Dr. Edward Cox of the Food and Drug Administration's Center for Drug Evaluation and Research said plague is extremely rare in most parts of the world -- including the United States -- with 1,000 to 2,000 cases worldwide each year.
The three most common forms of plague are bubonic plague, or infection of the lymph nodes; pneumonic plague, or infection of the lungs; and septicemic plague, or infection of the blood, Cox said.
Although primarily an animal disease, plague can be spread to humans via infected flea bites, contact with infected animals or humans, or laboratory exposure. However, it can also be a biological threat agent, officials said.
Because plague is such a rare disease, it would not be possible to conduct adequate efficacy trials in humans, so Levaquin's approval was based on an efficacy study conducted in African green monkeys infected with the plague bacterium a laboratory, Cox explained.
Animals were randomly selected to receive a 10-day regimen of Levaquin or placebo within 6 hours of the onset of fever after being infected.
The trial found that of the 17 monkeys treated with Levaquin, 94 percent survived, while one of the seven monkeys treated with the placebo survived.