the former Soviet Union is known to have weaponized at least 30 biological agents, including several vaccine- or drug-resistant strains.

Many bioterrorist scenarios are possible; two examples are aerosol and foodborne attacks. Aerosols exhibit wide-area coverage, and their small particle size allows them to deposit very deeply in the lung tissue, which is where many agents, including anthrax, induce maximal damage (see Chapter 3). A large amount of agent disseminated under ideal meteorological conditions over a city of substantial size could have considerable downwind reach, resulting in large numbers of casualties. Foodborne bioterrorism, which could encompass a variety of agents, must also be considered a likely threat. Agents that cause foodborne illness are easy to obtain from the environment and often have very low-dose requirements. Foodborne pathogens may in fact be the easiest bioterrorism agent to disseminate.

Consideration of the various bioterrorism agents and some of their properties is the first step in prioritizing defenses against them. Each has unique properties as we see them today, and thus each presents a distinct threat with different opportunities for control.

Anthrax

Bacillus anthracis is a highly stable organism because of its ability to sporulate. Most naturally occurring anthrax cases are cutaneous and are transmitted from agricultural or other occupational exposure. Under natural circumstances, humans become infected through contact with infected animals or contaminated animal products. The incidence of infection is unknown; most cases occur in developing countries.

Several characteristics of B. anthracis make it a potentially very lethal bioweapon (Inglesby et al., 2002). Most important are its stability and infectivity as an aerosol and its large footprint after aerosol release. Anthrax is also widely distributed in nature and thus readily available to terrorists in virulent form. The spores are extraordinarily stable, making them relatively easy to store or transport as an aerosol. An aerosol release of anthrax could potentially affect millions of individuals.

Currently, three types of preventative or therapeutic countermeasures exist against anthrax: vaccination, antibiotics, and various adjunctive antitoxin treatments. Since the late 1930s, attenuated strains of B. anthracis have been used throughout the world as live veterinary vaccines and have proven to be highly effective in controlling disease in domesticated animals. Since the 1950s, one of these strains has been used as a live attenuated vaccine in humans in countries of the former Soviet Union. An inactivated cell-free product is currently used in the United States to vaccinate military personnel and laboratory workers. The molecular pathogenesis of anthrax, including the exact target of its lethal factor, is largely unknown. However,



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