As many as 75 scientists from the Centers for Disease Control and Prevention may have been exposed to live anthrax bacteria after potentially infectious samples were sent to laboratories unequipped to handle dangerous pathogens, a spokesman for the federal health agency said Thursday.
The agency was testing a new way to kill anthrax, which it discovered did not work as well as expected.
None of the potentially infected scientists have any symptoms, but a number of them are being treated with antibiotics “out of an abundance of caution,” the spokesman, Thomas Skinner, said.
The lapse occurred sometime between June 6 and June 13. Workers in three labs who were not wearing protective gear moved and experimented with samples of the highly infectious bacteria that were supposed to have been deactivated, the agency said.
It added in a statement that procedures used in two of those laboratories in Atlanta, where the C.D.C. is based, may have “aerosolized the spores,” essentially blowing the bacteria into the air. The exposure was discovered June 13, when the bacterial plates were collected for disposal and live B. anthracis colonies, or anthrax bacteria, were found.
“The likelihood that anyone was actually exposed is very small,” Mr. Skinner said.
Anthrax infects humans by touch, by inhalation or by consuming it. The inhaled form is the most dangerous, and among the 18 such cases identified in the United States during the 20th century, the fatality rate was around 75 percent, according to the C.D.C.’s website. After the terrorist attack in fall 2001 in which B. anthracis spores were released through the mail, five of the 11 people who were ill died.
The incubation period for anthrax is usually more than two weeks, so it is possible the scientists could still come down with symptoms. The incubation period can sometimes take months, according to the agency’s website.
Lab and hallway areas were being decontaminated after environmental sampling was conducted, the agency said. It added that the areas would be reopened when agency officials considered it safe to operate. The agency said that it “believes that other C.D.C. staff, family members and the general public are not at risk of exposure and do not need to take any protective action.”
The error arose, according to Paul J. Meechan, the agency’s director of health and safety, as scientists were testing a new way to kill anthrax bacteria with chemicals instead of radiation.
C.D.C. scientists were developing a way for state and local laboratories to rapidly test mysterious powders or liquids that might or might not contain anthrax. For safety, especially in state labs that do not have containment apparatus, the samples must be sterilized before they are tested.
Radiation is the most foolproof way to do that, but many state labs also do not have expensive irradiation machines, so the intention was to kill with chemicals.
After the bacteria were chemically treated, samples were put on agar plates and incubated 24 hours. When no anthrax colonies grew on them, the scientists assumed the bacteria were dead.
“It didn’t work as well as they thought,” Dr. Meechan said.
The supposedly dead bacteria were sent to C.D.C. laboratories that usually work with low-risk organisms, where workers are not normally vaccinated against anthrax and not expected to use advanced protective gear.
Six days later, when scientists started to dispose of the agar plates, they saw anthrax colonies growing on them, proving that some of the bacteria had survived.
The bacteria were from the lethal Ames strain. Research is often conducted with the much safer Sterne strain, which is used in animal vaccines. It can infect, but it cannot keep reproducing, because it does not have the outer coating that protects it from the animal’s immune system.
The dangerous Ames strain was used, Dr. Meechan said, because the test being developed was to detect dangerous ones.
Asked what lesson the agency would take from the accident, Dr. Meechan said: “It’s too early to say what exactly went wrong, but we will find what did and we will fix it. Our people are too important to us for this to happen.”