Twenty-two persons developed anthrax infection after coming in contact with Bacillus anthracis spores from a bioterrorism act in October and November of 2001. Inhalational anthrax, the most serious mode of infection, occurred in 11 of these cases. Patients became critically ill; five subsequently died. Kuehnert and colleagues at the Centers for Disease Control and Prevention collected clinical data from these cases and present a description of features that may help distinguish this rapidly lethal disease from other routine respiratory infections.

Inhalational anthrax appears to be a biphasic illness. The initial infection is marked by nonspecific symptoms similar to influenza, which then is followed by rapid progression to severe pneumonia and septic shock. The investigators compared clinical data from the 11 inhalational anthrax cases with 2,203 patients with influenza-like illness (ILI) who participated in a recent trial of a drug for influenza treatment. They also compared anthrax cases with 784 persons with community-acquired pneumonia (CAP) from the North American Pneumonia Etiology Study. Only patients 40 years of age and older were used in the comparison studies to match the age distribution of the 11 anthrax cases.

After various statistical modeling and data analysis methods, the authors developed a scoring system to differentiate inhalational anthrax from ILI and CAP. A score of four or higher for the ILI system would have correctly identified all 11 anthrax cases and excluded 96 percent of those with routine ILI. For the pneumonia phase of the illness, a score of three or higher correctly included nine of the 11 anthrax cases and excluded 81 percent of routine CAP cases.

The authors conclude that there were similarities in clinical signs, symptoms, and laboratory markers among persons with inhalational anthrax that may help distinguish this illness from other routine respiratory infections.