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The newspaper of The Johns Hopkins University August 4, 2003 | Vol. 32 No. 41
Sixty-Day Antibiotic Treatment Not Enough to Prevent Anthrax in Some Cases

By Tim Parsons
School of Public Health

The 60-day dose of antibiotics recommended by doctors to prevent anthrax may not be long enough in some cases, according to an analysis conducted by researchers at the Johns Hopkins Bloomberg School of Public Health.

Mathematical analysis shows that prophylaxis antibiotics may be needed for as long as four months for people exposed to high levels of anthrax spores. The new model also provides a valuable tool for estimating exposure doses in future outbreaks that can then be used to determine the optimum window for antibiotic therapy. The analysis appears in the July 28 issue of the Proceedings of the National Academy of Sciences.

"Anthrax spores are expelled from the lungs at a specific rate. Our model calculates the time it takes the body to expel these spores. A single spore left behind can germinate to cause disease," explained Ron Brookmeyer, lead author of the study and professor of biostatistics at the School of Public Health. "The model we developed shows that larger exposures to anthrax spores take longer to expel and require longer antibiotic therapy. Sixty days of antibiotics provides adequate protection in outbreaks with low exposure to spores. However, four months of antibiotics are required for outbreaks in which persons are exposed to large numbers of spores."

Brookmeyer and his colleagues tested their model using data from the 2001 anthrax attack on the United States and the 1979 anthrax outbreak near a biological weapons plant in Sverdlovsk, Russia. In both cases, the model's predictions were consistent with the actual illness rates.

"During the 2001 U.S. anthrax attacks, many people failed to take the recommended 60-day course of antibiotics. Full compliance was about 60 percent, or less in some cases. Our model showed that the reason they didn't get sick even without the antibiotics was because the anthrax spore exposure levels were very low. If the exposure levels were higher, there would have been more casualties," Brookmeyer said.

The study was funded by the National Institute of Allergy and Infectious Diseases.