Environmental Engineering Reference
In-Depth Information
Plague
The etiologic agent of plague is the gram negative, facultatively anaerobic, non-
motile, coccobacillus,
Yersinia pestis
. Plague is a vectorborne disease that mani-
fests itself in three clinical forms; bubonic, septicemic, and pneumonic. Bubonic
plague has the greatest notoriety, having been the cause of great historic pan-
demics, such as the sixth-century pandemic that killed 100 million people and the
fourteenth-century “Black Death” pandemic that claimed 40 million people.
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The bubonic form of plague has a 75 percent fatality rate. No bacterial disease
in history has been more devastating.
Y. pestis
is a zoonotic pathogen, and the
reservoirs of
Y. pestis
are various rodents. Infected rodents transmit the pathogen
to other animals, most notably domestic rats, through the bite of fleas. Domestic
rats are susceptible to the plague and will die. In areas of poor sanitation and liv-
ing conditions, as characterized much of Europe and Asia in the Dark and Middle
Ages, domestic rat populations abounded among human squalor. As domestic rat
populations dwindled, owing to loss of members to the plague, fleas carrying
Y. pestis
infected humans. The flea carries a high density of
Y. pestis
following a
blood meal on an infected rat and can deposit the bacteria at the site of a human
bite, both by regurgitation and fecal deposition.
The term
bubonic
comes from the word “bubo,” which refers to the enlarged
nodule that forms as a result of
Y. pestis
growth in lymph nodes. The human
(host) defense system, through the action of polymorphonuclear leucocytes
and macrophages, attack the infectious bacteria. Bacteria phagocytized by
macrophages produce toxins that spare them from enzymatic destruction. Other
bacteria (e.g.,
Legionella pneumophila
) have similar defense strategies. The
bacteria contained in the macrophages survive and grow and are delivered
to lymph nodes and various organs of the body by the macrophages in the
bloodstream. The hemorrhaging (gangrene) that occurs beneath the skin over
various parts of the body appears dark — hence the term
Black Death
(recall
a similar visible effect to the lesions developed in anthrax infections). More
fatal than the bubonic form of plague is pneumonic plague; a manifestation
of the disease caused by the migration of the infectious bacteria to the lungs.
Untreated pneumonic plague is 100 percent fatal. Septicemic plague, which
results either upon inoculation of the bacteria directly into the blood stream or
as secondary complications from bubonic or pneumonic forms, progresses from
the multiplication of the infectious bacteria in the bloodstream and is essentially
always fatal.
As a bioweapon, it is likely that an attack would involve dissemination of the
infectious bacteria in aerosol form. The respiratory consequences of inhalation
would be expressed as pneumonic plague, which is the most contagious form of
plague.
Assuming
the availability of swift medical attention and effective hospital
care, the fatality rate from such an attack might be held to 25 percent of the
infected portion of the population. First indications of an attack would be a burst
in incidence of the disease, especially in places free of animal reservoirs such as
a metropolitan area. The incubation period of the disease would be short, likely
in the range of two to four days. Despite the high fatality rate and contagious
