CASE-FATALITY BY ANTHRAX TYPE AND SOURCE OF EXPOSURE

Among the 21 anthrax cases where source of exposure is likely known (includes cases 5, 7 and 9 but excludes case  22), the spore-laden letters of September 18 were more associated with cutaneous than inhalational anthrax.  Conversely, among those who had contact with the October 9 letters, inhalational anthrax was more common.  This suggests that the spores were smaller and more easily aerosolized in the October 9 mailing as compared to the September 18 letters, allowing easier access to the lower lungs.  The resultant case-fatality once the spores reached the lungs, however, appears similar (1/2 = 50% vs 3/8 = 38%).    

For the likely September 18th mailing, the case-fatality was 9% (different from 0% for the known September 18 mailing).  Conversely for the likely October 9th mailing, the case-fatality was 30% (different from 20% for the known October 9 mailing).