pecialists
on bioterrorism disagree about the likelihood of a mass-casualty bioterrorist
attack. Some point to the small number of such incidents that have occurred
to date, whereas others contend that, for a number of reasons, the threat
is increasing.
Although a large-scale bioterrorist attack remains a frightening possibility,
it is important to take a sober and realistic view of the threat. Would-be
terrorists would have to overcome a series of major technical hurdles
before they could inflict mass casualties through the release of a disease
agent. The most prominent hurdles are summarized below.
Acquisition
of a Dangerous Pathogen
CDC staff and Zairian scientists take
samples from animals collected
near Kikwit, Zaire, 1995
With the exception of the smallpox virus, most bioterrorism threat agents
can be isolated from natural sources such as diseased animals, patients,
or even contaminated soil in the case of anthrax spores. Nevertheless,
more than 85 different strains (varieties) of anthrax bacteria have
been identified in nature, and only a few of these strains are highly
virulent, or capable of causing disease. For this reason,
considerable trial and error would be required to obtain a particularly
virulent strain. Some means of testing the virulence of isolated strains
would also be required. Alternatively, terrorists might attempt to steal
well-characterized strains from a research laboratory or a culture collection.
Agent
Production
Once terrorists acquired a "seed culture" of a virulent
pathogen, they
would need to cultivate the agent in laboratory glassware or a small
stainless steel fermentation tank. Producing a sufficient quantity of
agent would require extensive knowledge of fermentation techniques and
conditions. The terrorists would also need to take measures to protect
themselves from infection, such as vaccination or antibiotic prophylaxis,
wearing a gas mask, or using a containment system such as a biosafety
cabinet.
Weaponization
Next, the terrorists would need to weaponize the agent, a process involving
several steps. The goal of weaponization is to convert the agent into
a form in which it can be dispersed as an aerosol cloud of microscopic
particles, ranging in size from one to five microns (thousandths of
a millimeter). Only particles with these dimensions are small enough
to lodge in the tiny air sacs of the victims' lungs to cause infection.
In the nutrient-rich environment of the human body, anthrax spores for
example would germinate and begin to multiply, causing disease and death.
Spores of Bacillus anthracis,
the bacterium that causes
anthrax
In the case of anthrax, the first weaponization step would be to induce the bacteria to sporulate, a process in which the cells stop dividing and grow a tough outer shell that is resistant
to the external conditions. Spore formation enables the bacterium
to survive for long periods in a nutrient-poor environment by reverting
to a state of suspended animation (vital functions are temporarily
stopped). Anthrax spores can survive for decades in soil and for hours
in an airborne aerosol. Furthermore, anthrax spores can survive environmental
contaminants and potentially become re-aerosolized.
Terrorists could attempt to deliver anthrax spores as a slurry (watery mixture), but this physical form of the
agent is difficult to disseminate by passing it through a sprayer
system to create an infectious aerosol. Alternatively, terrorists
could dry the spores and process them into a fine microscopic powder. Although a dry powder of
anthrax would be much harder to produce than a wet slurry, it would
be easier to deliver as an aerosol.
Sophisticated terrorists might perform
additional
processing steps on the dried spores to enhance their dissemination
as an aerosol. These steps include adding chemicals to remove excess
moisture or to neutralize the electrostatic charges on the particles,
preventing them from clumping.