The blacklegged ticks that transmit Lyme disease are now established in twice as many counties as the last nationwide survey showed, back in 1998.

Now, almost half of U.S. counties have reported the ticks, according to a Centers for Disease Control and Prevention survey released today.

The numbers correlate with an upsurge in Lyme disease: in 2013, the CDC announced that the disease was probably 10 times as prevalent as previous estimates, affecting about 300,000 Americans per year.

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And, the number of counties with ticks is probably higher than reported, the researchers note, because not all counties track ticks.

“I think the numbers could be higher,” said lead author Dr. Rebecca Eisen, a research biologist at the CDC. “We have good confidence where it’s reported or established, but low confidence in the areas with no records.”

Thomas Mather, a University of Rhode Island professor and director of its TickEncounter Resource Center, agrees, saying that his organization’s TickSpotters citizen-scientist program has received pictures of ticks from some of the counties listed as having zero records.

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“We live in a more ticks, more places world,” he said.

Does that mean everyone should be concerned about Lyme disease? Unless you live in an area that doesn’t have potential for tick habitat, you should be vigilant, tick experts say.

That’s especially true for people who live in newly-affected areas, such as the Appalachian West and the Ohio River Valley, where medical care providers might not be on the lookout for Lyme disease.

The top map shows distribution of ticks in the United States from 1907–1996. The bottom map shows counties classified as established (red or green) for a given tick species had at least six ticks or two life stages recorded within a single calendar year. Counties with fewer ticks of a single life stage were classified as reported (blue or yellow) for the tick species. Dennis et al. 1998/CDC

Most people are not yet “tick-literate,” Mather said. He regularly gets emails, he said, that say, “‘Oh, my doctor says it can’t be a deer tick because it doesn’t occur here.’ Doctors who are not entomologists don’t consider Lyme disease if they believe that vector ticks aren’t present.”

That’s troubling because effective treatment of the disease requires a quick response. But equally troubling is that some people are mistakenly treated for Lyme disease. While TickSpotters has received over 20,000 submissions over two years, over 50 percent wrongly identify the tick in the picture.

“How many people have said in an email that I was diagnosed with Lyme disease and the picture was of a dog tick” - which doesn’t carry Lyme disease, he said.

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Since there is no vaccine and no universally accepted diagnostic test for Lyme disease, prevention and vigilance are the best precautionary measures. The CDC recommends avoided woody and brushy areas if possible, using insect repellents on skin and clothing, and checking yourself, pets and clothing for ticks.

Historically, black-legged ticks actually roamed in a bigger area of the country, according to a 2010 study, but were wiped out of some areas that were deforested. Now, experts expect tick expansion to continue as warming trends help increase their range.

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“Where the climate is conducive we would expect to see continued expansion, but not in places like Colorado or the arid intermountain west because it’s too dry,” Eisen said.

The behavior of ticks varies based on habitat: southern ticks like to hang out deeper in the leaf litter, and are thus less likely to encounter humans, so people are more likely to get bitten in the north. That may change, however, as the range of the northern ticks stretches southward, Mather said.

Although Lyme is the most prevalent disease caused by blacklegged ticks, they can also carry Human Anaplasmosis, Babesiosis and Powassan disease.