • Through inhalation (most serious)

  • Through the gastrointestinal tract (rare)

Skin anthrax

Most anthrax cases involve the skin. A painless, itchy, red-brown bump appears 1 to 10 days after exposure. The bump forms a blister, which eventually breaks open and forms a black scab (eschar), with swelling around it. Nearby lymph nodes may swell, and people may feel ill—sometimes with muscle aches, headache, fever, nausea, and vomiting. It may take several weeks for the bump to heal and the swelling to go down.

About 10 to 20% of untreated people die, but with treatment, death is rare.

Inhalation anthrax (woolsorter’s disease)

Inhalation anthrax is the most serious. It results from inhaling anthrax spores, almost always when people are working with contaminated animal products (such as hides).

Spores may stay in the lungs for weeks but eventually enter white blood cells called macrophages, where they germinate, and the resulting bacteria multiply and spread to lymph nodes in the chest. The bacteria produce toxins that make the lymph nodes swell, break down, and bleed, spreading the infection to nearby structures. Infected fluid accumulates in the space between the lungs and the chest wall.

Symptoms develop 1 day to 6 weeks after exposure. Initially, they are vague and similar to those of influenza, with mild muscle aches, a low fever, chest discomfort, and a dry cough. After a few days, breathing suddenly becomes very difficult, and people have chest pain and a high fever with sweating. Blood pressure rapidly becomes dangerously low (causing shock), followed by coma. These severe symptoms probably result from a massive release of toxins.

Gastrointestinal anthrax or an infection of the brain and the tissues covering the brain and spinal cord (meninges)—an infection called meningoencephalitis—may develop.

Many people die 24 to 36 hours after severe symptoms start, even with early treatment. Without treatment, all people with inhalation anthrax die. In the 2001 outbreak in the United States, 45% of people treated for inhalation anthrax died.

Gastrointestinal anthrax

Gastrointestinal anthrax is rare. When people eat contaminated meat, the bacteria grow in the mouth, throat, or intestine and release toxins that cause extensive bleeding and tissue death. People have a fever, a sore throat, a swollen neck, abdominal pain, and bloody diarrhea. They also vomit blood.

Even with treatment, about half of infected people die, probably because they have already become very ill before the diagnosis is made.

Did You Know...

  • Anthrax spores are not easily killed by cold or heat and can survive for decades.

  • Over 1.25 million people have received the anthrax vaccine without having a serious adverse reaction.


  • Examination or culture of samples of infected skin, fluids, or stool

  • Sometimes blood tests

Doctors suspect skin anthrax based on its typical appearance. Knowing that people have had contact with animals or animal products or were in an area where other people developed anthrax supports the diagnosis.

If inhalation anthrax is suspected, chest x-ray or computed tomography (CT) is done.

Samples from infected skin, fluids around the lungs, or stool are removed and examined with a microscope or cultured (enabling bacteria, if present, to multiply). Anthrax bacteria, if present, can be readily identified.

If people have inhalation anthrax and symptoms (such as confusion) suggesting that the brain may be affected, doctors may also do a spinal tap (lumbar puncture) to obtain a sample of the fluid that surrounds the brain and spinal cord (cerebrospinal fluid). The sample is examined and analyzed.

Blood tests may be done to check for fragments of the bacteria’s genetic material or antibodies to the toxins produced by the bacteria.


  • Vaccination

  • Preventive antibiotics and sometimes other drugs

A vaccine against anthrax can be given to people at high risk of infection. Because of anthrax’s potential as a biological weapon, most members of the armed forces have been vaccinated. To be effective, the vaccine must be given in five doses. A booster shot, given yearly, is also recommended. Despite widely publicized anxiety, over 1.25 million people have received the anthrax vaccine without having serious adverse reactions.

People who are exposed to inhalation anthrax are given an antibiotic by mouth, usually ciprofloxacin, levofloxacin, or doxycycline or, if they cannot take these antibiotics, amoxicillin. The antibiotic is continued for at least 60 days to prevent the infection from developing. These people are also given three doses of the vaccine. If these treatments are not available or people cannot receive them, they may be given injections of raxibacumab or obiltoxaximab (antibodies that can bind anthrax toxins in the person's system).


  • Antibiotics

  • Sometimes other drugs

The longer anthrax treatment is delayed, the greater the risk of death. Thus, treatment is usually started as soon as doctors suspect that people have anthrax:

  • Skin anthrax is treated with ciprofloxacin, levofloxacin, or doxycycline given by mouth for 7 to 10 days.

  • Inhalation, gastrointestinal, or severe skin anthrax is treated with a combination of two or three antibiotics, including intravenous ciprofloxacin or doxycycline plus another antibiotic, such as ampicillin, clindamycin, rifampin, or others.

  • Inhalation anthrax can also be treated with a combination of antibiotics and injections of raxibacumab or obiltoxaximab (monoclonal antibodies that bind anthrax toxins in the person's system) or with a combination of antibiotics and intravenous anthrax immune globulin.

  • If the brain and meninges are affected or if fluid has accumulated around the lungs, corticosteroids may help.

Other treatments may include mechanical ventilation to help with breathing and fluids and drugs to increase blood pressure.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Gammagard S/D
No US brand name
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