More than a quarter of workers at the Centers for Disease Control and Prevention in Atlanta — and more than a third of its medical officers — will be eligible to retire in the next five years. Replacing them will be difficult because of a national shortage of public health professionals, according to a new report by the Government Accountability Office.

In addition, the agency has dramatically increased the number of people it employs from outside contracting firms — and may not be adequately ensuring their proper use, the GAO auditors said in a report scheduled to be released today.

"GAO's report presents a disturbing picture of the CDC's work force, which needs a stronger foundation to meet global public health threats such as pandemic influenza, food-borne illnesses and other infectious diseases," said Sen. Daniel Akaka (D-Hawaii), chairman of a Senate subcommittee on management of the federal work force.

CDC spokesman Tom Skinner said the work force changes at the CDC are not unusual and reflect contracting trends at other federal agencies. The CDC's mission has expanded, he said, with the increased threat of bioterrorism and emerging global diseases.

"We're being asked to do more with less, especially when it comes to having resources to hire full-time employees," he said.

The CDC has 9,000 employees and 5,000 contract workers; about two-thirds of the blended work force is in Atlanta, where the agency is headquartered.

Between fiscal years 2000 and 2006, the number of contractors working at the CDC increased 139 percent, compared with a 3.5 percent increase in the CDC's staff of federal workers, the GAO report said. The GAO is the investigative arm of Congress.

The GAO said the CDC is doing a better job planning for the future of its work force, but that the agency's strategic plan doesn't address the challenges posed by one-third of its workers being contractors.

Oversight of contractors is "critical," the GAO wrote, to ensure they are not improperly influencing or directing the CDC's mission.

Federal regulations prohibit contractors from performing "inherently governmental functions," such as determining agency policies and priorities, directing federal employees and awarding or administering contracts.

In some branches of the CDC, contractors outnumber federal employees, the report said.

According to a GAO analysis of CDC data: In the Office of the Chief Operating Officer, responsible for overall operations at the agency, 61 percent of 3,152 workers are contractors.

In the Office of the Chief Science Officer, responsible for providing scientific vision and leadership, 67 percent of 114 workers are contractors.

Skinner said these percentages could be misinterpreted because they probably reflect where the contract originated but not where the contract employees are physically assigned to work. In the report, CDC officials disputed that they don't provide adequate oversight of contractors. They said they watchdog contracts but note they are prohibited from directly supervising contract employees, who must be managed by their companies.

The CDC will be providing training to managers and updating its employee database to better reflect where contractors work and what they're doing.

"We really value the work that's done by our contractors," Skinner said. "They're a vitally important part of CDC's ability to accomplish its mission."

Sen. George Voinovich of Ohio, the ranking Republican on the Senate federal work force subcommittee, said:

"Going forward, CDC must place greater emphasis on recruiting the next generation of scientists and ... improving the management of its contractor work force to ensure our nation is prepared to meet current and future threats."

To reach staff writer Alison Young, call 404-526-7372.