As many as 75 scientists working in US government laboratories in Atlanta may have been exposed to live anthrax bacteria, and are being offered treatment to prevent infection from the deadly organism, the US Centers for Disease Control and Prevention (CDC) says.
The potential exposure occurred after researchers working in a high-level biosecurity laboratory at the agency's Atlanta campus failed to follow proper procedures to inactivate the bacteria. They then transferred the samples, which may have contained live bacteria, to lower-security labs not equipped to handle live anthrax.
Two of the three labs conducted research that may have aerosolised the spores, the CDC said on Thursday.
Environmental sampling was done and the lab areas have been closed until decontamination is complete.
Dr Paul Meechan, director of the environmental health and safety compliance office at the CDC, said the agency discovered the potential exposure on June 13 and immediately began contacting individuals working in the labs who may have unknowingly handled live anthrax bacteria.
"No employee has shown any symptoms of anthrax illness," Meechan told Reuters news agency.
Meechan said the CDC was conducting an internal investigation to discover how the exposure occurred and said disciplinary measures would be taken if warranted.
The normal incubation period for anthrax can take up to five to seven days, though there are documented cases of the illness occurring about 60 days after exposure, Meechan said.
As many as seven researchers may have come into direct contact with the live anthrax, he said. But the agency is
casting as wide a net as possible to make sure all employees at the agency who may have walked into any of the labs at risk are being offered treatment.
Around 75 people are being offered a 60-day course of treatment with the antibiotic ciprofloxacin as well as an injection with an anthrax vaccine.
Meechan said it was too early to determine whether the transfer was accidental or intentional. He said that all employees who were doing procedures to inactivate the bacteria were working in a biosecurity laboratory and had passed a security check.
The CDC said in a statement it has reported the lab-safety incident to the Federal Select Agent Program, which oversees the use and transfer of biological agents and toxins that pose a severe threat to the public.
The FBI is working with the US Centers for Disease Control and Prevention to examine government scientists' possible exposure to anthrax at a lab in Atlanta, but has found no evidence of wrongdoing, a spokesman for the agency said on Thursday.
"We're aware of it and working with CDC," FBI spokesman Paul Bresson said in an email. "Nothing leads us to believe it's anything criminal at this point."
Anthrax is a potentially deadly infectious disease caused by exposure to the bacterium Bacillus anthracis. The bacteria most commonly affect hoofed animals such as goats, but people can also become infected.
Infection can occur through a cut in the skin, breathing in anthrax spores or eating tainted meat.
Meechan said CDC workers in the lower-security labs were likely not wearing masks.
With anthrax, the biggest threat is inhalation anthrax, in which bacterial spores enter the lungs where they germinate before actually causing disease, a process that can take one to six days. Once they germinate, they release toxins that can cause internal bleeding, swelling and tissue death.
About 90 percent of people with second-stage inhalation anthrax die, even after antibiotic treatment.