Decontamination of Anthrax

In the setting of an announced alleged anthrax release,  any person coming in direct physical contact with a substance alleged to be anthrax should perform thorough washing of the exposed skin and articles of clothing with soap and water.  Further decontamination of directly exposed individuals or of others is not indicated.  In addition, any person in direct physical contact with the alleged substance should receive post-exposure antibiotic prophylaxis until the substance is proved not to be anthrax.  If the alleged substance is proved to be anthrax, immediate consultation with experts at the CDC and USAMRIID should be obtained.

The greatest risk to human health following an intentional aerosolization of anthrax spores occurs during the period in which anthrax spores remain airborne, called primary aerosolization. The duration for which spores remain airborne and the distance spores travel before they become noninfectious or fall to the ground is dependent on meteorological conditions and aerobiological properties of the dispersed aerosol.   Under circumstances of maximum survival and persistence, the aerosol would be fully dispersed within hours to 1 day at most, well before the first symptomatic cases would be seen. Following the discovery that a bio-weapon has been used, anthrax spores may be detected on environmental surfaces using rapid assay kits or culture, but they provide no indication as to the risk of reaerosolization.

The risk that anthrax spores might pose to public health after the period of primary aerosolization can be inferred from the experience following an accident at a Russian bio-weapon procesing facility that resulted in a release of anthrax to the surrounding community of Sverdlovsk, Russia and modeling analyses by the US Army.  At Sverdlovsk, new cases of inhalational anthrax developed as late as 43 days after the presumed date of release, but none occurred during the months and years afterward.  Some have questioned whether any of those cases with onset of disease beyond 7 days might have represented illness following resuspension of spores from the ground or other surfaces, a process that has been called secondary aerosolization. While it is impossible to state with certainty that secondary aerosolizations did not occur, it appears unlikely.  It should be noted that few efforts were made to decontaminate the environment after the accident and only 47 000 of the city’s 1 million inhabitants were vaccinated.  The epidemic curve is typical for a common source epidemic, and it is possible to account for virtually all patients having been within the area of the plume on the day of the accident. More over, if secondary aerosolization had been important, new cases almost certainly would have continued for a period well beyond the observed 43 days.

Although persons working with animal hair or hides are known to be at increased risk of developing inhalational or cutaneous anthrax, surprisingly few of those exposed in the United States have developed disease.  During the first half of this century, a significant number of goat hair mill workers were likely exposed to aerosolized spores. Mandatory vaccination became a requirement working in goat hair mills only in the 1960s. Meanwhile, many unvaccinated person-years of high-risk exposure had occurred, but only 13 cases of inhalational anthrax were reported.  One study of environmental exposure was conducted at a Pennsylvania goat hair mill at which workers were shown to inhale up to 510 B anthracis particles of at least 5 µmin diameter per person per 8-hour shift. These concentrations of spores were constantly present in the environment during the time of this study, but no cases of inhalational anthrax occurred.

Modeling analyses have been carried out by US Army scientists seeking to determine the risk of secondary aerosolization. One study concluded that there was no significant threat to personnel in areas contaminated by 1 million spores per square meter either from traffic on asphalt-paved roads or from a runway used by helicopters or jet aircraft.   A separate study showed that in areas of ground contaminated with 20 million Bacillus subtilis spores per square meter, a soldier exercising actively for a 3-hour period would inhale between 1000 and 15 000 spores.

If an environmental surface is proved to be heavily contaminated with anthrax spores in the immediate area of a spill or close proximity to the point of release of an anthrax aerosol, decontamination of that area may decrease the slight risk of acquiring anthrax by secondary aerosolization. However, decontamination of large urban areas or even a building following an exposure to an anthrax aerosol would be extremely difficult and is not indicated. Although the risk of disease caused by secondary aerosolization would be extremely low, it would be difficult to offer absolute assurance that there was not risk whatsoever.  Post-exposure vaccination, if vaccine were available, might be a possible intervention that could further lower the risk of anthrax infection in this setting.

(The preceding material was extracted from the American Medical Association's consensus statement, Anthrax as a Biological Weapon: Medical and Public Health Management, Journal of the American Medical Association, May 12, 1999, Volume 281, Number 18, pages 1735-1745.)

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