Lyme disease, a mysterious illness carried by ticks common on the North Coast, is 10 times more prevalent than previously reported, according to a new federal study.

As many as 300,000 Americans are diagnosed with it every year, the Centers for Disease Control and Prevention announced Monday. Previous estimates had pegged Lyme disease infections at 20,000 to 30,000 a year, but experts had surmised that many cases went unreported.

"It's about time," said Thora Graves of Santa Rosa, who said she's been treated for Lyme disease for 26 years. "Now the truth is finally out."

Graves, an Oakmont resident, said she got the disease from a tick bite while hiking in the hills near Sonoma and has suffered from encephalitis, among other conditions, as a result.

Dr. Karen Holbrook, Sonoma County's deputy health officer, said the CDC announcement was an acknowledgment that Lyme disease "is a bigger problem than we thought it was."

"I see this as a positive development," she said, expressing hope that it will heighten public awareness of Lyme disease and boost research to improve diagnosis and treatment.

"There is a sense of frustration in the community," Holbrook said, referring to the woes of patients who go untreated or undiagnosed.

CDC's new estimate was based on a review of medical billing records of 22 million people, a survey of laboratories processing blood tests and a national patient survey.

"It's giving us a fuller picture and it's not a pleasing one," said Dr. Paul Mead, who oversees the agency's tracking on Lyme disease.

Lyme disease was recognized as a specific illness in Lyme, Conn., in 1975, and three years later the first human case in Sonoma County was confirmed, Holbrook said.

The county has averaged nine cases a year since 2005, she said. The North Coast, from Marin to Humboldt County, has a high rate of the disease compared to the rest of California.

Official surveillance figures for communicable diseases are sometimes "just the tip of the iceberg," Holbrook said.

The bacteria that causes Lyme disease in California is transmitted to people through bites by the western black-legged tick, also known as the deer tick.

A bulls-eye skin rash is the typical symptom of an infectious bite, and early treatment with antibiotics typically is effective.

But if it goes untreated, Lyme disease can result in serious conditions, such as joint pain, nerve pain and neurological problems like depression and anxiety, said Dr. Moses Goldberg, a naturopathic doctor in Santa Rosa.

The disease is difficult to diagnose and the blood test is unreliable, he said. Patients may be forced to pay for treatment out of pocket because health insurance typically will not cover an undiagnosed condition, Goldberg said.

The CDC's announcement, Goldberg said, should prompt more physicians "to look at their cases and see if Lyme disease is a consideration."

Meanwhile, Holbrook advised residents to learn about preventing the disease by checking the website at: www.sonoma-county.org/health/topics/lymedisease.asp.