Human cases of rabbit fever, or tularemia, have surged this year, and experts have not been able to determine why, according to a report released Friday by the U.S. Centers for Disease Control and Prevention.
There had been an average of 125 reported cases in the U.S. annually for the past 20 years. This year, however, there are already 100 reported cases in just four states -- Colorado, Nebraska, South Dakota and Wyoming -- as of the month of September.
One fatality, an 85-year-old man, was recorded, with 48 others being hospitalized. The infected patients' ages ranged from 10 months to 89 years.
From 2004 to 2014, there were only four cases reported in Colorado, but that number rose to 43 in 2015. A similar pattern was observed in Nebraska (from seven to 21 cases), South Dakota (from seven to 20 cases) and Wyoming (from two to 16 cases).
"This was something we noticed happening here in Nebraska, and when we contacted our colleagues in neighboring states, they were having similar experiences," Dr. Caitlin Pedati from the Nebraska Department of Health and Human Services and lead author of the CDC report told Live Science.
States bordering the affected regions have not reported a similar increase in rabbit fever cases.
Tularemia is a serious, yet rare, disease. It is caused by the bacterium Francisella tularensis. The disease affects rabbits, rodents and hares, but it gets transmitted to humans through bites of ticks and deer flies, touching infected animals, drinking contaminated water or inhaling contaminated aerosols.
The infection is not transmitted from human to human. It is life-threatening, but it can be cured by antibiotics, according to the CDC.
The agency reminded health care practitioners to "be aware of the elevated risk for tularemia" and issued warnings to residents in the affected areas.
"Residents and visitors to these areas should regularly use insect repellent, wear gloves when handling animals, and avoid mowing in areas where sick or dead animals have been reported," the CDC said.
The cause of the surge in tularemia cases is still unknown, but the agency is considering factors like increased rainfall, increased populations of rabbits and rodents and increased awareness of the disease.