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Notice to Readers: Interim Guidelines for Investigation of and Response to Bacillus Anthracis Exposures

Environmental Sampling. Environmental testing to detect B. anthracis on surfaces or in the air can be used to investigate known or suspected exposure events. The highest priority of an investigation is to evaluate the risk for exposure to aerosolized B. anthracis spores. Persons collecting and testing samples should 1) obtain adequate samples, 2) avoid cross-contamination during processing, and 3) ensure proficient laboratory testing and interpretation of test results. A positive laboratory test for B. anthracis from a sample of an environmental surface may be caused by cross-contamination from an exposure vehicle (e.g., contact with an envelope containing B. anthracis), background occurrence of B. anthracis spores in the environment, or previously aerosolized B. anthracis that has settled onto environmental surfaces. Laboratory test results of environmental surface samples should not be the only criterion for starting, continuing, or stopping antimicrobial prophylaxis for inhalational disease.

Environmental sampling can be directed, prospective, or random. In directed sampling, air and/or surface samples are obtained as part of an investigation of a specific threat, a known exposure, or of persons with bioterrorism-related anthrax. Directed environmental sampling may play a critical role in characterizing potential exposures and guiding public health action (Box 1).

Prospective environmental sampling is defined as ongoing sampling and testing of air or surfaces for B. anthracis spores. The value of prospective sampling is not known. Current technologies for monitoring air for B. anthracis and other agents are not validated and their performance has not been assessed during bioterrorism events. Prospective environmental sampling of surfaces may have a role in detecting B. anthracis contamination, especially at facilities or events determined to be at high risk for bioterrorism (Box 1).

The testing of random environmental samples (i.e., sampling air or surfaces of facilities that are not directly associated with confirmed anthrax disease or a known B. anthracis exposure) is of uncertain utility in detecting past exposures. Random positive tests for B. anthracis spores may represent cross-contamination from an exposure vehicle (e.g., letter) that poses negligible risks for inhalational anthrax. These positive test results may prompt more extensive evaluation to direct cleanup, if needed.

Nasal Swab Cultures. Nasal swab cultures should not be used to diagnose cases of anthrax or to evaluate whether a person had been exposed. Nasal swab cultures may be useful in the investigation of known or suspected airborne B. anthracis (Box 1). Because the sensitivity of nasal swab cultures decreases over time, cultures should be obtained within 7 days of the exposure. The presence of B. anthracis from a nasal swab culture cannot be determined by gram stain or colony characteristics alone and requires confirmatory testing by qualified laboratories.

Antimicrobial Prophylaxis. Antimicrobial prophylaxis is used to prevent cases of inhalational anthrax (Box 1). Public health authorities often start prophylaxis before the extent of exposure is known. Subsequent epidemiologic and laboratory test data may indicate that some persons started on prophylaxis were not exposed. These persons should stop antimicrobial prophylaxis. Persons who were exposed should complete 60 days of therapy. No shorter course of antimicrobial prophylaxis exists. The choice of an antimicrobial agent should be based on antimicrobial susceptibility, the drug's effectiveness, adverse events, and cost. B. anthracis isolates from patients with bioterrorism-related anthrax have been susceptible to ciprofloxacin, doxycycline, and other agents; the use of doxycycline may be preferable to prevent development of ciprofloxacin resistance in more common bacteria (1). Respiratory transmission of B. anthracis from person-to-person does not occur; no antimicrobial prophylaxis is indicated.

Closing Facilities. The decision to close a facility is made to prevent cases of inhalational anthrax (Box 1). The facility should remain closed until the risk for inhalational disease is eliminated.


  1. CDC. Update: investigation of bioterrorism-related anthrax and interim guidelines for exposure management and antimicrobial therapy, October 2001. MMWR 2001;50:909--19.

Box 1

BOX 1. Interim guidelines for investigation of and response to B. anthracis exposures

Environmental Sampling

Directed sampling of environmental surfaces may be indicated:

  • To identify a site or source of Bacillus anthracis exposure that has resulted in a case(s) of anthrax
  • To trace the route of an exposure vehicle (e.g., a powder-containing letter)
  • To obtain the B. anthracis strain when isolates from patients are not available
  • To guide cleanup activities in a contaminated area or building
  • To assess biosafety procedures in laboratories processing B. anthracis specimens

Prospective sampling of environmental surfaces may be indicated:

  • To identify receipt of a contaminated exposure vehicle in high risk facilities (e.g., mailrooms of targeted persons or groups)
  • To detect aerosolized B. anthracis in high risk areas or events

Laboratory testing of environmental surface samples should not be the only means to determine the need for antimicrobial prophylaxis.

Nasal Swab Cultures

Collection of nasal swabs for culture of B. anthracis may be useful:

  • To help define an area of exposure to aerosolized B. anthracis
  • To help ascertain where a person with inhalational anthrax was exposed if the time and place of exposure are not already known

Collection of nasal swabs for culture of B. anthracis is not indicated:

  • To diagnose anthrax
  • To determine a person's risk of exposure and the need for antimicrobial prophylaxis
  • To determine when antimicrobial prophylaxis should be stopped
  • To supplement random environmental sampling

Antimicrobial Prophylaxis

Antimicrobial prophylaxis may be initiated pending additional information when:

  • A person is exposed to an air space where a suspicious material may have been aerosolized (e.g., near a suspicious powder-containing letter during opening)
  • A person has shared the air space likely to be the source of an inhalational anthrax case

Antimicrobial prophylaxis should be continued for 60 days for:

  • Persons exposed to an air space known to be contaminated with aerosolized B. anthracis
  • Persons exposed to an air space known to be the source of an inhalational anthrax case
  • Persons along the transit path of an envelope or other vehicle containing B. anthracis that may have been aerosolized (e.g., a postal sorting facility in which an envelope containing B. anthracis was processed)
  • Unvaccinated laboratory workers exposed to confirmed B. anthracis cultures

Antimicrobial prophylaxis is not indicated:

  • For prevention of cutaneous anthrax
  • For autopsy personnel examining bodies infected with anthrax when appropriate isolation precautions and procedures are followed
  • For hospital personnel caring for patients with anthrax
  • For persons who routinely open or handle mail in the absence of a suspicious letter or credible threat

A positive test for B. anthracis from a randomly collected specimen does not require implementation of antimicrobial prophylaxis or the closing of a facility.

Closing a Facility

Closing a facility or a part of a facility may be indicated:

  • After an inhalational anthrax case is detected and a probable site of exposure in the facility is identified
  • When there is a known aerosolization of B. anthracis in the facility
  • When evidence strongly suggests an aerosolization of B. anthracis in the facility
  • As determined by law enforcement authorities in a criminal investigation

Closing a facility is not indicated:

  • Based only on the identification of B. anthracis from samples of environmental surfaces
  • Based only on the identification of a cutaneous anthrax cases

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