The number of human tularemia cases in 2015 has increased significantly in Colorado, Nebraska, South Dakota and Wyoming, but the cause remains unclear.

“Our report seeks to inform providers about this increase in tularemia cases, especially in these four states, in order to ensure that it is being recognized, reported, and treated appropriately,” said Dr. Caitlin Pedati from the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia.

“We would like providers to keep tularemia on their differential for patients who present with compatible signs and symptoms so that we can continue to gather valuable information and move toward a better understanding of what might be contributing to this trend,” she told Reuters Health by email.

Over the past two decades, the number of cases of tularemia reported in the U.S. has hovered around 125, but as of September, 2015, 100 cases have been reported among residents of Colorado (43 cases), Nebraska (21 cases), South Dakota (20 cases), and Wyoming (16 cases), Dr. Pedati’s team reports.

These represent increases over the mean annual number of cases between 2004 and 2014 of 975 percent (Colorado), 200 percent (Nebraska), 186 percent (South Dakota), and 70 percent (Wyoming), the researchers write in the December 4 Morbidity and Mortality Weekly Report (MMWR).

Forty-eight individuals required hospitalization, and one person (an 85-year-old man) died.

Possible exposures in these individuals included animal contact, environmental aerosolizing activities, and arthropod bites.

Francisella tularensis, the causative gram-negative coccobacillus, can be isolated in culture or detected by polymerase chain reaction (PCR) from patients’ blood. The drug of choice for treatment is streptomycin, which can be difficult to obtain and is associated with frequent ototoxicity. Alternatives include gentamicin, doxycycline, and ciprofloxacin.

Although the cause for the increased incidence of tularemia is unclear, possible explanations could include increased rainfall promoting vegetation growth, pathogen survival, and increased rodent and rabbit populations, as well as increased awareness and testing since tularemia was reinstated as a nationally notifiable disease in January 2000.

“Traditionally it can be complicated to pinpoint a single cause for something like tularemia that has several routes of exposure and can present with slightly non-specific symptoms such as cough, fever, or lymph node swelling,” Dr. Pedati said. “The state health departments and CDC are continuing to investigate this increase and are working together to gather as much information about these cases as possible.”

She concluded, “We are hoping that by sharing this report with providers, they will keep tularemia in mind as a diagnosis, and we can gather more clinical, laboratory, and epidemiological information to help clarify what might be going on.”