General Information on Anthrax


Anthrax is an acute infection caused by the Gram-positive, spore forming, bacteria

Bacillus anthraci
s. Anthrax naturally infects many species of grazing mammals such as sheep, cattle and goats, which are infected through ingestion of soil contaminated by
B. anthracis
spores. Human infection is acquired by skin contact with, inhalation or ingestion of
B. anthracis
spores, usually from infected animals or animal products. In over 95% of cases the infection is cutaneous, due to inoculation of spores into small abrasions on the skin.

Deliberate release of anthrax

The threat of a deliberate release of anthrax is of the release of large quantities of spores in an aerosol. This threat is considered serious because:


The organism is relatively easy to cultivate from environmental sources.


The inhalation form of disease has a high mortality rate.

Despite this, the creation of an infective anthrax aerosol is not easy – particles need to be between 1 and

mm in size and sufficient energy is required to disperse them. It is unlikely therefore that contact with letters and packages, which sometimes claimed to be contaminated, constitutes a significant exposure risk.


Anthrax is a zoonosis to which most mammals, especially grazing herbivores, are susceptible. Human infections usually result from contact with infected animals or animal products. Direct exposure to secretions from cutaneous anthrax lesions may result in secondary cutaneous infection, but there have been

no known cases of person-to-person transmission of inhalation diseas


The spores of

B. anthracis
are extremely durable. Modes of transmission include:


Cutaneous contact with spores, spore contaminated materials or infected skin lesions. Infection required an existing break in the skin.


Inhalation of spores.


Ingestion of contaminated meat.

Infectious dose

The ID50 for inhalation anthrax is estimated at 10,000 spores. (This is the infectious dose required to cause disease in 50% of those exposed by inhalation).

Incubation period

1 day to 8 weeks (mode 5 days), dependent on dose and exposure route.


3-7 days following cutaneous exposure.


1-6 days following inhalation exposure.


1-7 days following ingestion.

Period of communicability


Transmission of anthrax infection from person to person is highly unlikely.


Contact with skin lesions can result in subsequent cutaneous infection.


Airborne transmission from person to person does

Clinical features

Human anthrax can occur in three forms: inhalation/pulmonary, cutaneous or gastrointestinal, depending on the route of exposure, and details of these diseases are given below.

It can be expected that any malicious or deliberate release of anthrax spores will involve aerosol exposure. Clinicians should be aware of the possibility of cases of inhalation anthrax, and any previously healthy patient with the following clinical presentations should be immediately reported to the Consultant in Communicable Disease Control or their deputy.


A severe, unexplained febrile illness or febrile death.


Severe sepsis or respiratory failure with a widened mediastinum.


Severe sepsis with
Bacillus species or
gram-positive rods identified from clinical specimens into the blood or cerebrospinal fluid.



Non-specific prodrome of flu-like illness following inhalation of spores with fever, headache, myalgia and non-productive cough. Two to four days after initial symptoms, there is
abrupt onset of respiratory failure
and on chest X-ray a
widened mediastinum
is often present, suggestive of mediastinal lymphadenopathy and haemorrhagic mediastinitis.


Gram-positive bacilli seen in blood cultures, usually after 2-3 days of onset of illness.


Treatment may be successful in the prodromal stage, but by the time respiratory or bacteraemic symptoms develop, treatment may not arrest the disease before a fatal outcome.



Local skin involvement after direct contact.


Commonly seen on hands, forearms and head.


Three days after exposure a raised, itchy, inflamed pimple appears followed by a papule that turns vesicular and then 2-6 days later a black eschar develops.  Extensive oedema accompanies the lesion.


Responds to oral antibiotics.


Rarely may progress to bacteraemia or meningitis without treatment.





Characterised by severe abdominal pain, nausea and vomiting with watery or bloody diarrhoea.


2-3 days after onset bacteraemia may develop.


Usually fatal if it progresses to bacteraemia.


Systemic infection resulting from inhalation of the organism has a mortality rate approaching 100%, with death usually occurring within a few days after the onset  of symptoms. Cutaneous anthrax, the most common form, is usually curable with antibiotics. The mortality rate among people with infection resulting from ingestion is variable, but may also approach 100%.

Organism survival

Anthrax endospores do not divide, have no detectable metabolism, and are resistant to drying, heat, UV light, gamma irradiation and many disinfectants. In some types of soil, anthrax spores can remain dormant for decades.

Antimicrobial susceptibilities

Most naturally occurring anthrax strains are sensitive to penicillin which historically has been the preferred therapy for the treatment of anthrax. There are no clinical studies of the treatment of inhalational anthrax in humans. Thus, antibiotic regimens commonly recommended for first line treatment of sepsis have not been studied in this setting. Natural strains of

B. anthracis
are resistant to extended-spectrum cephalosporins.

In studies of small numbers of monkeys infected with susceptible strains of

B. anthraci
s, oral doxycycline has proved efficacious. Doxcycline is therefore the preferred option from the tetracycline class of antibiotics because of its proven efficacy in monkey studies and its ease of adminstration. Other members of this class of antibiotics are suitable alternatives.

Although treatment of anthrax infection with ciprofloxacin has not been studied in humans, animal models suggest excellent efficacy. In-vitro data suggest that other fluroquinolone antibiotics would have equivalent efficacy in treating anthrax infection, although no animal data exist for fluoroquinolones other than ciprofloxacin. Pharmacokinetic studies of ciprofloxacin in humans have demonstrated excellent penetration into lung tissue following oral administration.  Ciprofloxacin has the added advantage that it is also the first line prophylactic treatment for other potential bioterrorist agents such as plague and tularaemia.


Source:  The U.K. Public Health Laboratory Service, "Provisional Guidelines for Action in the Event of a Deliberate Release.  Anthrx:  Issue 1/Version2, October 12, 2001.