As many as 300,000 Americans are actually diagnosed with Lyme disease each year, the Centers for Disease Control and Prevention announced.
Usually, only 20,000 to 30,000 illnesses are reported each year. For many years, CDC officials have known that many doctors don't report every case and that the true count was probably much higher.
The new figure is the CDC's most comprehensive attempt at a better estimate. The number comes from a survey of seven national laboratories, a national patient survey and a review of insurance information.
"It's giving us a fuller picture and it's not a pleasing one," said Dr. Paul Mead, who oversees the agency's tracking of Lyme disease.
The ailment is named after Lyme, Conn., where the illness was first identified in 1975. It's a bacteria transmitted through the bites of infected deer ticks, which can be about the size of a poppy seed.
Symptoms include a fever, headache and fatigue and sometimes a telltale rash that looks like a bull's-eye centered on the tick bite. Most people recover with antibiotics. If left untreated, the infection can cause arthritis and more severe problems.
In the U.S., the majority of Lyme disease reports have come from 13 states: Connecticut, Delaware, Maine, Maryland, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Vermont, Virginia and Wisconsin.
The new study did not find anything to suggest the disease is more geographically widespread, Mead said.
Online CDC report: http://www.cdc.gov/lyme/