The three general modes of infection by Bacillus anthracis (“B. anthracis”), the bacterium which causes anthrax infection, are by inhalation, ingestion or skin contact with anthrax spores. Inhalation is the form of infection most likely to be lethal. Inhalational anthrax occurs when anthrax spores become airborne and enter a person’s body through the lungs. Inhalational anthrax is usually fatal if left untreated, and has approximately a 50% mortality rate in patients treated aggressively with antibiotics and supportive care.
The U.S. Department of Defense estimates that up to ten countries may possess anthrax weapons and an undetermined number of individuals and terrorist groups could have access to anthrax. Anthrax is an effective bioterrorism agent because the spores are stable, can be milled to a fine powder and may be dispersed widely with readily available instruments and machinery. The World Health Organization estimates that 50 kilograms of B. anthracis spores released upwind of a city of 500,000 people could result in up to 95,000 fatalities, with an additional 125,000 persons being incapacitated.
Following exposure, but prior to the onset of symptoms, antibiotics like ciprofloxacin, doxycycline or penicillin can be used as post-exposure prophylaxis with the goal of preventing progression of the infection with a recommended antibiotic course of treatment of 60 days, sometimes in combination with the administration of anthrax vaccine. However, both compliance and side effects are problematic for anyone asked to take antibiotics for such an extended period of time. Furthermore, antibiotic resistance, whether naturally occurring or genetically engineered, is a concern. Consequently, medical countermeasures like PharmAthene’s rPA-based anthrax vaccine candidate and monoclonal human antibody treatment, Valortim®, with a prolonged half-life, might allow for a shorter duration of antibiotic dosing to achieve adequate post-exposure prophylaxis.