Biological Warfare and Vaccines: Anthrax
Meryl Nass and Barbara Price
Part 1. Pathogenicity and Virulence of Anthrax
The information on the pathogenicity was taken from numerous reference materials, various Internet sources, and Medical Aspects of Chemical and Biological Warfare, Office of the Surgeon General, Department of the Army, USA (1997).
The Infecting Organism
Anthrax is a disease primarily of mammals and birds caused by the anthrax bacterium (Bacillus anthracis). The B. Anthracis, is a large, Gram-positive, spore-forming, nonmotile, rod-shaped bacillus (1-1.5 µm by 5 -10 µm), which grows on ordinary nutrient media and in soil, aerobically and anaerobically. Its genotype and phenotype are similar to Bacillus cereus, a bacteria readily isolated from soil cultures.
Exposure and Clinical Symptoms
Humans become infected with anthrax:
- cutaneously, through cuts or abrasions on skin when handling infected animals, carcasses, meat, or contaminated soil,
- through digestion of infected meat, (oropharyngeal and gastrointestinal anthrax) or
- by inhalation of anthrax spores, usually on dust from animal hair or hides ("Woolsorter's disease").
Cutaneous anthrax is characterized by a small papule that forms at the site of the infection associated a gelatinous edema caused by the germination of the spores. The papule develops into a vesicle and finally into a necrotic ulcer. There is often an edema around the ulcer, which can encompass the affected limb. If untreated, septicemia may develop, but with treatment mortality is less than 1%.
Anthrax caused by ingestion is very rare in humans.
Oropharyngeal anthrax develops with a sore throat, fever
and swelling of the neck due to swollen lymph nodes and
edema. Gastrointestinal anthrax progresses via nausea and
vomiting followed by abdominal pain and bloating from
accumulation of fluid in the peritoneum. The mortality
for digestive forms of anthrax is estimated as 50% or
higher.
Inhalation anthrax may begin 1 - 6 days after exposure
with a general malaise and chest pain. There is
increasing respiratory distress and high fever and may be
edema of the neck or chest, excessive sweating, and
cyanosis. Inhalation anthrax is almost invariably fatal.
Weaponized anthrax would most likely be inhalation
anthrax.
Virulence
Until the 1950s, it was thought that anthrax fatalities
resulted from capillaries blocked by large numbers of
B.anthracis bacteria. However, anthrax bacilli release an
exotoxin that diffuses into the blood and causes symptoms
of anthrax when injected into test animals. Since then,
three different antigenic components, proteins with
molecular weights about 80kDa have been found to make
anthrax toxin. Although the exact pathogenic mechanism of
the disease is not known, the virulence of anthrax
appears to be a result of the toxins and the protective
capsule around the organism.
Anthrax is one of the bacteria that forms a capsule
around the organism to protect it from being ingested by
phagocytes. The anthrax bacterium's cell wall is made of
an antigenic polysaccharide. The capsule is made of poly
D-glutamate polypeptide and is also antigenic. All
virulent anthrax forms this capsule. This capsule is
important in establishing the infection. In addition to
the antigenic capsule anthrax has three antigenic
components of an exotoxin, anthrax toxin.
The protective antigen (PA) is an immunogenic protein.
The PA is a B protein (a cell-binding protein) and works
with each of the two other virulence factors, which are A
proteins, to elicit toxic responses. Lethal Factor (LF)
is a protein that combines with PA to form a toxin that
is essential for the lethal effects of anthrax (lethal
toxin). Edema Factor (EF) is an inherent adenylate
cyclase, similar to Bordetella pertussis's (whooping
cough) adenylate cyclase toxin. EF, in combination with
PA, forms the edema toxin responsible for producing the
edema in anthrax. Neither LF nor EF are inherently toxic;
only in combination with PA. EF+PA produces edema toxin;
LF+PA produces lethal toxin; EF+LF appears to be
inactive; and EF+LF+PA produces edema and necrosis and is
lethal. All three genes for PA, LF and EF are on one
plasmid, pX01, separate from the plasmid that has the
genes for the capsule, pX02.
The exact pathogenic mechanisms for EF, LF and PA in the
disease are still being researched. On April 30, 1998, in
Science (Duesbury, N. S., Science 280, p.734,
"Proteolytic Inactivation of MAP-Kinase-Kinase by
Anthrax Lethal Factor"), the results of recent
studies on LF were presented. LF has been shown to
prevent frog oocytes from maturing into eggs, indicating
possible blockage of the MAPK pathway, a major pathway
for regulating cell proliferation and growth. The LF
protein clips off a piece of the enzyme responsible for
activating MAPK. By identifying a target molecule for LF,
researchers can then look for drugs that interfere with
LF's enzymatic activity. This approach is different from
the approach used for vaccines.
Vaccines
From Current Med Talk: Vaccines are a mixture of live,
live-attenuated (decreased virulence), killed, complete
or incomplete microorganisms or products derived
therefrom that contains antigens capable of stimulating
the production of specific protective antibodies against
a microorganism. Modern understanding goes even further
with some authors acknowledging the role immunology plays
by requiring a vaccine to contain a sufficient number of
different T-cell epitopes (sites, "hills and
ridges," that elicit an antigenic response) so that
T-cell responses are achieved in all members of a
genetically diverse outbred population. A vaccine should
generate a large pool of memory T- and B-cells, so that
the greater the size of the pool, the more rapid the
appearance of effector T-cell activity from the memory
cells. The titer of antibodies should be high enough so
that most of a challenge organism is prevented from
infecting susceptible cells. Newer recombinant and DNA
vaccines may also include other properties.
Adjuvants are substances administered with an antigen to
elicit an immune response to the antigen. Adjuvants are
sometimes used in vaccines to stimulate the immunsystem
and make the vaccine more effective. Adjuvants can be
derived from materials as diverse as silica gel,
synthetic polymers and dead mycobacteria. Adjuvants must
be selected carefully because some may induce diseases.
Part 2. History of Anthrax Vaccine Development
Part 3. Side Effects of Anthrax Vaccine and
Possible Connections with Gulf War Syndrome
References:
- Medical Aspects of Chemical and Biological Warfare, Office of the Surgeon General, Department of the Army, USA (1997).
- Current Med Talk: A Dictionary of Medical Terms, Slang and Jargon, J.C. Segen, Appleton & Lange, Stamford, CT (1995).
- Molecular Biology and Biotechnology: A Comprehensive Desk Reference, Ed. R.A. Meyers, VCH Publishers (1995).
- Prof. Jiri Bajgar, the Bard of ASA, gives to us
his paraphrase (or perhaps soliloquy) on
Shakespeare and the never ending quest of trying
to find and promulgate the perfect word(s) and
wording which in themselves might signify BTWC-VP
progress.
98-4, issue no. 67