Lyme disease is gradually spreading from the Northeast and becoming more common farther south and east, government researchers reported Wednesday.
A county-by-county look at the infections shows it's found in four times as many counties now as it was in 1993, a team from the Centers for Disease Control and Prevention found.
It's not clear why — experts say climate change, forest regrowth and the spread of deer might all be factors. What is clear is that many more people than before need to watch out for the ticks that carry the infection, CDC says.
"Over time, the number of counties identified as having high incidence of Lyme disease in the northeastern states increased more than 320 percent: from 43 (1993-1997) to 90 (1998-2002) to 130 (2003-2007) to 182 (2008-2012)," Kiersten Kugeler of the CDC's center in Forth Collins, Colorado, and colleagues write in their report.
The northern coast of New Jersey is no longer a hotbed of new Lyme infections, but now east-central Pennsylvania is, they said.
Lyme disease is caused by bacteria called Borrelia burgdorferi, carried by blacklegged ticks. It was first recognized in the Lyme, Connecticut, area in 1975 and it's spread from there to the northeast and to the mid-Atlantic and upper Northwest regions and elsewhere.
The CDC found high-risk counties in 17 states, including Connecticut, Massachusetts, New Hampshire, Maine, Vermont, Pennsylvania, Virginia, New York, Iowa, Michigan and Minnesota.
Infection causes fever, headache, fatigue, and a characteristic skin rash called erythema migrans. The infection can spread to joints, the heart and the nervous system if it's not treated. On rare occasions, it can kill.
More than 30,000 cases of Lyme disease are reported to CDC every year, making it the most commonly reported tick-borne illness in the United States, the CDC says. But some reports have suggested it is far more common than that maybe as many as 300,000 cases.
Kugeler's team came up with a new method to identify high-risk counties, and they used it to track the spread of Lyme since 1993.
"Our results show that geographic expansion of high-risk areas is ongoing, emphasizing the need to identify broadly implementable and acceptable public health interventions to prevent human Lyme disease," they write in the CDC journal Emerging Infectious Diseases.
The team found that Lyme isn't spreading much in Wisconsin, staying in the same cluster of northwestern counties,but didn't determine why.
"I think everyone agrees that climate change has something to do with it (the overall spread)," said Dr. Alan Barbour, who wasn't involved in the study but who studies tick-borne diseases at the University of California Irvine, and who wrote a book on Lyme disease.
Barbour says many of the affected areas also have seen a return of brushy and forested areas as small farms have gone out of operation and the land returned to a more natural state. That, in turn, allows the return of rodents, which carry the infection and spread it to ticks, and the deer who help the ticks complete their life cycle.
In some parts of the Southwest, Lyme is less commonly reported than it was before, Kugeler's team said. That's probably because a different tick-borne disease, southern tick-associated rash illness or STARI, was being mistaken for Lyme.
"Patients with this illness have rash similar to that of Lyme disease, but the condition is not caused by B. burgdorferi bacteria," the CDC team wrote.
Treatment of Lyme disease can be as simple as a single dose of the antibiotic doxycycline or it can involve a three-week to one-month course of antibiotics, depending on how long it takes to diagnose after infection. Infectious disease experts disagree on whether longer courses of treatment are helpful. Up to 20 percent of patients have long-term symptoms, the CDC says.
There used to be a vaccine, but its maker stopped manufacturing it because too few people asked for it. The CDC recommends using an insect repellent that contains DEET, careful checking for ticks after being outdoors, and staying out of bushy, wooded areas.