Anthrax, the spectrum of diseases caused by infection with Bacillus anthracis
, is not considered a communicable disease but is generally acquired via environmental exposures. Many anthrax cases through history have been the result of agricultural or industrial exposure to B. anthracis
). The disease most often presents itself as a cutaneous infection; however, there are both gastrointestinal and inhalational forms of the disease. Inhalational anthrax is typically rapidly fatal, even with treatment. In general, inhalation exposures require specific conditions, such as poor ventilation and activities that disturb dust containing B. anthracis
Because diagnosing anthrax in its early stages in human and animal hosts is difficult and B. anthracis
spores are extremely stable in the environment, this microorganism has been investigated, developed, and deployed as a biological weapon throughout the 20th century. Use of this microorganism has seen varied success during World War I (9
) and subsequently. It is generally accepted that there was an accidental release of B. anthracis
spores from a weapons manufacturing or development facility in 1979 in Sverdlovsk, USSR (now Yekaterinaburg, Russia) (10
). In 1993, an attempt by a civilian group, Aum Shinrikyo, to use this microorganism to attack a civilian population in a Tokyo suburb did not result in any casualties (22
In 2001, envelopes containing a powder formulation of B. anthracis
were mailed in the United States to several individuals. These letters were the presumed cause of 22 cases of clinical anthrax, 11 inhalational and 11 cutaneous, with 5 fatalities, all of whom suffered from inhalational disease (34
). According to congressional testimony, the powdered spore suspension was “easily dispersed into the air” (29
). Of the 11 individuals with inhalational disease, 2 had no history of handling mail or having any other direct contact with these threat letters (11
). Of the remaining nine individuals, eight were thought to have been exposed through handling or processing mail (20
) but may never have picked up or directly handled the actual threat letters. Thus, some individuals who contracted inhalational disease may have been exposed to aerosols that were generated from residual spore material deposited on contaminated surfaces. This conclusion was borne out by a study conducted on the scene of one contamination incident, which demonstrated that spores could be reaerosolized from surfaces during simulated office activities—e.g., paper handling, foot traffic, moving containers—after a period of no entry and no ventilation for several days (38
). McCleery et al. (25
) found that reaerosolization of spores is possible in postal facilities.
In the mail-related instance of 2001, aerosol exposures occurred. Since spore-contaminated surfaces can become sources for aerosol generation, nonporous surfaces (walls, desks, lockers, etc.) were decontaminated to reduce risk while porous surfaces (draperies and sofas) were removed. To determine the efficacy of decontamination, contaminated buildings were first sampled for the presence of B. anthracis
spores followed by treatment by a variety of techniques. Postdecontamination sampling was used to determine efficacy (37
) and to assess the safety for reoccupancy.
The Government Accountability Office (GAO) reported that additional methodological validation of sampling collection and analytical methods should be conducted to enhance the interpretation of negative sampling results because initial samples from two postal facilities were negative, but later samples were positive (17
). The GAO (17
) report defined validation as “… a formal and independently administered empirical process. For validation, the overall performance characteristics of a given method must be certified as meeting the specified requirements for intended use and as conforming with applicable standards.” Currently, there is no preexisting standard for a presumable safe level of surface contamination with B. anthracis
spores that may be assessed through sampling and analysis.
Development of independent standards for assessing the requirements for surface sampling methods requires an understanding of the rate at which spores leave surfaces to become entrained in aerosols, the potential for aerosol exposure by humans, and the infectivity of inhaled spores. Inhalation infectivity has been researched, but estimates of a lethal dose vary (14
). Bartrand et al. (5
) conducted a risk analysis on the mortality of guinea pigs and rhesus monkeys exposed to B. anthracis
spores and found a 50% lethal dose (LD50
; i.e., the dose at which 50% of subjects die) of about 100,000 spores inhaled for 1-μm particles. Limitations of relating exposure to inhalation infectivity include quantification of the ability of spores to move from stasis on a surface to entrainment as an aerosol, quantification of exposures to the resultant aerosol, uptake by humans, room size and ventilation characteristics, and exposure time. Despite these limitations, it is necessary to standardize the performance of surface sampling methods.
Brown et al. evaluated wipe (6
), swab (7
), and vacuum (8
) spore collection methods with B. atrophaeus
. These studies have added significant information to the understanding of recovery efficiencies for these three sampling methods; however, sampling performance was not evaluated at very low spore surface loading concentrations. Sampling performance measures at very low surface loading of B. anthracis
are needed to aid in the decision making for decontamination and other interventions (31
The goal of this study was to evaluate the current CDC environmental surface sampling methods for B. anthracis
) as slightly modified based on subsequent CDC research (19
). We estimated B. anthracis
Sterne sampling limit of detection (LOD), recovery efficiency (RE), and measurement precision for three sampling methods (swab, wipe, and vacuum) and two surfaces (steel and carpet) by allowing spores to settle from an aerosol in a controlled environment. In addition, we compared sample analyses performed at three laboratories to determine the level of interlaboratory variability.