Mathematical model may help health officials respond to anthrax attack
In order to protect the public from infection, healthcare providers and officials must know the likelihood of infection, how many cases to expect and what kind of drugs should be used to combat the outbreak, Infection Control Today reports.
The researchers used their model to estimate that, for an individual to have a 50 percent chance of becoming infected with anthrax--known as ID50--he or she would have to inhale 11,000 spores of the bacteria.
The inhalation of 1,700 spores would lead to a 10 percent chance of being infected, and an individual would have a one percent chance of infection by inhaling 160 spores. The median time for anthrax symptoms to appear is 9.9 days, and the optimal time to take antibiotics is 60 days.
"Anthrax is a well-studied disease and experimental animal data exist, but there is no real good information on dose response for the disease in humans," Adi V. Gundlapalli, an infectious diseases specialist and epidemiologist, an associate professor of internal medicine at the University of Utah School of Medicine and a staff physician at the Salt Lake City George E. Wahlen Department of Veterans Affairs Medical Center, said, according to Infection Control Today. "We don't want to be overly fearful, but we need to be prepared in the event of a bioterrorism attack with anthrax."
Animals studies usually involve vaccine testing and not exposure amounts for infection, making it difficult to glean information from accidental exposures in isolated cases. Gundlapalli and Toth combined both forms of data, including animal studies and information from a exposure at a Soviet Union bioterrorism plant that occurred in Sverdlovsk, Russia, in 1979.
Anthrax is found on the skin of dead animals, and its spores can live for thousands of years, infecting people who come into close contact with them, such as farmers who may deal with dead animals. Anthrax can also be manufactured in laboratories and spread in other says, such as through infected letters or aerosol.
"Our study, for the first time, takes all the best data and modeling techniques available on dose response and evaluates them critically," Toth said, Infection Control Today reports. "No one study satisfied all our criteria to be the best model, so we refined the available information to develop our model."
The Sverdlovsk exposure incident occurred when a filter was accidentally left off of a piece of equipment at a plant that was developing anthrax as a bioterrorism weapon. Spores were released into the air near the town of Sverdlovsk, leading to the deaths of up to 100 people. The Soviets eventually let outside experts study the accident, and scientists were able to plot where people were in the surrounding area when it occurred. They also looked at weather records to identify wind currents, using the information to plot how the spores were scattered in relation to people who became infected.
The timing and geographical pattern of the best-documented cases from Sverdlovsk were consistent with the shape of the dose-response curve and the distribution of incubation periods produced by the new model, which also sheds light on how long after an exposure antibiotics should be given in order to decrease the chances of infection.
"By combining the data from Sverdlovsk and prior studies, we can make defensible estimates on how scenarios might play out if anthrax were released in a terrorist attack," Gundlapalli said, according to Infection Control Today. "How many cases could we expect? When would be expect to see the cases? How long should we treat those exposed with preventive antibiotics? Our model provides real answers to help policy makers when they need that information."
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