MICROBIAL BIOTERRORISM: INTRODUCTION
Descriptions of the use of microbial pathogens as potential weapons of war or terrorism date from ancient times. Among the most frequently cited of such episodes are the poisoning of water supplies in the sixth century b.c. with the fungus Claviceps purpurea (rye ergot) by the Assyrians, the hurling of the dead bodies of plague victims over the walls of the city of Kaffa by the Tartar army in 1346, and the efforts by the British to spread smallpox via contaminated blankets to the native American population loyal to the French in 1767. Although the use of chemical weapons in wartime took place in the not-too-distant past (Chap. 215), the tragic events of September 11, 2001, followed closely by the anthrax attacks through the U.S. Postal System, dramatically changed the mindset of the American public regarding both our vulnerability to microbial bioterrorist attacks and the seriousness and intent of the Federal government to protect its citizens against future attacks. Modern science has revealed methods of deliberately spreading or enhancing disease in ways not appreciated by our ancestors. The combination of basic research, good medical practice, and constant vigilance will be needed to defend against such attacks.
Although the potential impact of a bioterrorist attack could be enormous, leading to thousands of deaths and extensive morbidity, acts of bioterrorism would be expected to produce their greatest impact through the fear and terror they generate. In contrast to biowarfare, where the primary goal is destruction of the enemy through mass casualties, an important goal of bioterrorism is to destroy the morale of a society through fear and uncertainty. While the actual biologic impact of a single act may be small, the degree of disruption created by the realization that such an attack is possible may be enormous. This was readily apparent with the impact on the U.S. Postal System and the functional interruption of the activities of the legislative branch of government following the anthrax attacks noted above. Thus, the key to the defense against these attacks is a highly functioning system of public health surveillance and education so that attacks can be quickly recognized and effectively contained. This is complemented by the availability of appropriate countermeasures in the form of diagnostics, therapeutics, and vaccines, both in response to and in anticipation of bioterrorist attacks.
The Working Group for Civilian Biodefense has put together a list of key features that characterize the elements of biologic agents that make them particularly effective as weapons (Table 214-1). Included among these are the ease of spread and transmission of the agent as well as the presence of an adequate database to allow newcomers to the field to quickly apply the good science of others to bad intentions of their own. Agents of bioterrorism may be used in their naturally occurring forms or they can be deliberately modified to provide maximal impact. Among the approaches to maximizing the deleterious effects of biologic agents are the genetic modification of microbes for the purposes of antimicrobial resistance or evasion by the immune system, creation of fine-particle aerosols, chemical treatment to stabilize and prolong infectivity, and alteration of host range through changes in surface proteins. Certain of these approaches fall under the category of weaponization, which is a term generally used to describe the processing of microbes or toxins in a manner that would ensure a devastating effect of a release. For example, weaponization of anthrax by the Soviets comprised the production of vast amounts of spores in a form that maintained aerosolization for prolonged periods of time; the spores were of appropriate size to reach the lower respiratory tract easily and could be delivered in a massive release, such as via widely dispersed bomblets.
Table 214-1 Key Features of Biologic Agents Used as Bioweapons
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High morbidity and mortality
Potential for person-to-person spread
Low infective dose and highly infectious by aerosol
Lack of rapid diagnostic capability
Lack of universally available effective vaccine
Potential to cause anxiety
Availability of pathogen and feasibility of production
Environmental stability
Database of prior research and development
Potential to be “weaponized”
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Source:
From L Borio et al: JAMA 287:2391, 2002; with permission.
The U.S. Centers for Disease Control and Prevention (CDC) classifies potential biologic threats into three categories, A, B, and C (
Table 214-2). Category A agents are the highest-priority pathogens. They pose the greatest risk to national security because they (1) can be easily disseminated or transmitted from person to person, (2) result in high mortality rates and have the potential for major public health impact, (3) might cause public panic and social disruption, and (4) require special action for public health preparedness. Category B agents are the second highest priority pathogens and include those that are moderately easy to disseminate, result in moderate morbidity rates and low mortality rates, and require specifically enhanced diagnostic capacity. Category C agents are the third highest priority. These include certain emerging pathogens, to which the general population lacks immunity, that could be engineered for mass dissemination in the future because of availability, ease of production, ease of dissemination, potential for high morbidity and mortality, and major public health impact. A potential pandemic strain of influenza, such as avian influenza, is one such example. It should be pointed out, however, that these designations are empirical, and, depending on evolving circumstances such as intelligence-based threat assessments, the priority rating of any given microbe or toxin could change. The CDC classification system also largely reflects the severity of illness produced by a given agent, rather than its accessibility to potential terrorists.
Table 214-2 CDC Category A, B, and C Agents
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Category A
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Anthrax (Bacillus anthracis)
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Botulism (Clostridium botulinum toxin)
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Plague (Yersinia pestis)
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Smallpox (Variola major)
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Tularemia (Francisella tularensis)
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Viral hemorrhagic fevers
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Arenaviruses: Lassa, New World (Machupo, Junin, Guanarito, and Sabia)
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Bunyaviridae: Crimean Congo, Rift Valley
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Filoviridae: Ebola, Marburg
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Category B
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Brucellosis (Brucella spp.)
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Epsilon toxin of Clostridium perfringens
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Food safety threats (e.g., Salmonella spp., Escherichia coli 0157:H7, Shigella)
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Glanders (Burkholderia mallei)
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Melioidosis (B. pseudomallei)
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Psittacosis (Chlamydophila psittaci)
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Q fever (Coxiella burnetii)
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Ricin toxin from Ricinus communis (castor beans)
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Staphylococcal enterotoxin B
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Typhus fever (Rickettsia prowazekii)
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Viral encephalitis [alphaviruses (e.g., Venezuelan, eastern, and western equine encephalitis)]
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Water safety threats (e.g., Vibrio cholerae, Cryptosporidium parvum)
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Category C
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Emerging infectious diseases threats such as Nipah, hantavirus, SARS coronavirus, and pandemic influenza.
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Centers for Disease Control and Prevention and the National Institute of Allergy and Infectious Diseases.
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