5,000 Letters Laced With Anthrax?
Math model estimates cross-contamination in mail attacks

May 13, 2002
By Maggie Fox

As many as 5,000 letters may have been cross-contaminated with spores from anthrax-laced letters sent in attacks late last year, experts said Monday.
          
       THAT cross-contamination can explain the deaths of two women, they said in releasing a mathematical model showing how spores from letters sent to news organizations and U.S. senators last year may have transferred onto other pieces of mail and infected the pair.
       “If there were ever another outbreak this could help us more rapidly control it and maybe more quickly identify where it is coming from,” Dr. Martin Blaser, a professor of medicine and microbiology at the New York University School of Medicine who helped write the study, said in a telephone interview.
       The letters are believed to have infected at least 18 people and to have killed five of them. Another four cases of suspected skin, or cutaneous, anthrax infection have not been confirmed.
       More than 10,000 people including postal workers and Capitol Hill staffers took antibiotics to prevent infection in case they were exposed, and Blaser said this step saved lives.
       The greatest danger from the attacks was probably not to the people the letters were addressed to, but to postal workers who handled the mail and to people who got cross-contaminated mail, Blaser said in a study published in the Proceedings of the National Academy of Sciences.
       Blaser and Glenn Webb, a professor of mathematics at Vanderbilt University in Nashville, Tennessee, came up with the model after the letter attacks in October.
       Those who died were a newspaper photo editor in Florida, two Washington postal workers, a woman in Connecticut and a woman in New York. No one has been arrested.
       
SECOND DEATH HELD KEY
       The U.S. Centers for Disease Control and Prevention has assumed that mail cross-contamination was to blame for the deaths of the two women, but Blaser, an adviser to former New York mayor Rudolph Giuliani on bioterrorism even before the attacks, said it was not clear at first. He said he figured it out when the second woman died in November.
       “Then I called up Glenn Webb, with whom I had been working for several years,” Blaser said.
       Blaser and Webb developed a mathematical model based on the number of anthrax letters and how many “nodes” — such as a mailbox or a sorting center — they passed through.
       “As the contaminated letters travel through the nodes, there is a probability that they may transfer spores to other letters, and thus proliferate the number of contaminated letters in successive generations,” they wrote.
       For instance, letters addressed to U.S. senators contaminated a post office in New Jersey, affecting postal workers there.
       “In a less than one-minute period, 300 letters passed through the same sorting machines as each of those two original contaminated letters. These 600 letters (and more) were most likely first-generation cross-contaminated letters,” they wrote.
       “One was found to have gone to a recipient in Seymour, Connecticut, who lived about a mile from the 94-year-old woman in Oxford, Connecticut who died of inhalational anthrax on November 21.”
       She was probably infected by a “second-generation” cross-contaminated letter, they said.
       “There was also an issue early on about how many spores did it take to infect a person,” Blaser said. “Our model presumes that there is no one magic number.”
       Noting the two women who died were elderly, Blaser said older people in general have weaker immune systems and are probably more susceptible to inhaled anthrax.
       “The rapid and widespread usage of antibiotics among postal workers and persons in the immediate environment if the received original letters probably averted a substantial number of cases,” they wrote.
       It may be worth vaccinating postal workers, they added.
       “If, God forbid, there were another outbreak and cases started turning up in different localities, the model could be used to work backward to identify the groups of people who need to receive antibiotics or vaccine,” Blaser said.
       Last March a team at the Johns Hopkins University School of Public Health in Baltimore did calculations that showed at least nine more cases of inhaled anthrax infection would have occurred if people had not been given antibiotics.

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