FBI agents decontaminate offices in Florida Credit: AP Photo/Steve Mitchell

As Nature Medicine went to press, the number of people in the United States who had tested positive for exposure to anthrax had risen to 10—one of whom has died. Anthrax spores have been sent to victims in the mail and appear as white powder. The events are almost certainly acts of bioterrorism.

Two victims in Florida developed inhalation anthrax, a rare but deadly form of the disease. The spinal fluid of the man who died was cloudy and contained a high titer of immune cells in addition to rectangular rods of bacilli. One woman and a baby in New York have developed cutaneous anthrax after the spores entered their bodies through lesions in the skin. They are being treated with antibiotics and are expected to survive. So far no cases of gastrointestinal anthrax, which is acquired by eating infected meat, have been reported.

Scientists hope genetic fingerprints from the anthrax samples will provide clues to the source, says Steven Block, a biophysicist at Stanford University and a member of JASON, a scientific group that advises the military on sensitive national security problems. There are over 1,000 strains of anthrax, and if the infections were caused by one of the more common strains, the genetic sample will not be useful in pinpointing a source.

Primarily a disease of hoofed animals, anthrax becomes deadly when dormant spores of the organism Bacillus anthracis enter the body. Once inside, the bacteria produce a three-part toxin comprising a protective antigen, a lethal factor and an edema factor. Symptoms develop as the three combine to penetrate and destroy cells. The antibiotic ciprofloxacin can stop the process, but only during the brief window between exposure and infection. Sales of ciprofloxacin, produced by Bayer, have risen sharply.

The US Food and Drug Administration (FDA) approved an anthrax vaccine in 1970, but its efficacy in humans is uncertain. At the present time the Michigan-based biotechnology company, Bioport, is licensed to make an anthrax vaccine but only supplies the military, and production of vaccine batches is presently delayed because the company has been cited by the FDA for quality control problems.