Lyme disease is notoriously tough to diagnose. The symptoms often don't appear for one or two weeks after a bite and can vary from feeling flu-ish to longer-term neurological damage. And ticks seem to lie in wait throughout much of the U.S., prepared to pounce and infect a passerby.

Part of the difficulty in confirming the condition, which is caused by the bacterium Borrelia burgdorferi, is that its range has been unclear. Previous estimates were based on tick distribution and diagnostic reports. But as researchers have pointed out, reporting of tick populations can vary from county to county, and Lyme disease in humans is frequently under-, over- and misdiagnosed, skewing our understanding of its prevalence.

A new study skipped the human reports and went straight to the source: Ixodes scapularis (commonly known as blacklegged or deer ticks) nymphs infected with the pathogen. Teams of researchers and field workers swept through more than 300 sites in 37 states in the eastern half of the country, dragging large squares of corduroy during prime tick time—May through August—between 2004 and 2007 to see how many ticks would hop on. Collected ticks were tested for the bacterium. The results and a distribution map were published online Wednesday in the American Journal of Tropical Medicine and Hygiene.

"A better understanding of where Lyme disease is likely to be endemic is a significant factor in improving prevention, diagnosis and treatment," Maria Diuk-Wasser, an assistant professor at the Yale School of Public Health and co-author of the new study, said in a prepared statement. The areas of highest risk are southern Maine through Washington, D.C., as well as Minnesota and Wisconsin.

These findings should help doctors better evaluate a person's actual risk of having contracted Lyme disease, which is key to effective treatment and avoiding false positives. "Our sampling of tick populations at hundreds of sites suggests that any diagnosis of Lyme disease in most of the South should be put in serious doubt, unless it involves someone who has traveled to an area where the disease is common," Diuk-Wasser said. Folks in the South might be more likely to encounter the lone star tick (Amblyomma americanum), whose bite can be easy to confuse with an infected deer tick bite because it can also cause a rash that resembles the telltale Lyme disease "bull's eye." Such cases could result in mis-diagnosis with Lyme disease.

Lab tests are more likely to produce false-positive results in areas where Lyme disease is not endemic, the researchers noted. And with some 2.7 million tests for Lyme disease in the U.S. a year, false positives are a concern for patient safety. They also muddy the overall picture of where Lyme occurs nation-wide. Doctors "may act too aggressively and prescribe unneeded and potentially dangerous treatments if they incorrectly believe their patient was exposed to the pathogen," Diuk-Wasser said.

The new survey did not cover other parts of the country, such as the West Coast, where another species of tick, the western blacklegged tick (Ixodes pacificus), is known to carry Lyme disease. And as the ticks move with deer populations—which are, themselves, influenced by changes in land use and climate—the maps should be updated. Early evidence suggests that there are newer centers of Lyme disease in Illinois and Indiana as well as Michigan and even North Dakota.

The good news for those who live in endemic areas is that it takes more than a day post-attachment for a tick to infect a human, according to the U.S. Centers for Disease Control and Prevention. That makes regular tick checks a good preventive measure. And it doesn't have to be a solitary chore. As country singer Brad Paisley notes, "I'd like to walk you through a field of wildflowers, and I'd like to check you for ticks."