On October 15, 2001, at 9:45 a.m., a staff member on the 6th floor of the Hart Senate Office Building (HSOB) in the office of Senate Majority Leader Tom Daschle cut open a taped business envelope containing a letter and a powdery substance (Table 1). Upon noticing a burst of dust, she placed the letter on the floor and notified the U.S. Capitol Police. Within 5 minutes of being notified, officers were at the scene. The hazardous device unit of the Capitol Police arrived minutes later. The officers and emergency response personnel, referred to as first responders, arrived with respiratory personal protective equipment (PPE) on hand, but equipment was not put on until after arrival at the scene. These officers tested the powder for B. anthracis spores twice, using commercial rapid tests. Preliminary results obtained within 15 minutes suggested that the powder contained B. anthracis. Laboratories at the U.S. Army Medical Research Institute of Infectious Diseases (USAMRIID) in Fort Detrick, Maryland, later confirmed these preliminary results.

At approximately 10:30 a.m., the ventilation system was shut off. Medical staff from the Office of the Attending Physician (OAP), U.S. Capitol, began collecting nasal swabs for B. anthracis culture from staff members in Senator Daschle’s office, from staff in an adjacent office belonging to Senator Russell Feingold, and from the first responders; in addition, an initial 3-day antimicrobial postexposure prophylaxis regimen consisting of ciprofloxacin, 500 mg twice a day, was given to these persons. Only the person who opened the contaminated envelope removed and changed her clothing and was decontaminated with soap and water. All others washed their hands with soap and water.

Next, first responders led employees from the two 6th-floor offices to the 9th floor of the building, where further samples were taken from nares and clothing. After testing, these employees were led back to Senator Daschle’s 5th-floor office, where other staff members were detained. At approximately 3:00 p.m., the staff members were allowed to go home.

Employees in other offices continued working until the close of business. The southwest quadrant of the building was closed the morning of October 16, and a decision was made to close the entire HSOB that evening. During the next 3 days, OAP continued to collect nasal swabs for B. anthracis for all HSOB employees present on October 15 and for others on Capitol Hill who requested these tests. OAP also gave those tested an initial 3-day antimicrobial prophylaxis, pending final confirmation of the presence of B. anthracis spores and results of the epidemiologic investigation.

A team from the Centers for Disease Control and Prevention (CDC) arrived in Washington D.C., on the morning of October 16 to begin the epidemiologic investigation. To identify the group of persons who needed prolonged antimicrobial prophylaxis on the basis of likely exposure to B. anthracis spores, we sought to define an exposure area of higher risk.

To identify other facilities that may have been contaminated with B. anthracis spores, the contaminated envelope was traced back through the congressional mail distribution system before its arrival in Senator Daschle’s office. To define the exposure area for HSOB, we obtained floor diagrams for the 5th and 6th floors and information about the ventilation system from the Office of the Architect of the Capitol, which maintains and operates the U.S. Capitol complex. Multiple environmental samples were taken from these facilities by a variety of techniques (13).

The population at risk of developing anthrax was defined as persons in the exposed area during or after the time the contaminated envelope was processed or opened. To identify each person who may have been within the exposure area, employee lists were obtained from staff managers for each affected facility; in HSOB, managers for individual senators’ offices within the defined exposure area were contacted to obtain employee and visitor lists. We identified responders within HSOB, such as law enforcement and medical personnel, by contacting supervisors for a comprehensive list of those who were in the area. To identify other visitors or nonemployees, press conferences were used to relay the appropriate information.

Within 9 hours of the initial event, nasal swab specimens were collected for all persons in Senator Daschle’s and Senator Feingold’s offices and for all first responders. As mentioned earlier, further specimens were collected by OAP, for 4 days after the opening of the contaminated envelope, from employees of HSOB and others on Capitol Hill. Specimens were collected with Dacron fiber-tipped sterile swabs and sent for B. anthracis culture at the National Naval Medical Center in Bethesda, Maryland. Persons with initial positive nasal swabs for B. anthracis had repeat nasal swabs at 7 days postexposure and were administered a questionnaire about symptoms consistent with anthrax disease. In addition, serum specimens were obtained from these persons and tested at the CDC Meningitis and Special Pathogens Laboratory for the presence of immunoglobulin (Ig) G antibodies to B. anthracis protective antigen (anti-PA) at 7, 21, and 42 days postexposure.

In collaboration with OAP, efforts were made to ensure that all exposed persons were contacted and that they received appropriate prophylaxis with ciprofloxacin, or in the cases of persons unable to tolerate a quinolone, with doxycyline. OAP closely monitored persons who came to the clinic with respiratory symptoms; follow-up surveys were later conducted on persons receiving long-term antibiotic prophylaxis.

Among the series of bioterrorism incidents during 2001 related to B. anthracis–contaminated envelopes, this event was unique because it was the first with a known source of exposure, enabling a rapid public health response by a multidisciplinary team including law enforcement officers, medical and public health personnel, laboratory personnel, industrial hygienists, and engineers. The known source enabled us to assess the usefulness of nasal swab cultures in determining exposure to B. anthracis.

The contaminated letter purportedly contained about 2 g of powder, with each gram reported to contain between 100 billion to 1 trillion spores (14). The recovery of B. anthracis from nasal cultures was limited to persons who were inside Senator Daschle or Feingold’s offices or in the hallway joining the two offices. Nasal swab results suggest that the ventilation system played a very small role, if any, in the spread of anthrax spores in HSOB. Based on proportions of persons with positive nasal swabs, most dissemination likely occurred through room currents from the 6th to the 5th floor of the Daschle suite via an open staircase; closed doors that blocked air currents were most likely the reason a smaller proportion in Senator Feingold’s office had positive nasal cultures despite being adjacent to Senator Daschle’s office.

Swabs were taken within 1 day of the initial event from all 71 persons in the immediate exposure area, including those with positive nasal cultures for B. anthracis. However, in others with negative results, testing was not done for up to 4 days. Although these persons were located outside the immediate exposure area, it is uncertain whether prompt antibiotic administration, a delay in nasal swab testing, or both, may have had an effect on those nasal culture results. In one animal model involving macaques, large inhaled doses of anthrax spores in a controlled setting yielded B. anthracis in nasal swabs of all animals within 24 hours of exposure, and although sensitivity decreased as time progressed, positive nasal cultures were recovered in some macaques 1 week after exposure (15). In the Florida anthrax investigation, positive nasal cultures were detected in a person >1 week after presumed exposure (5). Repeat swabs from the persons with initially positive cultures were negative at 7 days postexposure, but prophylaxis administration may have influenced those results. The greatest sensitivity for recovery of B. anthracis can be achieved by obtaining nasal swab specimens as early as possible after recognized exposure.

Nasal swabs served as an epidemiologic tool; we considered the work locations of those with positive nasal swabs to be areas at risk for anthrax exposure. However, interpretation of positive or negative nasal swab results for individual risk assessment of anthrax disease has not been evaluated, and nasal swabs should not be used for this purpose. In the case of one person who died after exposure to anthrax, a nasal swab culture was negative (16). Likewise, environmental sampling may be a valuable component of assessment of areas of risk, but individual environmental samples are not sufficient to determine a person's risk for anthrax.

Two other issues deserve mention. First, the use of PPE may be an effective barrier to exposure to B. anthracis spores, although its efficacy could not be addressed in this investigation; no responder entering Senator Daschle’s office wore PPE before entering the office, and all had positive nasal swabs. Second, while subclinical anthrax infection has been documented in persons with continuous exposure to B. anthracis spores (9), the lack of serologic conversion in persons with positive nasal cultures suggests that no apparent asymptomatic infection occurred during this event, when prophylaxis was promptly initiated and continued.

Since the initial events of October 15, more information has become available—four cases of inhalational anthrax, two of them fatal, occurred in USPS employees from the Washington, D.C., Postal Distribution Center where Senator Daschle’s envelope was sorted (7,12,16), and a fifth case occurred in an employee of another mail facility, which receives government mail from the Washington, D.C., Distribution Center. These events led to new recommendations to expand the traceback for future events through the entire path to envelope origin. In addition, updated prophylaxis and treatment protocols, including options for vaccination, and subsequent recommendations for a comprehensive response to a bioterrorism attack involving B. anthracis have been published (17–21). In Table 4, specific recommendations are given for a comprehensive public health response and epidemiologic investigation that prevent further spread, identify and treat those at risk, and avoid mass administration of prolonged prophylaxis to persons not considered at risk for anthrax in the event of a future bioterrorist attack.

In conclusion, a rapid and coordinated public health response helped avert an anthrax outbreak by identifying and administering prophylaxis to persons at high risk for disease. Nasal swabs can provide useful information about the extent of exposure to B. anthracis spores to assist with defining groups at risk.

Epidemiologic assessment of risk for anthrax in persons in settings affected by a biological attack is complex, and much remains to be learned. In the meantime, a well-developed public health infrastructure, effective antimicrobial prophylaxis strategies, and effective guidelines for management based on past experiences are essential in our defense against future bioterrorism events.