An unusually high number of cases of plague have been reported in the United States this year. According to the Centers for Disease Control and Prevention, 11 people have been infected in six states since April 1.
Four cases have been reported in Colorado, two each in Arizona and New Mexico, and one each in California, Georgia, and Oregon. The Georgia and California cases both involved people who had visited Yosemite National Park this summer.
The average from 1970 to 2012 was seven cases per year. Ten cases were reported in 2014, and four cases were reported in both 2012 and 2013. Seventeen cases were reported in 2006. That was the last time more than 10 cases were reported in a single year.
Plague is endemic to rural and semirural areas in the western United States, although it is rare. The disease is most common from the spring through the fall. It is unclear why more cases have been reported this year.
Plague is caused by the bacterium Yersinis pestis, which is carried by rodents and fleas. People most commonly contract the disease after being bitten by fleas, but they can also get plague by coming in contact with infected animals or their tissue or fluids.
If it is not treated, plague kills 60 to 90 percent of people. If it is treated with antibiotics, the death rate falls to 16 percent. Three of the 11 plague patients this year have died.
The drought in California may be one reason for the increase in cases of plague there. If the drought is causing less food to be available for rodents, they could be searching for food in campgrounds and coming into contact with humans.
To reduce the risk of contracting plague, the CDC recommends that people who participate in outdoor activities where the disease is known to occur wear long pants, use insect repellent, and avoid contact with sick or dead animals. People should not feed squirrels, chipmunks, or other rodents.