Organism Details

Anthrax is an acute infection caused by the Gram-positive spore-forming bacterium Bacillus anthracis. 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. There are different forms of human disease depending on how infection is acquired: cutaneous, inhalation, ingestion or injection. In over 95% of naturally occurring cases the infection is cutaneous, acquired by inoculation of spores into small abrasions on the skin, usually during handling of untreated animal hides.

Deliberate release of anthrax

The deliberate release of anthrax would be by 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, creation of an infective anthrax aerosol is not easy – particles need to be between 1 and 5µm in size and sufficient energy is required to disperse them.

A deliberate release of anthrax spores occurred in 2001 in the USA. Letters containing the spores were sent through the postal system and resulted in 22 human cases: 11 with cutaneous disease and 11 with inhalation anthrax. Five of the patients with inhalation anthrax died: case-fatality rate of 45%.

Epidemiology

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 disease.

Transmission

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

• Cutaneous contact with spores, spore contaminated materials or infected skin lesions - infection usually requires an existing break in the skin, though this lesion may be so small as to be unnoticed
• Inhalation of spores.
• Ingestion of contaminated meat.

Infectious dose

The ID50 for inhalation anthrax (the infectious dose required to cause disease in 50% of those exposed by inhalation) is generally regarded to be around 10,000 spores; however it may be lower if the infectious particles have been modified or if the person is particularly vulnerable (e.g. the elderly). The infective dose for cutaneous or gastrointestinal anthrax is not known.

Incubation period

The incubation period varies and is dependent on dose and exposure route. The following incubation periods are generally accepted, although inhalation anthrax in particular may have a much longer incubation period of 43 days or more.

• 1-7 days following cutaneous exposure
• 1-6 days following inhalation exposure
• 1-7 days following ingestion
• 1-2 days or longer following injection

Modelling suggests that the incubation period for inhalational anthrax is dose-dependent and that a relatively long incubation period would be expected if the level of exposure was low (as in the Sverdlovsk incident). During the 2001 US deliberate release the last (eleventh) case of inhalation anthrax had an onset of symptoms 56 days after the first letters containing anthrax were mailed and at least 3 weeks after the onset of illness of the tenth case of inhalation anthrax. However, the actual incubation period could not be determined (Barakat et al 2002).

Period of communicability

• Transmission of anthrax infection from person to person is highly unlikely.
• Contact with skin lesions can occasionally result in subsequent cutaneous infection.
• Airborne transmission from person to person does not occur.

Organism survival

Anthrax spores 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.

Last reviewed: 23 December 2010