The rise of human plaque caused by Yersinia pestis bacterium in the Southwestern United States may be due to climate change, according to a recent study.

National Science Foundation researchers found that human plague cases in New Mexico occurred more frequently after wetter than average winter-spring time periods from October to May.

Most common in New Mexico, Colorado, Arizona and California, plague was introduced to the United States by infected rats from Asia. People usually get plague from being bitten by a rodent flea carrying the plague bacterium or by handling an infected animal. If the person is not treated immediately with antibiotics, the disease can be fatal. Globally, the World Health Organization reports 1,000 to 3,000 cases of plague every year, with an average of six cases a year in the United States.

"This disease forms a very interesting model for mammal hosts and insect vectors that are influenced by climate change," said Robert Parmenter, lead author of the paper and director of the Long Term Ecological Research field research station in Sevilleta, New Mexico.

Parmenter looked at the 215 cases that have occurred in New Mexico since 1949. By examining the actual location of the cases in relation to rainfall in that area, he found that wetter winter/spring conditions caused significantly more cases of the virus. In fact, during years much wetter than normal, a 60 percent rise in the number of cases of human plague resulted.

Scientists blame global warming for recent outbreaks of infectious diseases such as malaria and encephalitis. Some warn that long-term global warming could increase the risk of contracting such diseases.

"The recent El Niño events of the 1990s have been associated with large increases in rodent populations in both North and South America," explains Scott Collins, Long Term Ecological Research program director at NSF. "These increases in rodent populations have been shown to correlate with numerous diseases, including Lyme disease and hantavirus pulmonary syndrome. Climate studies may provide a forecasting tool for identifying periods of increased risk of disease transmission."