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NEW YORK (CBSDFW.COM/AP) — Health officials are seeing an increase of a rare illness called rabbit fever that was beaten back decades ago.

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In the last two decades, health officials saw an average of only about 125 cases each year of the illness — known to doctors as tularemia. But there have already been 235 cases this year, the Centers for Disease Control and Prevention reported Thursday. That’s the most since 1984.

Officials aren’t sure why cases are up, but speculate that it may have to do with weather conditions that likely helped rodents — and the bacteria — thrive in certain states.

Researchers at The Institute of Environmental and Human Health at Texas Tech University have found evidence of the  tularemia bacteria in feral hogs here in Texas.

At least 100 of this year’s cases have been in four states — Colorado, Nebraska, South Dakota and Wyoming. Among those cases was an elderly man who died.

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Ticks and deer flies pick up the bacteria from rabbits and other small mammals and then spread it when they bite humans. People can also get it from handling dead animals or breathing in the bacteria.

Symptoms include sudden fever, headaches, muscle aches, joint pain and weakness. It is treatable with antibiotics.

The government still looks for cases because officials worry it potentially could be used as an airborne bioterrorism weapon.

Before 1940, there were as many as 2,200 cases each year.

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