The CDC is providing the list in the February issue of its monthly journal, Emerging Infectious Diseases.  Intended to guide domestic preparedness efforts, the list presents a systematic assessment of the potentially most devastating agents for the United States.  The article is meant to refocus attention on analysis first prepared in 1999 by a collection of U.S. medical experts.

"Identifying these priority agents will help facilitate coordinating planning efforts among federal agencies, state and local emergency response and public health agencies, and the medical community," the article's authors said.

According to the analysis, the agents generally share a combination of characteristics that make them of particular concern, including:

• They have the greatest potential for mass casualties;
•   They have a moderate to high potential for delivery to large populations, possibly because they are infectious;
• They could prompt mass disruption and fear because of public perception of the particular agent; and
• They require the most special public health preparedness, including treatment stockpiles, enhanced surveillance or diagnosis.

Smallpox was found to pose the greatest threat, followed by anthrax, plague, botulism, Tularemia and viral hemorrhagic fevers such as Ebola and Marburg.  Smallpox was found to be decisively the most contagious, followed by plague and viral hemorrhagic fevers.  Anthrax had the greatest potential for mass production and dissemination to a large population.

The CDC list is in agreement with the focus of study at the Center for Civilian Biodefense Studies at Johns Hopkins University in Baltimore, Maryland.

"Historically, these are some of the agents that nations have used to develop biological weapons," said center spokesman Tim Parsons.

Jonathan Tucker, a bioterrorism expert at the Monterey Institute of International Studies, said that the list can be misleading about the dangers, because it does not evaluate the likelihood of the agents being available to and used by terrorists. 

"Anthrax is the most likely threat, because it is accessible, is a widely dispersed disease in livestock, is very easy to weaponize and can be delivered through the air with the appropriate dissemination technology," he said.  Some of the other agents, such as plague and Tularemia, are much more difficult to weaponize because "they tend to die off rapidly," he said.

The smallpox virus, Tucker said, is known to exist in only a few laboratories, and independent terror groups, such as al-Qaeda, are not believed to have it.  But it warrants attention because of the potential for massive consequences, he said.

The list also does not address genetically engineered agents, because of the difficulty in predicting the nature of those agents.  The analysis includes a secondary category of agents which have "some potential for large-scale dissemination with resultant illness, but generally cause less illness and death and therefore would be expected to have lower medical and public health impact."  These include Q fever, Brucellosis, Glanders, Melioidosis, Encephalitis, Typhus fever, toxic syndromes, Psittacosis and food and water safety threats.