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Anthrax as weapon

Print edition : Oct 27, 2001 T+T-

ANTHRAX, a severe illness caused by the bacterium Bacillus anthracis, is considered a prototypical biowarfare (B.W.) agent. The name derives from the Greek word for coal, anthrakis, because the disease causes black, coal-like skin lesions. In 1999, the Working Group on Civilian Biodefence in the United States considered numerous biological agents that may be used as weapons and identified a limited number of organisms that could cause disease and deaths in sufficient numbers to cripple a city or a region. The group concluded that anthrax was one of the most serious of these diseases.

In nature, anthrax is primarily a disease of cattle and sheep but can also infect humans. It can survive for long periods in soil in a dormant (spore) phase; after infection it reverts to an active phase when it multiplies rapidly in the host and secretes fatal toxins. Natural human infection can result either from skin contact with infected animals, ingestion of contaminated meat or inhalation of spores, usually from contaminated hides.

In humans, three types of anthrax infection occur: cutaneous, inhalational and gastrointestinal. Naturally occurring inhalational anthrax is rare in humans. Historically, wool sorters in industrial mills were at the highest risk. Hence the inhalational form is also called "woolsorter's disease". Cutaneous anthrax is the most common naturally occurring form. The largest reported epidemic occurred in Zimbabwe between 1979 and 1985 when more than 10,000 cases, nearly all of them cutaneous, were reported. Gastro-intestinal anthrax is also not common. However, outbreaks of this form have been reported in Africa and Asia arising mainly from ingestion of insufficiently cooked contaminated meat. It presents two distinct syndromes: oral-pharyngeal and abdominal. Cases of pulmonary - and sometimes gastrointestinal - anthrax are almost invariably fatal if not treated immediately with antibiotics. Inhalation of aerosolised spores would be the primary route of infection if the bacteria were used as a B.W. agent.

The recent incidents in the United States, however, do not involve aerosolised spores but merely milled spores in letters and packages. Most experts agree that the manufacture of a lethal anthrax aerosol is beyond the capacity of individuals or groups without access to advanced biotechnology. However, autonomous groups with substantial funding or contacts may be able to acquire the required materials for an aerosol attack. The Japanese terrorist group Aum Shinrikyo, which was responsible for releasing the poisonous chemical sarin in a Tokyo subway station in 1995, killing and injuring many, had successfully dispersed anthrax aerosols in various parts of Tokyo but the strain turned out to be non-virulent.

The accidental aerosolised release of anthrax spores from a military B.W. facility in Sverdlovsk in the former Soviet Union in 1979 resulted in at least 79 cases of infection and 69 deaths, thus demonstrating the lethal potential of anthrax aerosols. In this incident there were no deaths in patients developing cutaneous anthrax. The maximum latency period for this form was 12 days. In the event of an outbreak resulting from aerosolised anthrax release, cutaneous anthrax would be far less common than inhalational anthrax.

An anthrax aerosol would be odourless and invisible following release and would have the potential to travel many kilometres before disseminating. Evidence suggests that following an outdoor aerosol release, persons indoors too could be exposed to the threat. Extrapolation from animal studies suggests that inhalation of 1,000 spores or less can cause pulmonary anthrax in some members of an exposed population, while inhalation of about 8,000 spores - weighing about 0.08 microgram - is fatal within a week to a large proportion of those exposed.

A WHO study has estimated that if there is a release of 50 kg of anthrax aerosol from an aircraft over a deveeveloped urban population of five million, 250,000 persons would develop the disease, of whom 100,000 would die without treatment.

In case of a non-aerosolised spore release, as in the recent events, the lack of volatality of anthrax spores as well as their inability to penetrate the skin should be borne in mind. These factors make it unlikely, in most cases, that persons coming into contact with letters, packages and other devices possibly containing anthrax spores will be at risk. Moreover, because energy is required to aerosolise anthrax spores, opening a letter, even if it contains anthrax spores, will be unlikely to place a person at substantial risk. For these reasons post-exposure prophylaxis (vaccination) may not be necessary in such cases of anthrax release, according to Theodore Cislak and Edward Eitzen of the U.S. Army Medical Research Institute of Infectious Diseases, Maryland.

In addition to its lethality, anthrax has other characteristics that make it an effective B.W. agent. First, the disease is not contagious. As a result, anthrax would not spread far beyond the intended target. Second, anthrax is easy to produce. The organism and its spores can be easily produced in the laboratory in almost unlimited quantities and antibiotic resistant strains have been developed with standard selection strains. Third, when anthrax bacteria are incubated under certain conditions, they transform themselves into the rugged spore form, which has a long shelf life. Although most spores can be killed by boiling for 10 minutes, they can survive up to 20 years or longer in soil and animal hides. Indeed, during the Second World War, Britain detonated experimental anthrax bombs in Gruinard island off Scotland, releasing spores that remained in the top 15 to 20 cm of the soil for more than 40 years.

This spore-forming ability makes anthrax particularly well-suited for delivery even in the dry form by a variety of delivery systems, including missiles or bombs. The spores are stable when suspended in air, can survive detonation from a bomb or a shell and, unlike most pathogens, will live for several days if direct sunlight is avoided. Field tests have shown that anthrax spores decay at the rate of less than 0.1 per cent a minute, which is very slow for an organism. By infecting livestock, anthrax bacteria might also create reservoirs of the disease that could result in occasional outbreaks.

In sum, although anthrax may be the most viable of B.W. agents, it is also a weapon for which good anti-microbial therapy, a vaccine and post-exposure prophylaxis exist. However, given the relatively short incubation period and the rapid progression of the disease, identification of the exposed population within 24 to 48 hours, rapid diagnosis and employment of therapeutic and prophylactic strategies present the biggest challenge in case of a bio-terrorist attack with anthrax.

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