Bioterrorism Weapon --- Anthrax
From: Federation of American Scientists
Website: http://www.fas.org
Anthrax is a zoonotic disease
caused by Bacillus anthracis. There are two types of this disease:
cutaneous anthrax and inhalation anthrax. About 95%
of the human anthrax cases in the United States have been in the former
category. Cutaneous anthrax develops when a bacterial organism from
infected animal tissues becomes deposited under the skin. When a patient
contracts cutaneous anthrax, he develops a small elevated lesion on his
skin which becomes a skin ulcer, frequently surrounded by swelling or
edema. The lymph gland near the lesion may also swell from the infection.
If the lesion occurs on the neck or on or about the eye, it may cause
complications. The incubation period for cutaneous anthrax is from one to
seven days. When a patient does not receive an effective antibiotic, the
mortality rate for cutaneous anthrax is 10-20%. With treatment, the
mortality rate falls to less than 1%.
Inhalation anthrax develops when
the bacterial organism is inhaled into the lungs. A progressive infection
follows. Since inhalation anthrax is usually not diagnosed in time for
treatment, the mortality rate in the United States is 90-100%. A biological warfare attack with anthrax spores delivered by aerosol would
cause inhalation anthrax, an extraordinarily rare form of the naturally
occurring disease.
A lethal dose of anthrax is
considered to be 10,000 spores; 80 percent of a population that inhaled
such a dose would die. Less than one millionth of a gram is invariably
fatal within five days to a week after exposure. According to an estimate
by the US Congress's Office of Technology Assessment, 100 kilograms of
anthrax, released from a low-flying aircraft over a large city on a clear,
calm night, could kill one to three million people.
The disease begins after an
incubation period varying from 1-6 days, presumably dependent upon the
dose of inhaled organisms. Onset is gradual and nonspecific, with fever,
malaise, and fatigue, sometimes in association with a nonproductive cough
and mild chest discomfort. In some cases, there may be a short period of
improvement. The initial symptoms are followed in 2-3 days by the abrupt
development of severe respiratory distress with dyspnea, diaphoresis,
strider, and cyanosis. Physical findings may include evidence of pleural
effusions, edema of the chest wall, and meningitis. Chest x-ray reveals a
dramatically widened mediastinum, often with pleural effusions, but
typically without infiltrates. Shock and death usually follow within 24-36
hours of respiratory distress onset.
An epidemic of inhalation anthrax
in its early stage with nonspecific symptoms could be confused with a wide
variety of viral, bacterial, and fungal infections. Progression over 2-3
days with the sudden development of severe respiratory distress followed
by shock and death in 24-36 hours in essentially all untreated cases
eliminates diagnoses other than inhalation anthrax. The presence of a
widened mediastinum on chest x-ray, in particular, should alert one to the
diagnosis. Other suggestive findings include chest-wall edema, hemorrhagic
pleural effusions, and hemorrhagic meningitis. Other diagnoses to consider
include aerosol exposure to SEB; but in this case onset would be more
rapid after exposure (if known), and no prodrome would be evident prior to
onset of severe respiratory symptoms. Mediastinal widening on chest x-ray
will also be absent. Patients with plague or tularemia pneumonia will have
pulmonary infiltrates and clinical signs of pneumonia (usually absent in
anthrax).
Almost all cases of inhalation
anthrax in which treatment was begun after patients were symptomatic have
been fatal, regardless of treatment. Historically, penicillin has been
regarded as the treatment of choice, with 2 million units given
intravenously every 2 hours. Tetracycline and erythromycin have been
recommended in penicillin-sensitive patients. The vast majority of anthrax
strains are sensitive in vitro to penicillin. However,
penicillin-resistant strains exist naturally, and one has been recovered
from a fatal human case. Moreover, it is not difficult to induce
resistance to penicillin, tetracycline, erythromycin, and many other
antibiotics through laboratory manipulation of organisms. All naturally
occurring strains tested to date have been sensitive to erythromycin,
chloramphenicol, gentamicin, and ciprofloxacin.
Vaccines are available against
some forms of anthrax, but their efficacy against abnormally high
concentrations of the bacteria is uncertain. A licensed, alum-precipitated
preparation of purified B.anthracis protective antigen (PA) has
been shown to be effective in preventing or significantly reducing the
incidence of inhalation anthrax. Limited human data suggest that after
completion of the first three doses of the recommended six-dose primary
series (0, 2, 4 weeks, then 6, 12, 18 months), protection against both
cutaneous and inhalation anthrax is afforded. As with all vaccines, the
degree of protection depends upon the magnitude of the challenge dose;
vaccine-induced protection is undoubtedly overwhelmed by extremely high
spore challenge.
If there is information indicating that a
biological weapon attack is imminent, prophylaxis with ciprofloxacin (500
mg po bid), or doxycycline (100 mg po bid) is recommended. If
unvaccinated, a single 0.5 ml dose of vaccine should also be given subcutaneously. Should the attack be confirmed as anthrax, antibiotics should be
continued for at least 4 weeks in all exposed.
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