A week after six bioterrorism sensors detected the presence of a dangerous bacterium on the Mall, health officials said there are no reports that any of the thousands of people in the nation's capital Sept. 24 have tularemia, the illness that results from exposure to the bacteria.
Federal health officials are still testing the samples from air sensors on the Mall and in downtown Washington that collected a small amount of the tularemia agent, which can cause flulike symptoms and is usually treated with antibiotics.
The bacteria probably was not the result of nefarious activity, according to federal investigators. "There is no known nexus to terror or criminal behavior. We believe this to be environmental," said Russ Knocke, spokesman for the Department of Homeland Security.
The mission for health officials now is to figure out how the bacteria got there, why they were detected that day and whether they are from a strain that doesn't affect humans.
"Our purpose now is to reach into the medical community just to make sure nobody out there has any symptoms," said Von Roebuck, spokesman for the federal Centers for Disease Control and Prevention. "There is an incubation window of one to six days, but we still want to get this information into clinicians' hands."
Health officials in the Washington area were notified Friday that the filters on biohazard sensors that make up the BioWatch network detected the bacteria Sept. 24, when tens of thousands of people were on the Mall for antiwar demonstrations and the National Book Festival.
The samples were collected between 10 a.m. Saturday and 10 a.m. Sunday. The naturally-occurring biological agent -- which is on the "A list" of the Department of Homeland Security's biohazards, along with anthrax, plague and smallpox -- was detected in small amounts, said Gregg A. Pane of the D.C. Department of Health.
Detection of the bacteria turned into an incident with nationwide implications, because thousands of protesters had come from throughout the country.
The infection is not spread from person to person, but tracking potential patients became a coast-to-coast undertaking. Police said that more than 100,000 people attended the rally; organizers put the figure at 300,000.
After the filters were tested in Washington, further tests were done by CDC laboratories in Atlanta, Knocke said.
Meanwhile, the CDC was using its nationwide tracking system to look for unusual occurrences of pneumonia-like symptoms in every state, Roebuck said.
After seeing no cases and finding that the levels of tularemia in their samples were low, health officials at the CDC decided Friday night to release their findings in the event that there were cases, on the sunset of the incubation period, that weren't being detected, Roebuck said.
Pane said he learned of the biohazard alert that night, when CDC officials told him that sensors at the Lincoln Memorial, Judiciary Square and Fort McNair tested positive for the bacterium.
He immediately put out an alert to hospitals, clinics, doctors and pharmacies in the area to be on the lookout for symptoms, which also can resemble pneumonia. "The linchpin is that we're still in the incubation window, so we really don't know that there were no cases," he said.
Pane said one theory is that tularemia bacteria, which occur naturally in soil, might have been kicked up by the thousands of feet stomping on the Mall grounds that day.
Homeland Security and the CDC work together to operate the BioWatch sensors. The $60 million-plus system was created in 2001 to monitor air in more than 30 U.S. cities.
The system recently was criticized in a June report by the Government Accountability Office. It highlighted communication problems within the network and questioned the nation's capabilities in environmental surveillance, an emerging technical field.
A similar incident occurred in Houston in October 2003, when two air sensors detected fragments of tularemia bacteria. There were no human cases of tularemia reported after the incident, and some experts in the bioterror field said they believe the incident was spurred by a strain of the bacteria that does not affect humans.
"It's probably something that just lives in the environment," said Tara O'Toole, who is director of the Center for Biosecurity, sponsored by the University of Pittsburgh Medical Center. "We forget that microorganisms rule the world. Now we're looking and finding things we didn't know were there."
Tularemia, often called "rabbit fever" because small animals are often carriers in rural areas, was amassed by the U.S. military as a biological weapon in the 1960s.