Use as a Biological Weapon

Last updated May 25, 2011

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Aerosol Release of Anthrax Spores
Contamination of Food or Water
Historical Perspective
Bibliography

Aerosol Release of Anthrax Spores

Aerosol release of weaponized spores is the most likely mechanism for use of anthrax as a biological weapon (Inglesby 2002). Although there is no formal definition of weaponized anthrax, weaponization for aerosol release generally involves:

Use of small particles
A high concentration of spores
Treatment to reduce clumping
Neutralization of the electrical charge
Use of antimicrobial-resistant strains or genetic modification of the organism to increase virulence or escape vaccine protection

The impact of a large aerosol release of weaponized anthrax remains unknown; however, scenarios have been hypothesized, including:

A 1970 World Health Organization (WHO) report estimated that an aerosol release of 50 kg of dried powder containing 6 x 10¹⁵ anthrax spores over a city of 5 million people in an economically developed country (such as the United States) would produce 250,000 incapacitating illnesses and up to 100,000 deaths (WHO 1970).
A 1993 Office of Technology Assessment (OTA) study estimated that up to 3 million deaths could occur following the release of 100 kg of B anthracis (OTA 1993).

Experience with aerosol spraying of B thuringiensis in Canada to control the European gypsy moth demonstrated the following pertinent findings (Levin 2003):

Approximately 5 to 6 hours after the spray application began, indoor concentrations of airborne B thuringiensis actually exceeded outdoor concentrations, suggesting that the organisms entered homes and buildings after the aerosol release.
Although most of the particles were relatively large, particles with a medium diameter of 2 to 7 microns were detected both inside and outside of homes. The authors estimated that, at 5 to 6 hours after spraying, adults in the spray zone inhaled approximately 203 spores per hour.
Nasal swabs from asthmatic children in the spray zone were collected after spraying; Bacillus species with genetic patterns consistent with B thuringiensis were identified in 76.6% of isolates obtained from nasal passages.

The findings from this study suggest that it is technologically feasible to disseminate biological agents from aircraft; however, the applicability of this information to an intentional aerosol release of B anthracis is unknown. These data also raise concerns about indoor air safety should an intentional release of a biological agent occur.

Contamination of Food or Water

Deliberate contamination of food or water with anthrax spores also is a possibility. During World War II, the Japanese reportedly impregnated chocolate with anthrax to kill Chinese children. The apartheid government of South Africa also experimented with anthrax in chocolate (Sirisanthana 2002).

Even though contamination of a water supply is unlikely owing to the large volume of water involved and the chlorination process, contamination of smaller water sources is theoretically feasible since spore counts remain stable in water for at least several days following inoculation (Beatty 2003). Since B anthracis spores are not destroyed by pasteurization, contamination of milk is another theoretical possibility (Perdue 2003).

Historical Perspective

Although anthrax has not been responsible for the massive number of deaths associated with cholera, plague, or smallpox, it has played a prominent role in the history of infectious diseases. Anthrax was the first disease for which a microbial origin was definitively established. Robert Koch identified the bacterium that causes anthrax in 1877 (Martin 2010: Bacillus anthracis [anthrax], Purcell 2007). Anthrax also was the first disease for which an effective live bacterial vaccine was developed; Louis Pasteur developed this vaccine in 1881 and tested it in domesticated animals. Additionally, inhalational anthrax among wool and animal hide processors introduced the concept of occupational infectious disease risk.

Intentional spread of anthrax dates back to World War I, when German operatives infected horses and cattle with B anthracis while they were awaiting shipment overseas. During World War II, both the Axis and the Allies had biowarfare programs that involved anthrax (Martin 2010: Anthrax as an agent of bioterrorism). The US military has been concerned about anthrax as a potential biological weapon for many years because of its infectiousness via the aerosol route and the associated high mortality rate for untreated inhalational anthrax (Purcell 2007). B anthracis is readily accessible and easy to grow; in the spore form it is stable, easily stored, and portable. Spores may be dispersed or sprayed as a powder or liquid. Thus, anthrax remains the agent of greatest concern for bioterrorism (Martin 2010: Anthrax as an agent of bioterrorism).

In 1972, more than 140 countries signed the Biological and Toxin Weapons Convention, which called for termination of all offensive biological weapons research and development and destruction of existing biological weapons stocks. However, the former Soviet Union continued to expand its biological weapons program (which included weaponization of anthrax) throughout the 1980s and early 1990s.

After the demise of the Soviet Union, many of the scientists who worked in the biological weapons program left the country. The status of those scientists remains unknown; however, Iraq, Iran, Syria, Libya, and North Korea actively have recruited such experts (Henderson 1999). These countries and others have been suspected of ongoing development of offensive bioweapons programs. Reports from the past few years suggest that at least three countries have offensive biological weapons programs and at least an additional six have research programs with possible production of offensive weapons (MIIS).

Weaponized anthrax has caused two outbreaks of disease (key points from each are outlined in the sections below). In addition, in July 1993, the religious group Aum Shinrikyo aerosolized a liquid suspension of B anthracis from the roof of an eight-story building in the Kameido district of Tokyo, but the release did not cause any human cases. Factors contributing to failure of this bioterrorist act included use of an attenuated B anthracis strain, low spore concentration, ineffective dispersal, a clogged spray device, and inactivation of spores by sunlight (Takahashi 2004).

Sverdlovsk, USSR—1979

This outbreak in Sverdlovsk in the Union of the Soviet Socialist Republics (now Ekaterinburg, Russia) resulted from accidental release of anthrax spores from a military microbiological facility, Compound 19, where weaponized anthrax was being produced (Dembek 2007, Meselson 1994).
Seventy-seven human cases were reported and 66 of the patients died, for a case-fatality rate of 86%; 75 cases were inhalational and two were cutaneous (one on the back of the neck and one on the shoulder). A subsequent statistical analysis of available data suggests that 250 cases with 100 fatalities actually may have occurred (Brookmeyer 2001).
The mean incubation period was 9 to 10 days (range, 2 to 43 days), and the mean time between illness onset and death was 3 days.
Mean patient age for male cases was 42 years and for female cases was 55 years, and no cases occurred in children.
The geographic distribution of human and animal cases indicated that the outbreak was confined to a narrow zone, downwind from a point of origin in Sverdlovsk (approximately 4 km for humans and 40 km for animals). Historical meteorological data, combined with this case distribution, identified Compound 19 as the point of origin. These data also showed that the release most likely took place on April 2, 1979.
Approximately 2 weeks after the presumed date of exposure, a vaccination campaign of 59,000 eligible residents was begun; an estimated 80% of the target population received at least one dose of vaccine. Prophylactic antibiotics also were provided to both suspected and confirmed cases.
Investigators postulated that the weight of spores released as aerosol "could have been as little as a few milligrams or as much as nearly a gram."
Recent modeling studies suggest that the incubation period for anthrax is dose-dependent. The authors postulate that the relatively long incubation period for cases associated with Sverdlovsk was related to the level of exposure (Wilkening 2006).

United States—2001

An outbreak of cutaneous/inhalational anthrax occurred in the United States in 2001.
The outbreak predominantly involved direct exposure to mail that was deliberately contaminated with anthrax spores. Several contaminated letters were sent to members of Congress and media outlets, and one was reported to contain 2 g of powder, with 100 billion to 1 trillion anthrax spores per gram (Inglesby 2002).
The following features were noted in an epidemiologic report that summarized the outbreak findings (Jernigan 2002):

Twenty-two cases (11 inhalational and 11 cutaneous) were identified. Five of the patients with inhalational anthrax died, for a case-fatality rate of 45% among that group.
Cases occurred in residents of seven states along the East Coast (Connecticut, Florida, Maryland, New Jersey, New York, Pennsylvania, and Virginia), with illness onsets between September 22 and November 16, 2001.
Four contaminated letters were recovered; all four were mailed in or around Trenton, New Jersey. Two were postmarked September 18, 2001, and two were postmarked October 9, 2001.
Twenty of the patients were either mail handlers or were exposed to work sites where mail was handled or received; 1 of these cases was a 7-month-old infant. The remaining 2 cases had no apparent association with contaminated mail. These persons likely became exposed through cross-contamination of bulk mail that passed through contaminated mail facilities (Griffith 2003, Holtz 2003).
Illness in the 7-month-old infant with cutaneous anthrax was complicated by quick progression to severe microangiopathic hemolytic anemia despite early antibiotic treatment. The source is thought to be the workplace of the infant's mother, which the infant visited the day before symptom onset. One possible exposure scenario, according to the authors, is that spores on the hands of someone in the workplace who lifted or held the child may have contacted an exposed or possibly abraded area of the child's skin (Freedman 2002).
B anthracis isolates were obtained from the 4 contaminated letters, 17 clinical specimens from cases, and 106 environmental samples collected along the mail path of the contaminated letters; all were identical by molecular subtyping.

Following recognition of anthrax cases in postal workers, air sampling was conducted before and during activation of a contaminated mail-sorting machine at a Washington, DC, postal facility. This testing (which was conducted several weeks after the contaminated mail passed through the machine) demonstrated that a mail-sorting machine can remain contaminated for many days after processing mail contaminated with B anthracis (Dull 2002).
The outbreak demonstrated several important points about weaponized anthrax:

Mail can be an effective vehicle for disseminating anthrax spores.
Cross-contamination of mail likely can occur within postal facilities.
Persons who handle or process unopened contaminated mail are at risk of acquiring anthrax.
Substantial environmental contamination can occur in facilities handling contaminated mail or in offices where contaminated mail is opened.

Following the outbreak, a case of cutaneous anthrax occurred in a laboratory worker in Texas who was working at a private laboratory that was processing environmental samples from the CDC investigations (CDC 2002: Suspected cutaneous anthrax in a laboratory worker—Texas, 2002).
A 1-year health assessment of adult anthrax survivors demonstrated that survivors continued to report moderate to severe health complaints affecting multiple organ systems and had significantly greater overall psychological distress compared with US referent populations (Reissman 2004). Fifty-three percent had not returned to work since their infection.
The alleged perpetrator and the source of the anthrax in this outbreak have been identified. Bruce Ivins, the scientist named by Federal Bureau of Investigation (FBI) investigators as the sole orchestrator of the attack, committed suicide before he was charged. Ivins worked at the US Army Medical Research Institute of Infectious Diseases (USAMRIID) and had access to the spores used in the attack. Sequence analyses of anthrax strains allowed investigators to trace the spores to two flasks, one at USAMRIID that was under Ivin's charge and one flask from another laboratory. Several factors continue to be discussed, including the basis on which the FBI ruled out spores from the other flask and ruled out other individuals who had access to the spores. Some scientists remain skeptical of the FBI statements, in light of the circumstantial nature of the the evidence. The FBI closed the case on February 19, 2010, in spite of the remaining unanswered questions (Bhattacharjee 2008, Warrick 2010).
In response to this skepticism, the FBI requested that the National Research Council (NRC) of the National Academy of Sciences (NAS) launch an independent review of the scientific approaches used and the conclusions reached during the FBI investigation. On Feb 15, 2011, the NRC completed its review; in its report, the NRC ruled that the genetic analyses conducted by the FBI were scientifically sound but the results could not by themselves conclusively link the anthrax strain to the flask (designated "RMR-1029") in Ivins' custody (NRC 2011). "Spores in the mailed letters and in RMR-1029 … share a number of genetic similarities consistent with the FBI finding that the spores in the letters were derived from RMR-1029," the NAS said in a press release (NAS 2011). "However, the committee found that other possible explanations for the similarities—such as independent, parallel evolution—were not definitively explored during the investigation." The FBI replied in a statement, "The FBI has long maintained that while science played a significant role, it was the totality of the investigative process that determined the outcome of the anthrax case" (FBI 2011).
In March 2011, scientists involved in the anthrax investigation reported how, through whole-genome sequencing and comparative genomics, they were able to identify four mutations in spore material from letter samples. The mutations were related to a regulatory protein involved in sporulation and were found only in spores linked to the B anthracis in Ivins' custody (Rasko 2011). Only 8 of the 947 isolates they studied had all four mutations, and these all came direcly or indirectly from the "parent" strain from Ivins' lab (see Feb 15, 2011, CIDRAP News story).
The original Ames strain came from a laboratory in College Station, Texas (rather than Ames, Iowa). Several distinct Ames strains have been identified.
As a result of this outbreak, the US Postal Service has deployed an autonomous biodetection system (BDS) for anthrax in mail-processing systems across the United States (CDC 2004: Responding to detection of aerosolized Bacillus anthracis by autonomous detection systems in the workplace).


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