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It wasn't the kind of seminar where a terrorist might
slip in with a tape recorder and furtively learn how to create
biological mischief; they don't offer that sort of fare over the
NIH videocast system after all. Rather, Dr. Arturo Casadevall of
Albert Einstein College of Medicine offered a sweeping intellectual
consideration of how we view risk in a July 14 talk in Bldg. 50
titled, "The Weapon Potential of a Microbe."
Continued...
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| Dr. Arturo Casadevall of Albert Einstein
College of Medicine |
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In an immune-compromised patient, for example, something as otherwise
innocuous as brewer's yeast or yogurt (containing, respectively, S.
cerevisiae and L. acidophilus) could be pathogenic,
argued Casadevall, who also displayed a slide of his humble sedan
alongside a caption, "The civilian car is the most effective weapon
of war in Iraq." If cars, yogurt and yeast have "dual use" as potential
weapons, what lines can modern society draw as to their availability?
In the good old days, a weapon was defined as a club, knife or
gun. But biological weapons come in a staggering variety not subject
to the usual restraints imposed by the laws of physics. It is naïve,
Casadevall said, to pick and choose among the many thousands of
microbes, labeling this one a danger and another harmless; after
all, most organisms are only one "virulence factor" away from being
pathogenic. (Using the metaphor of playing cards, he showed that
most pathogens represent a hand that is only one card — or
virulence factor — short of becoming significantly more dangerous.)
Granted, the bugs on the government's Select Agents and Toxins
list and its various categories are undeniably prime candidates
for weaponization and should be guarded against, he said. But it
makes more sense to craft a broader framework for evaluating risk,
taking into account the intricacies of host-microbe interaction,
including communicability, stability, inoculum (the actual material
that transmits disease such as spore or bacteria) and symptoms.
Casadevall and his colleagues have constructed a simple formula
to determine the virulence potential of a biological agent. The
calculation yields numbers that, in themselves, are not especially
meaningful, but enable one to rate relative danger. Time is a crucial
variable in the math — a virus such as HIV might rate low
on the communicability scale, but given many years to work its
way through a given population can eventually claim cultures and
continents, as is happening in Africa, Casadevall said.
Ironically, science's successes in wiping out biological threats
eventually serve to create new ones: "In 2020, measles and polio
will have weapon potential (because vaccines eliminated them in
earlier generations) — every public health success generates
a new weapon."
In summary, Casadevall noted that "all pathogenic microbes are
potential weapons." He argued that regulations inhibiting research
on biological agents invariably leave society more vulnerable to
them, and that the weapon potential of microbes changes over time. "The
line that we draw in the sand [with respect to microbes] can't
be fixed," he concluded.
Casadevall offered a postlude on the role of fungi as a cause
of disease. There are some 1.5 million fungal species, but only
It is naïve to pick and choose among the many thousands
of microbes, labeling this one a danger and another harmless;
after all, most organisms are only one “virulence factor” away
from being pathogenic.
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about 10 are common pathogens; most can't survive the 37 degree Celsius
(body temperature) barrier. Fungi, he theorized, might have contributed
to extinction of the dinosaurs in the following scenario: millions
of years ago, at the Cretaceous-Tertiary Boundary, a global dusk
set in, caused by Earth's collision with an asteroid (Alvarez hypothesis)
or by a spate of volcanic eruption. The event turned the planet into
a compost heap, teeming with fungi. Any surviving animals would have
been exposed to large numbers of spores, but those that developed
higher core body temperatures (such as mammals and birds) might have
been better able to survive fungal invasion via a process known as "endothermal
exclusion." Assuming that the dinosaurs did not enjoy high body heat,
they could have succumbed over time to fungi. Mammals, on the other
hand, flourished.
"We're hot on the inside and this helps keep out fungi," explained
Casadevall. "Most caloric intake is used to maintain body temperature,
not to yield the energy for walking around. That's the reason we
eat three times a day — to keep fungi away."
The entire talk can be viewed at www.videocast.nih.gov. 
