As many as 75 scientists and staff in US government laboratories in Atlanta may have been exposed to live anthrax bacteria after researchers failed to follow safety procedures, prompting a federal investigation.
Researchers working in a high-security bio-terror response laboratory at the US Centres for Disease Control and Prevention (CDC) were preparing what they thought were inactivated samples of the deadly organism, the organisation said on Thursday.
However, the bacteria may still have been infectious when the samples were transferred to lower-security areas not equipped to handle live anthrax.
Two of the three laboratories conducted research that may have aerosolised the spores, the CDC said. The agency first detected the exposure on June 13, when live bacteria were found on the original slides used by scientists. Environmental sampling was done and the areas remain closed for decontamination.
"No employee has shown any symptoms of anthrax illness," said Dr Paul Meechan, director of the environmental health and safety compliance office at the CDC.
"This should not have happened," he said. For those exposed, "we're taking care of it. We will not let our people be at risk."
Meechan said CDC workers in the lower-security areas were not likely to be wearing masks. Around 75 people are being offered a 60-day course of treatment with an antibiotic and an anthrax vaccination.
The safety breach raises new doubts about security measures at the CDC, whose infection-control protocols are held up as a model to the world.
The FBI was helping to investigate the incident, but had no evidence of foul play, a spokesman said.
"There is no room for error or negligence when it comes to bioterror research and every precaution must be taken to ensure the safety of our scientists," federal legislator and energy and commerce committee chairman Fred Upton said.
Meechan said the CDC was holding an internal investigation and that disciplinary measures would be taken if warranted. He said there was no public risk.
The normal incubation pe-riod for anthrax can last up to seven days, although there were documented cases of the illness occurring 60 days after exposure, Meechan said. Infection can occur through a cut in the skin, breathing in anthrax spores or eating tainted meat.
Of the three, the biggest threat is inhalation anthrax, in which bacterial spores enter the lungs. Once they germinate, they release toxins that can cause internal bleeding, swelling and tissue death.
Inhalation anthrax occurs in two stages. In the first stage, symptoms resemble a cold or the flu. In the second stage, anthrax causes fever, severe shortness of breath and shock.
About 90 per cent of people with second-stage inhalation anthrax die, even after antibiotic treatment.
Meechan said it was too early to determine whether the transfer of live bacteria was accidental or intentional. All employees who were doing procedures to inactivate the bacteria had passed a security check, he said.