Anthrax is one of the great infectious diseases of antiquity. The fifth and sixth
plagues in the Bible's book of Exodus (1) may have been outbreaks of anthrax in cattle and
humans, respectively. The "Black Bane," a disease that swept through Europe in
the 1600s causing large numbers of human and animal deaths, was likely anthrax. In 1876,
anthrax became the first disease to fulfill Koch's postulates (i.e., the first disease for
which a microbial etiology was firmly established), and 5 years later, in 1881, the first
bacterial disease for which immunization was available (2). Large anthrax outbreaks in
humans have occurred throughout the modern eraŚmore than 6,000 (mostly cutaneous) cases
occurred in Zimbabwe between October 1979 and March 1980 (3), and 25 cutaneous cases
occurred in Paraguay in 1987 after the slaughter of a single infected cow (4).
Anthrax, in the minds of most military and counterterrorism planners, represents the
single greatest biological warfare threat. A World Health Organization report estimated
that 3 days after the release of 50 kg of anthrax spores along a 2-km line upwind of a
city of 500,000 population, 125,000 infections would occur, producing 95,000 deaths
This number represents far more deaths than predicted in any other scenario of agent
release. Moreover, it has been estimated (6) that an aerial spray of anthrax along a
100-km line under ideal meteorologic conditions could produce 50% lethality rates as far
as 160 km downwind. Finally, the United States chose to include anthrax in the now-defunct
offensive biological weapons program of the 1950s, and the Soviet Union and Iraq also
admitted to possessing anthrax weapons. An accident at a Soviet military compound in
Sverdlovsk in 1979 resulted in at least 66 deaths due to inhalational anthrax, an
inadvertent demonstration of the viability of this weapon. The epidemiology of this
inadvertent release was unusual and unexpected. None of the persons affected were children
(7). Whether this is due to differences in susceptibility between children and adults or
purely to epidemiologic factors (children may not have been outdoors at the time of
release) is unclear.
Anthrax is caused by infection with Bacillus anthracis, a gram-positive
spore-forming rod. The spore form of this organism can survive in the environment for many
decades. Certain environmental conditions appear to produce "anthrax zones,"
areas wherein the soil is heavily contaminated with anthrax spores. Such conditions
include soil rich in organic matter (pH <6.0) and dramatic changes in climate, such as
abundant rainfall following a prolonged drought. Partly because of its persistence in
soil, anthrax is a rather important veterinary disease, especially of domestic herbivores.
In addition to encountering anthrax while grazing in areas of high soil contamination,
these herbivores may also acquire the disease from the bite of certain flies (8). Vultures
may mechanically spread the organism in the environment (9). Anthrax zones in the United
States closely parallel the cattle drive trails of the 1800s (10).
Anthrax spores lend themselves well to aerosolization and resist environmental
degradation. Moreover, these spores, at 2-6 microns in diameter, are the ideal size for
impinging on human lower respiratory mucosa, optimizing the chance for infection. It is
the manufacture and delivery of anthrax spores in this particular size range (avoiding
clumping in larger particles) that presents a substantial challenge to the terrorist
attempting to use the agent as a weapon. The milling process imparts a static charge to
small anthrax particles, making them more difficult to work with and, perhaps, enabling
them to bind to soil particles (11). This, in part, may account for the relatively low
secondary aerosolization potential of anthrax, as released spores bind to soil, now
clumping in particles substantially in excess of 6 microns. This clumping tendency,
together with a high estimated ID50 of 8,000-10,000 spores may help explain the
rarity of human anthrax in most of the Western world, even in areas of high soil
contamination. Other potential bioweapons, such as Q fever and tularemia, have ID50
values as low as 1 and 10 organisms, respectively.
Most endemic anthrax cases are cutaneous and are contracted by close contact of abraded
skin with products derived from infected herbivores, principally cattle, sheep, and goats.
Such products might include hides, hair, wool, bone, and meal. Cutaneous anthrax is
readily recognizable, presents a limited differential diagnosis, is amenable to therapy
with any number of antibiotics, and is rarely fatal. While common in parts of Asia and
sub-Saharan Africa, cutaneous anthrax is very rare in the United States; the last case was
reported in 1992 (12). Inhalational anthrax, also known as woolsorters' disease, has been
an occupational hazard of slaughterhouse and textile workers; immunization of such workers
has all but eliminated this hazard in Western nations. As a weapon, however, anthrax would
likely be delivered by aerosol and, consequently, be acquired by inhalation. A third type
of anthrax, acquired through the gastrointestinal route (e.g., consuming contaminated
meat) is exceedingly rare but was initially offered by Soviet scientists as an explanation
for the Sverdlovsk outbreak.
Inhalational anthrax begins after exposure to the necessary inoculum, with the uptake
of spores by pulmonary macrophages. These macrophages carry the spores to tracheobronchial
or mediastinal lymph nodes. Here, B. anthracis finds a favorable milieu for growth
and is induced to vegetate. The organism begins to produce an antiphagocytic capsule and
at least three proteins, which appear to play a major role in virulence. These proteins
are known as edema factor (EF), lethal factor (LF), and protective antigen (PA). Following
the A-B model of toxicity (13), PA serves as a necessary carrier molecule for EF and LF
and permits penetration into cells. Edema toxin results from the combination of EF + PA,
lethal toxin results from the combination of LF + PA. These toxins result in necrosis of
the lymphatic tissue, which in turn causes the release of large numbers of B.
anthracis. The organisms gain access to the circulation, and an overwhelming fatal
septicemia rapidly ensues. At autopsy, widespread hemorrhage and necrosis involving
multiple organs is seen.
Inhalational anthrax generally occurs after an incubation period of 1 to 6
During the Sverdlovsk outbreak, however, spontaneous cases appeared to arise as late as 43
days after the assumed release date (7). Such late cases are unexplained but have
potentially serious implications for postexposure management of victims of aerosol
exposure. After the incubation period, a nonspecific flulike illness ensues, characterized
by fever, myalgia, headache, a nonproductive cough, and mild chest discomfort. A brief
intervening period of improvement sometimes follows 1 to 3 days of these prodromal
symptoms, but rapid deterioration follows; this second phase is marked by high fever,
dyspnea, stridor, cyanosis, and shock. In many cases, chest wall edema and hemorrhagic
meningitis (present in up to 50% of cases ) may be seen late in the course of disease.
Chest radiographs may show pleural effusions and a widened mediastinum, although true
pneumonitis is not typically present. Blood smears in the later stages of illness may
contain the characteristic gram-positive spore-forming bacilli. Death is universal in
untreated cases and may occur in as many as 95% of treated cases if therapy is begun more
than 48 hours after the onset of symptoms.
While early recognition of anthrax is likely to require a heightened degree of
suspicion, the diagnosis is supported by gram-positive bacilli in skin biopsy material (in
the case of cutaneous disease) or in blood smears. A preponderance of gram-positive
bacilli in swabs of the nares or in appropriate environmental samples might support a
diagnosis of anthrax where intentional release is suspected. Chest radiographs exhibiting
a widened mediastinum in the proper setting of fever and constitutional signs and in the
absence of another obvious explanation (such as blunt trauma, deceleration injury, or
postsurgical infection) should also lead to a diagnosis of anthrax. This finding is only
likely to occur late in the course of disease. Confirmation is obtained by culturing B.
anthracis from blood.
While endemic strains of B. anthracis are typically sensitive to various
antibiotics, including penicillin G, antibiotic-resistant strains do (on rare occasion)
occur naturally (16) and can be readily isolated in laboratories. For this reason, as well
as the convenience of twice-daily dosing, many experts consider ciprofloxacin (400 mg
intravenously (i.v.) q 12 h) the drug of choice for treating victims of terrorism or
warfare. Doxycycline (100 mg i.v. q 12 h) is an acceptable alternative, although rare
doxycycline-resistant strains of B. anthracis are known. Conversely, however, the
much lower cost of tetracyclines compared to quinolones may factor into therapeutic
decisions, especially where large numbers of patients are involved. These recommendations
are based solely on in vitro data and data from animal models (17); no human clinical
experience with these regimens exists. In cases of endemic anthrax, or where organisms are
known to be susceptible, penicillin G (2 million units i.v. q 2 h or 4 million units i.v.
q 4 h) is recommended.
Postexposure prophylaxis against anthrax may be achieved with oral ciprofloxacin (500
mg orally q 12 h) or doxycycline (100 mg orally q 12 h), and all persons exposed to a
bioterrorist incident involving anthrax should be administered one of these regimens at
the earliest possible opportunity. In cases of threatened or suspected release of anthrax,
chemoprophylaxis can be delayed 24 to 48 hours, until the threat is verified.
Chemoprophylaxis can be discontinued if the threat is found to be false. Levofloxacin and
ofloxacin would be acceptable alternatives to ciprofloxacin. In addition to receiving
chemoprophylaxis, exposed persons should be immunized. On the basis of animal data
(wherein an appreciable number of unvaccinated primates died when antibiotics were
withdrawn after 30 days of therapy) (18), chemoprophylaxis is best continued until the
exposed persons has received at least three doses of vaccine (thus, for a minimum of 4
weeks). If vaccine is unavailable, some recommend that chemoprophylaxis be continued for 8
weeks (19). The available vaccine was licensed (for preexposure prophylaxis) by the U.S.
Food and Drug Administration in 1970 and is prepared from a formalin-treated culture
supernatent of an avirulent B. anthracis strain. It is given in a preexposure
regimen at 0, 2, and 4 weeks, and at 6, 12, and 18 months. Persons at continuing risk for
exposure should receive yearly boosters. Exposed persons should receive at least three
doses (at 0, 2, and 4 weeks), assuming no further exposure is likely, before discontinuing
Recently, a number of hoaxes involving a threatened release of anthrax have been
promulgated (19,20), and guidelines have now been published to assist in the management of
such threats (19). When evaluating a threatened release of anthrax, the lack of volatility
of the disease, as well as its inability to penetrate intact skin, should be taken into
account. These factors make it unlikely, in most cases, that persons coming in contact
with letters, packages, and other devices purported to contain anthrax will be at risk for
aerosol exposure. Moreover, because energy is required to aerosolize anthrax spores,
opening a letter, even if it contained anthrax, would be unlikely to place a person at
substantial risk. For these reasons, postexposure prophylaxis may not be necessary in many
cases of threatened anthrax dissemination.
Anthrax has little potential for person-to-person transmission; standard precautions
are thus adequate for health-care workers treating anthrax patients. Anthrax, as well as
other bacteriologic and viral weapons, has an incubation period of >24 hours. This
characteristic is not shared by conventional, chemical, and nuclear weapons and makes
decontamination of infected persons admitted to hospitals days after exposure unnecessary
in most cases. However, in certain cases, such as exposure to a threat letter involving an
unidentified substance, where anthrax cannot readily be ruled out by Gram stain or other
rapid diagnostic procedures, decontamination may be warranted. In such cases,
decontamination may be accomplished by removing clothing, sealing it in a plastic bag, and
showering with copious amounts of soap and water. Environmental surfaces and personal
effects may be treated with 0.5% hypochlorite after the area in which the agent was
released is investigated (19).
In summary, even though anthrax may be among the most viable of biological weapons, it
is also a weapon for which a licensed vaccine and good antimicrobial therapy and
postexposure prophylaxis exist. Given the relatively short incubation period, and rapid
progression of disease, however, identification of the exposed population within 24 to 48
hours and employment of therapeutic and prophylactic strategies are likely to present a
challenge. Good intelligence regarding the capabilities of terrorist groups, as well as
heightened awareness of the threat on the part of clinicians, first responders, and public
health personnel remains a cornerstone of bioterrorism defense.
- Exodus 9:1-12.
- Pasteur L, Chamberlain C-E, Roux E. Compte rendu sommaire des experiences faites a
Pouilly-le-Fort, pres Melun, sur la vaccination charbonneuse [French]. Comptes Rendus des
seances De L'Academie des Sciences 1881;92:1378-83.
- Turner M. Anthrax
in humans in Zimbabwe. Cent Afr J Med 1980;26:160-1.
- Harrison LH, Ezzell JW, Abshire TG, Kidd S, Kaufmann AF. Evaluation
of serologic tests for diagnosis of anthrax after an outbreak of cutaneous anthrax in
Paraguay. J Infect Dis 1989;160:706-10.
- Report of a WHO group of consultants. Health aspects of chemical and biological
weapons. Geneva: World Health Organization; 1970. p. 97-9.
- Science Applications International Corporation. Effectiveness of medical
intervention against battlefield levels of Bacillus anthracis. 1993.
- Meselson M, Guillemin J, Hugh-Jones M, Langmuir A, Popova I, Shelokov A, Yampolskaya
O, et al. The
Sverdlovsk anthrax outbreak of 1979. Science 1994;266:1202-7.
- Turell MJ, Knudson GB. Mechanical
transmission of Bacillus anthracis by stable flies and mosquitoes. Infect Immun
- Titball RW, Turnbull PCB, Hutson RA. The
monitoring and detection of Bacillus anthracis in the environment. Journal of
Applied Bacteriology 1991; Suppl 70:9S-18.
- Coker PR, Smith KL, Hugh-Jones ME. Anthrax in the USA. Proceedings of the Third
International Conference on Anthrax, Plymouth, England, September 7-10, 1998:44
- Sidell FR, Patrick WC, Dashiell TR, editors. Jane's chem-bio handbook. Alexandria
(VA): Jane's Information Group; 1998. p. 229-44.
- Centers for Disease Control and Prevention. Summary of notifiable diseases, United
States, 1997. MMWR Morb Mortal Wkly Rep 1998;46:74.
- Gill DM. Seven toxic peptides that cross cell membranes. In: Jeljaszewicz J,
Walstrom T, editors. Bacterial toxins and cell membranes. New York: Academic Press; 1978.
- Brachman PS, Friedlander AM. Anthrax. In: Plotkin & Mortimer, editors.
Vaccines. Philadelphia (PA): W.B. Saunders; 1994. p. 730.
- Abramova FA, Grinberg LM, Yampolskaya OV, Walker DH.
Pathology of inhalational anthrax in 42 cases from the Sverdlovsk outbreak of 1979. Proc
Natl Acad Sci U S A 1993;90:2291-4.
- Lightfoot NF, Scott RJD, Turnbull PCB. Antimicrobial susceptibility of Bacillus
anthracis. Salisbury Medical Bulletin Suppl 1990;68:95-8.
- Kelly DJ, Chulay JD, Mikesell P, Friedlander AM.
Serum concentrations of penicillin, doxycycline, and ciprofloxacin during prolonged
therapy in rhesus monkeys. J Infect Dis 1992;166:1184-7.
- Friedlander AM, Welkos SL, Pitt MLM, Ezzell JW, Worsham PL, Rose KJ, et al. Postexposure
prophylaxis against experimental inhalation anthrax. J Infect Dis 1993;167:1239-42.
- Centers for Disease Control and Prevention. Bioterrorism
alleging use of anthrax and interim guidelines for management-United States, 1998.
MMWR Morb Mortal Wkly Rep 1999;48:69-74.
- Sanchez R. California anthrax threats spawn costly wave of fear. Washington Post,
January 11, 1999, section A, page 1.