Mathematical model suggests six anthrax letters spread spores to 5,000 more

May 28, 2002 (CIDRAP News) – A mathematical model developed by two researchers suggests that six letters used in last fall's anthrax attacks spread anthrax spores to more than 5,000 other pieces of mail and led to the deaths of two women who had no known exposure to the pathogen.

The model suggests that a larger anthrax attack—one starting with 100 letters—could cross-contaminate millions of pieces of mail and lead to thousands of cases of illness, according to the report by Glenn F. Webb of Vanderbilt University in Nashville and Martin J. Blaser of New York University and the Veterans Administration Medical Center in New York City. Their report was published recently in the Proceedings of the National Academy of Sciences.

Webb and Blaser's model starts with the assumptions that six contaminated letters were mailed and that each contained more than 10 billion (1010) anthrax spores. Each letter is assumed to have passed through five postal "nodes," including mailboxes, local post offices, and regional processing centers, where cross-contamination occurred. Further, each cross-contaminated letter spread anthrax spores to other letters as it passed through the nodes, but the number of spores decreased with the distance from the original letters. For example, a "second-generation" letter (where a first-generation letter is one that came in contact with one of the original letters) would be likely to have only 10 to 100 spores.

In addition, the researchers assumed that older recipients of contaminated mail were more likely to get sick. They assumed that 35% of recipients were between ages 25 and 44, 35% were between 45 and 65, and 25% were older than 65.

The model suggests that 5,100 letters were contaminated with between 10 and 100 anthrax spores when they reached their destinations. Another 432 letters carried 100 to 1,000 spores, and 36 carried 1,000 to 10,000, the researchers estimate. "The model demonstrates that the observed number of cases in mail recipients is consistent with the cross-contamination of thousands of piece of mail, most with very low numbers of spores," probably fewer than 1,000 per letter, the report states.

Further, the article states, "The model predicts that a larger scale attack could result in millions of contaminated letters and thousands of infections, which could necessitate the complete suspension of the U.S. postal system. The model provides a means to quickly assess critical steps and magnitude of risks if such an attack occurs."

Given this prediction, Webb and Blaser suggest that the Postal Service should identify and curtail practices that can cause the airborne spread of letter-borne infectious particles in postal facilities. They also suggest that anthrax vaccination may be desirable for all postal workers. Periodic serologic screening of postal workers may be useful to determine the extent of contamination from last year's attack and to provide baseline data in case of future attacks, they add.

The authors state that though the two deaths that apparently resulted from cross-contamination in the 2001 attack were in older people, it is not safe to assume that a larger attack would affect only older people. "If the assumptions of the model are correct, approximately one-third of the cases would be in the middle age range in a larger scale attack," they say.

Webb and Blaser acknowledge that their model does not prove that the two unexplained cases of anthrax were caused by cross-contaminated mail. Another possible explanation is that the two victims simply inhaled windborne spores, because wind patterns on Oct 9, the day two anthrax-laden letters were processed in Trenton, N.J., were such that spores could have been blown from there to the victims in New York City and Connecticut.

Webb GF, Blaser MJ. Mailborne transmission of anthrax: modeling and implications. Proc Nat Acad Sci 2002;99(10):7027-32

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