US authorities have increased to 84 people their count of government workers potentially exposed to live anthrax at three laboratories in Atlanta.
The US Centre for Disease Control and Prevention said the number of lab workers who may have been exposed rose from the 75 people first disclosed.
As of early Friday, 32 staff were taking the powerful antibiotic ciprofloxacin, or Cipro, and 20 were taking another antibiotic called doxycycline, CDC press officer Benjamin Haynes said in a statement.
In addition, as many as 27 people were getting the anthrax vaccine to prevent infection.
No illnesses have been reported, but the agency expects more people to step forward now that news of the anthrax scare is public. The agency did not give reasons why other workers were not taking medication.
The safety breach, which originated in the CDC’s bioterror lab, raised new concerns about the way laboratories around the world conduct research into the deadliest known pathogens, from anthrax to Ebola and avian flu.
The CDC has already faced repeated scrutiny over security lapses and mechanical malfunctions at some of its labs dating back to at least 2007.
The CDC said it will cede control of the investigation to the US Department of Agriculture “to avoid potential conflicts of interest”.
President Barack Obama was briefed on the matter on Friday by his homeland security and counterterrorism adviser, Lisa Monaco, the White House said.
The incident also is drawing scrutiny from Congress about whether the CDC has the appropriate safety procedures in place to protect federal employees from contamination.
Dr Paul Meechan, director of the CDC’s environmental health and safety compliance office, first disclosed the possible anthrax exposure on Thursday.
The agency discovered the lapse on June 13. It has since launched an internal investigation and is cooperating in a probe by the Federal Select Agent Program, which oversees the use and transfer of biological agents. It is also working with the FBI.
Live bacteria sent instead of samples
According to Dr Meechan, researchers in the CDC’s high-security Bioterror Rapid Response and Advanced Technology laboratory realised they had sent live anthrax bacteria, instead of what they thought were harmless samples, to fellow scientists in two lower-security labs at the agency.
The initial safety lapse occurred as scientists in the bioterror lab were trying out a new protocol for inactivating anthrax, using chemicals instead of radiation.
Scientists in the Bioterror Rapid Response unit had been preparing an especially dangerous Ames strain of the bacteria for use at the two lower-security CDC labs, the Biotechnology Core Facility and the Special Bacteriology Reference Laboratory, Dr Meechan said.
The strain had been used in a bioterror attack in the United States in 2001.
Those teams were experimenting with methods to more quickly identify anthrax in substances and powders sent to CDC from authorities across the country.
“If there was a bioterrorism incident, we could more quickly identify yes or no, this sample has anthrax,” Dr Meechan said.
Mr Meechan said the team in the bioterror lab used a new process to purify anthrax samples. To check their work, they took a sample of what they thought was dead bacteria and put it on a nutrient-rich lab dish called an agar plate to ensure that the bacteria wasn’t still live.
“They waited 24 hours,” Dr Meechan said.
“They took a look at the plate, and they didn’t see any new growth. At that point, they assumed the material was safe.”
‘Clearly something needs to be altered’
Researchers took the samples to the two lower-security CDC labs, which were developing the new tests. Their experiments did not work, and a week later, one of the labs asked for additional inactivated samples.
That is when researchers in the bioterror lab discovered that they had left the agar plates containing the bacteria in an incubator for an additional week, Dr Meechan said. As they were about to dispose of them, the researchers noticed growth on one of the plates, which turned out to be live anthrax.
At that time, the scientists realised the samples they sent to the two lower-security labs might have contained live anthrax bacteria. People working in those labs take fewer safety precautions and were unlikely to be wearing a respirator, putting them at higher risk for infection.
The team immediately pulled back the samples and contacted the staff members who had handled them.
CDC reached some of the lab workers that same night. Since then, they have been interviewing managers and using electronic surveillance and keycard data to identify anyone who might have been inside one of the two labs testing the samples.
Agency spokesman Tom Skinner said the internal probe is focusing on why safety protocols were not followed to the letter. An FBI official said there was no evidence of criminal activity.
“Clearly something needs to be altered,” Mr Skinner said.
“We are going to find out what went wrong and we are going to fix it. If that requires discipline, that will happen in due time.”
The Office of Inspector General in the US Department of Health and Human Services said it is also evaluating the incident, but gave no further details.
ABC/Wires