What is Anthrax?

The bacterium that causes anthrax can infect the skin, mouth, throat, intestines, bloodstream, meninges and/or lungs. Anthrax is caused by a bacterium called Bacillus anthracis (figure 1). This bacterium can is present nearly anywhere in the world. It exists in two forms: as a vegetative cell and as a spore (figure 2; 9, 47). 

When there are plenty of nutrients the majority of the Bacillus anthracis cells are in the form of the vegetative cell. Vegetative cells actively multiply and are easily killed by antibiotics and disinfectants. When nutrients become scarce most of the vegetative cells stop multiplying and develop into the spores. The spore forms inside the vegetative cell. Another name for the spore is endospore (endo= inside). Spores do not multiply or grow but rather are a means the bacteria use to survive in harsh conditions. Anthrax spores can survive for many years in the soil and can withstand drying, high temperatures, lack of nutrients and treatment with antibiotics.  

The spore is the most common form of the bacteria in the soil and is the infectious form of the bacterium. A small number of spores are needed to cause an infection in humans. A dose of between 2,500 and 55,000 spores given to a group of animals has been shown to kill at least 50 percent them. However, after the 2001 events some investigators are estimating that a few as 1-3 spores might cause infection in certain people (elderly). 

Anthrax is a rare cause of human disease. From 1979-2004 there have only been 32 cases of anthrax reported to the U.S. Centers for Disease Control (59). Twenty-two of those cases occurred in 2001 when several anthrax-laced letters were sent to various people around the United States (30). 

Anthrax is much more common in domesticated and wild animals like sheep, cattle, goats, horses, and swine (pigs). These herbivores are more likely to ingest anthrax spores present in the soil while grazing. 

Humans are usually infected by Bacillus anthracis following contact with soil containing spores, infected animals or spore contaminated animal products (wool, goat hair and hides). There are three routes of human infection: inoculation into the skin (cutaneous anthrax), by inhalation (inhalation anthrax), and by ingestion (oropharyngeal (disease of mouth and throat) or gastrointestinal (infection of intestines) anthrax). Most BW experts believe that an aerosol release of Bacillus anthracis would cause the highest number of human causalities. 

Cutaneous anthrax

Usually more than 95% of all cases of anthrax are cutaneous infections. A small cut in the skin allows the spores to get in and cause damage to the skin resulting in a skin lesion called a black eschar (figure 3). The black appearance of the lesion gives anthrax its name from the Greek word anthrakos meaning coal. 

Infection begins when the spores enter the skin through small cuts or scraps. The spores become vegetative cells in the host (human or animal) and produce bacterial toxins. One to five days after the spores get in the skin a pimple-like sore develops. Over the next 1-2 days the pimple will become an itchy painless fluid filled sore with swelling around it. Sometimes the swelling can be severe and affect a person's entire face or limb. The bacterial toxins cause swelling, bleeding, and eventually tissue death (black center of the lesion) at the site of infection.

Patients may have fever, malaise, and a headache. Once the fluid-filled sore opens, it forms the black eschar. All around the black area is a ridge of red swollen tissue (figure 3). After 2-3 weeks, the black tissue falls off, leaving a scar. Occasionally, the bacteria can get into the bloodstream leading to septic shock and death. Infection of the meninges can occur if the organisms get into the bloodstream. With adequate treatment, less than 1% of people infected with cutaneous anthrax die.  If not treated with antibiotics the mortality rate can be as high as 20%.

Inhalation anthrax

Anthrax spores are inhaled and phagocytized by the alveolar macrophages in the alveolar sacs of the lungs and taken to the lymph nodes that are between the left and right lung (mediastinal lymph nodes). The alveolar macrophages cannot kill the spores. The spores sensing that they are in a nutrient rich environment become vegetative cells (exsporulate). The vegetative cells produce toxins and cause bleeding and swelling inside the chest cavity. The bacteria can also then get into the bloodstream and cause septic shock and blood poisoning, which may lead to death. 

Another name for this form of anthrax is woolsorter's disease. This is because it was at one time a common disease in people that sorted wool. The time from inhalation of the spores to symptoms appearing (incubation time) can be from one to six days or as long as 60 days. Initial symptoms are similar to many other respiratory diseases and include headache, tiredness, body aches, and fever. The victim may have a dry cough and mild chest pain. These symptoms usually last for 2-3 days. 

Some people may then feel better for a couple of days followed by the sudden onset of increased trouble breathing (dyspnea), shortness of breath, bluish skin color (cyanosis), increased chest pain, and sweating. A chest X-ray oftentimes shows the area between the two lungs is wider than usual (mediastinal widening; figure 4) demonstrating swelling of the mediastinal lymph nodes due to infection by Bacillus anthracis. Swelling of the chest and neck may also occur. 

The bacteria can also get into the bloodstream and be taken by the blood to the brain where the bacteria can infect the membranes that cover the brain (meningitis). Once in the bloodstream shock and death may follow within 24-36 hours. Even with appropriate treatment once severe symptoms begin (e.g. dyspnea, cyanosis, chest pain) most patients will die. Without treatment the death rate is close to 100%. While in the bloodstream the bacteria can also infect the meninges (see below). 

Inhalation anthrax is not spread from person to person because rather than being in the alveolar sacs of the lungs the bacteria are in the mediastinal lymph nodes and do not get into the air when a person with inhalation anthrax coughs. 

Oropharyngeal and gastrointestinal anthrax

When a person eats uncooked or undercooked meat of an animal infected with anthrax they are more likely to get this form of the disease. After an incubation period of 2-5 days, patients with oropharyngeal disease develop a severe sore throat or sores in the mouth or on the tonsils. Fever and neck swelling may occur and the patient may have trouble breathing. 

Gastrointestinal anthrax begins with symptoms similar to some other gastrointestinal diseases: nausea, vomiting, and fever. These symptoms are followed in most persons by severe abdominal pain. The person may also vomit blood and have diarrhea. The bacteria can then get into the bloodstream causing septic shock and death. The death rate of this form of anthrax is close to 100%. The bacteria can also get into the meninges (see below).  

Meningeal Anthrax

One complication of all forms of anthrax is that the bacteria can get into the bloodstream. The blood then takes the bacteria to the meninges that cover the brain. Symptoms of meningeal irritation include headache, fever and neck stiffness. Some people that have inhaled Bacillus anthracis do not have any symptoms associated with their respiratory tract but only show symptoms of meningeal anthrax. Death is common and usually occurs in 1 to 6 days following the start of symptoms. 

Why is anthrax an attractive BW?

Bacillus anthracis if used in an aerosol attack could cause many cases of inhalation anthrax. If the bacterial spores are inhaled symptoms might not appear for up to 60 days. Some estimate that if 100 kg of spores were released over a city the size of Washington D.C. 130,000 to 3 million deaths could result depending on weather and wind conditions and the type of anthrax produced (dry aerosol with particle size from 0.5-5 microns; 7). Fortunately, inhalation anthrax cannot be transmitted person-to-person. The spores that are released are infectious for years and the area would have to be decontaminated before it could be reoccupied.  

Laboratory Diagnosis
It is common in all forms except the cutaneous form of anthrax for the bacteria to get into the bloodstream. Usually some blood is taken and placed in blood culture bottles. Samples from skin lesions, respiratory secretions (sputum) or the fluid that surrounds the brain and is in the spinal cord (cerebrospinal fluid) can be used to grow the bacteria on sheep blood agar plates. It can take from 24 to 48 hours to identify the organism in the blood. A quicker method involves looking for antibodies to Bacillus anthracis in a personís blood. Assays of clinical and environmental samples from the 2001 anthrax-letter events revealed that real-time PCR was as sensitive and specific as culture in clinical specimens and nearly as specific as culture from environmental samples and could be performed in much less time (3-4 hours) (70). 

Samples needed differ a little depending the form of anthrax: Cutaneous - vesicular fluid from the blisters and blood; Inhalational - blood, cerebrospinal fluid (if infection has gotten into the meninges) or chest X-ray; Gastrointestinal Ė blood and stool specimens. Nasal swabs are not used to determine if a person is infected with Bacillus anthracis but are most helpful in determining where the organism came from since swabbing environmental sources (desks, floors, air vents) aids public health workers in determining the source of the infections. 


The sooner treatment is started the more likely a person with anthrax will survive. All forms of anthrax can be treated with anyone of several different antibiotics. The most commonly used antibiotics are doxycycline, penicillin or ciprofloxacin. 

Treatment of cutaneous anthrax does not affect the skin lesion that develops but it does eliminate any bacteria that may have gotten into the bloodstream and will significantly improve the personís chances of recovery from the infection. Oftentimes the antibiotic is given orally. People with the other forms of anthrax are given antibiotics intravenously. 


Since most cases of anthrax are the result of animal contact or consumption then control of the disease in animals will prevent most cases of anthrax in humans. All domesticated animals (cattle, goats, pigs, sheep) in the United States have to be vaccinated and that has been effective in lowering the number of human anthrax cases. 

There is an anthrax vaccine for use in humans and it is effective in preventing invasive disease (bloodstream infections, meningitis). However, due to the limited supply and adverse effects of the vaccine it is reserved for those most likely to have contact with this organism. 

High-risk people include (9):

Unvaccinated people that have been exposed to anthrax spores can be given antibiotics that will kill the organisms before they have a chance to cause disease. This method of protecting people exposed to an illness but not yet ill is called chemoprophylaxis or prophylaxis. This method was used quite successfully to protect people in 2001 exposed to the aerosols from the anthrax-laced letters.


© 2005 Neal Chamberlain. All rights reserved. 
Site Last Revised 5/2/05
Neal Chamberlain, PhD. A. T. Still University of Health Sciences/Kirksville College of Osteopathic Medicine.

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