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Doctors have reported 15 cases of Lyme disease in Montana this year - a sharp spike from prior years. But health officials say most appear to be recently diagnosed older illnesses and that all likely originated outside the state.

The disease is passed to humans by ticks and can cause headaches, fever, fatigue and a distinctive, bulls-eye shaped rash. The overwhelming majority of cases occur on the East Coast and in the Great Lakes states.

The 15 cases of Lyme reported so far this year to the Montana Department of Health and Human Services compares with four cases each in 2007 and 2008. Only one case this year was reported as a recent infection; the others were thought to be infections from two to 20 years old that had previously escaped detection.

Lyme disease is a contentious medical issue, with sharp disagreements among health professionals over the reliability of tests used to diagnose the illness.

Health officials said that if the latest reports are accurate, the increase in cases could be simply a result of more tests being administered in Montana, where the disease has not been given much attention in the past.

At least eight of the reports this year have come from a single Bozeman doctor's practice, Gallatin Valley Natural Medicine, said Health and Human Services Department disease specialist Elton Mosher. He said at least three doctors outside the Bozeman practice also have reported cases, including one that came from the VA medical clinic in Billings.

But Mosher said the tick species that carries Lyme disease has never been documented in Montana, including during an extensive 2004 study. Most of the people recently diagnosed, he said, likely contracted the disease during a visit to Connecticut or some other place where the disease is prevalent.

"I can't stress that enough," Mosher said. "These are older cases. I don't want the public to think they're going to go outside and get Lyme disease when there's far greater risks out there for them."

Mosher said of greater concern was a spike in Rocky Mountain Spotted Fever - another tick-borne illness with sometimes similar symptoms. He said there had been nine cases of Rocky Mountain Spotted Fever this year, versus zero over the last two years.

Robin Thomson, a naturopathic physician at Gallatin Valley Natural Medicine, said in an interview she had found more than a dozen cases of Lyme since coming to Montana from Minnesota earlier this year. She said a lack of awareness about Lyme disease in Montana is causing missed diagnoses and leading to patients getting the wrong treatment.

"People still tell me Lyme disease doesn't exist in Montana," she said. "Wrong. Very, very wrong. A lot of people are walking around very, very sick."

Still, that doesn't mean the disease necessarily originated in the state, said Dr. Paul Mead, medical officer in the infectious disease division of the Centers for Disease Control and Prevention.

Mead, who is based in Fort Collins, Colo., said Lyme disease is found in only a few small pockets in the West, including parts of Washington, California and Oregon. He said there has never been a documented case of the disease being contracted in Montana.

"We are aware of the occasional bona fide cases in Montana," he said. "However, those have all been associated with travel to a recognized endemic state" where the ticks that carry Lyme disease live.

CDC-recommended steps to prevent any tick borne illness include using insect repellent, removing ticks promptly and consulting with a doctor following a tick bite.