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,
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
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.
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%.
If not treated with antibiotics the mortality rate can be as high as 20%.
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
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.
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).
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.
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.
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
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
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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