By David Ruppe

Global Security Newswire

The U.S. Centers for Disease Control and Prevention is uncertain about what level of exposure to inhaled anthrax constitutes a fatal risk to humans, an official said Wednesday.

She assured, however, that very low, or “trace” levels of anthrax detected in an environment would not warrant administering antibiotics to people in that environment, saying there is no epidemiological or scientific evidence the anthrax would pose a risk.

“Right now, because we don’t have a threshold, if we see anything beyond trace contamination, we’re erring on the side of decontaminating the environment and assessing for aerosol risk and treating accordingly,” said Julie Gerberding, the acting deputy director of the National Center for Infectious Diseases, during a teleconference with reporters Wednesday.

Gerberding said antibiotics are not recommended if: there is no case of inhalation anthrax; no powdered anthrax that could aerosolize was in the area; or human and environmental testing finds “trace” contamination, for instance when “we only see light [petri dish] growth when we do a sample.”

“We would not recommend people in those kinds of environments to receive the 60-day regimen because we have no epidemiological or scientific evidence that that particular finding poses a risk,” she said.

Insufficient Testing

Gerberding’s comments come as some scientists suggest authorities may be underestimating the dangers posed by exposure to a small number of anthrax spores when deciding whether to administer antibiotics to potentially exposed people.

CDC Director Jeffrey Koplan told reporters Monday that inhalation anthrax requires both aerosolization and “some reasonable dose of spores, thought to be many thousand.”

Gerberding said the centers are working with experts on risk assessment and evaluation of environmental safety “to try to see if we can provide any more specificity around these estimates.”

The conventional wisdom has been that a lethal dose of around 8,000-10,000 spores would cause fatalities in 50 percent of a large group of people exposed to that many spores.

That estimate, however, was based on a study performed by the United States in the 1950s on a single species of monkey, cynomolgus, using a single type of anthrax.

“As far as I know, there are simply no reliable figures on this for human beings. So we simply don’t know how susceptible human beings are,” said microbiologist Mark Wheelis of the University of California-Davis.

Calling that estimate’s usefulness into question, other experiments with cynomolgus and other species of monkeys, have found that as few as 2,000 and as many as 50,000 spores were needed to kill 50 percent of those exposed, according to a recent analysis of the issue by Harvard Professor Matthew Meselson.

Numerous variables in the conditions of the test, such as spore size or monkey type and age, could explain the wide range results, he noted.

Similarly, many factors would impact the fatal dosage for humans, experts say. They include the type of anthrax strain, the method of spore preparation and storage, the aerosol suspension liquid and aerosol particle size distribution, whether the person is a nose or mouth breather and the person’s health and age.

Meselson also observed that although the inhalation of a smaller number of spores would likely reduce the probability of a fatal infection, it would not eliminate it.

“For both experimental and theoretical reasons the assumption of a threshold is untenable. It should be presumed that even one spore can initiate infection, albeit with very low probability,” Meselson wrote. “Understanding this is essential for the design of sound policy and procedures.”

Balancing Considerations

U.S. health authorities have expressed concern that the widespread administration of antibiotics could result in an antibiotics-resistant strain of anthrax to emerge.

A lower threshold for serious infection, experts say, could mean a greater probability of cross- or secondary-contamination, as is suspected to have occurred with the letter sent to U.S. Senator Tom Daschle (D-S.D.) in September, possibly exposing several Washington mail handlers. That would suggest more widespread antibiotics use.

“You’ve got to recognize it’s a practical issue,” says Wheelis. “The issue is where do you draw the line. It’s human beings making these decisions and they’re making them on the basis of a very complex calculus that involves sociological factors as well as medical factors.”

Another factor, he notes, is the side effects caused by antibiotics. “There’s always going to be harm caused by the treatment itself. So you have to balance the harm that the treatment is going to cause versus the good in preventing the disease.”

Lynn Klotz, a biotechnology consultant in Somerville, Mass., recently questioned the assumption that widespread treatment of persons suspected of secondary anthrax exposure would not necessarily be excessive.

In an e-mail posted to a discussion group, for instance, he roughly calculated that the amount of antibiotics widely provided for preventive treatment to1,000 employees of 50 facilities nationwide would be trivial compared to the estimated 5 million penicillin treatments administered annually.

32,000 treated

As evidence of suspected cross contamination has been uncovered, authorities have become more liberal in their administration of antibiotics, experts say.

The CDC currently recommends administering antibiotics if an inhalation case is identified at a facility, or if environmental testing finds anthrax along the path that a known anthrax-tainted latter has traveled, and for persons exposed to an air space known to be contaminated with the aerosolized bacteria.

The CDC also recommends preventive antibiotics for workers, such as mail handlers, whose jobs may have put them in contact with the spores.

Since Sept. 11, approximately 300 postal and other facilities have been tested for anthrax spores and approximately 32,000 people have begun preventive antibiotic treatment following potential exposure to at workplaces in Florida, Washington, New Jersey and New York City, the CDC reported Wednesday.

Many were taken off the antibiotics early, after it was determined they were not at risk of infection.

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