Bacillus anthracis and Anthrax (page 4)
(This chapter has 5 pages)
© Kenneth Todar, PhD
Immunity to Anthrax
Considerable variation in genetic susceptibility to anthrax exists
animal species. Resistant animals fall into two groups: (1) resistant
establishment of anthrax but sensitive to the toxin and (2) resistant
the toxin but susceptible to establishment of disease. This is
in the table below. Neither the source of the inoculum (spores or
cells or a mixture) nor the route of inoculation (subcutaneous,
or inhalational) is stated. The infectious dose of anthrax is expected
to vary widely based on these parameters, as well.
Table 2. The infectious
of B. anthracis and the lethal dose of toxin varies
within animal species. The data do not specify the route of infection
whether spores or vegetative cells were used in the inoculum.
||Toxic dose causing death
||Bacteria per ml blood at time death
Animals surviving naturally-acquired anthrax are immune to
Second attacks are extremely rare. Permanent immunity to anthrax seems
to require antibodies to both the toxin and the capsular polypeptide,
the relative importance of the two kinds of antibodies appears to vary
widely in different animals.
Vaccines composed of killed bacilli and/or capsular antigens
produce no significant immunity. A nonencapsulated toxigenic strain has
been used effectively in livestock. The Sterne Strain of Bacillus
anthracis produces sublethal amounts of the toxin that induces
of protective antibody.
The anthrax vaccine for humans, which is used in the U.S.,
a preparation of the protective antigen recovered from the
filtrate of an avirulent, nonencapsulated strain of Bacillus
that produces PA during active growth. Anthrax immunization consists of
three subcutaneous injections given two weeks apart followed by three
subcutaneous injections given at 6, 12, and 18 months. Annual booster
of the vaccine are required to maintain a protective level of immunity.
The vaccine is indicated for individuals who come in contact in the
workplace with imported animal hides, furs, bone, meat, wool, animal
(especially goat hair) and bristles; and for individuals engaged in
or investigational activities which may bring them into contact with
spores. Otherwise, it has been indicated for the military
the current era of biological warfare.
The vaccine should only be administered to healthy individuals from
18 to 65 years of age, since investigations to date have been conducted
exclusively in that population. It is not known whether the anthrax
can cause fetal harm, and pregnant women should not be vaccinated.
A new type of passive vaccine to anthrax is currently on the
horizon. This was recently announced by R.G. Crystal and colleagues
the Medical College of Cornell University, in the February, 2005 issue
of the journal, Molecular Therapy. They demonstrated that mice
with a human adenovirus expressing a single-chain antibody directed
protective antigen (PA) became immune to anthrax within 24 hours of
This is much quicker than is possible with existing anthrax vaccines,
are a relatively crude preparation of PA.
Currently available anthrax vaccines have limited use in a
attack because they are active vaccines in which multiple doses are
over several months to elicit protective immunity against anthrax.
vaccines, on the other hand, introduce fully formed antibodies directly
the body and immunity is achieved much sooner.
In mice receiving the adenovirus-based anti-PA vaccine, PA-specific
serum antibodies were detectable within 24 hours. These antibodies had
neutralizing activity that protected mice from an intravenous lethal
challenge administered 1-14 days post vaccination.
Crystal, et al envision a possible scenario wherein both the passive
and active vaccine might be given. Passive vaccines lose their
fairly rapidly over time, whereas active vaccines do not. The passive
could provide protection that would last a couple of weeks, but that
provide a safety margin for development of more active, long-term
stimulated by the active vaccine.
Passive immunotherapy with such adenovirus-based vectors expressing
anti-PA antibody, either alone or in combination with antibiotics, may
be a rapid, convenient, and highly effective strategy to protect
or treat anthrax in a bioterrorism attack.
Also, in cases of anthrax, coadministration of the passive vaccine
antibiotics may maximize the utility of antibiotic therapy.
would counter the effects of lethal toxin, and likely prolong the time
frame for effective antibiotic treatment and/or reduce the amount of
Treatment of Anthrax
Antibiotics should be given to unvaccinated individuals exposed to
anthrax. Penicillin, tetracyclines and fluoroquinolones are effective
administered before the onset of lymphatic spread or septicemia,
to be about 24 hours. Antibiotic treatment is also known to lessen the
severity of disease in individuals who acquire anthrax through the
Inhalation anthrax was formerly thought to be nearly 100% fatal despite
antibiotic treatment, particularly if treatment is started after
appear. A recent Army study resulted in successful treatment of monkeys
with antibiotic therapy after being exposed to anthrax spores. The
therapy was begun one day after exposure.