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New Map Shows Most Lyme-Infected Ticks Are in the Northeast, Northern Midwest

The views expressed are those of the author and are not necessarily those of Scientific American.

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female blacklegged tick

Female blacklegged tick courtesy of Graham Hickling/University of Tennessee

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.

Lyme disease risk map

New map of Lyme disease risk courtesy of Maria Diuk-Wasser/Yale School of Public Health

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.”

Katherine Harmon Courage About the Author: Katherine Harmon Courage is a freelance writer and contributing editor for Scientific American. Her book Octopus! The Most Mysterious Creature In the Sea is out now from Penguin/Current. Follow on Twitter @KHCourage.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. Richard Pollack 5:55 pm 02/1/2012

    This excellent report confirms what many knew or suspected, that risk of Lyme disease is greatest in certain areas of the northern US. Readers should note that finding and promptly removing ticks (from a person or pet) can dramatically reduce risk of infection. Once the tick has been removed, have it identified. Only certain kinds of ticks can transmit the agents of Lyme disease, babesiosis and anaplasmosis. Other ticks transmit other infections. The longer the tick is attached, the greater the risk of infection. Physical samples can be sent, or digital images uploaded, for a rapid, confidential, independent and expert evaluation. For more educational information and help with identification, visit
    Richard Pollack, PhD (IdentifyUS LLC)

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  2. 2. QuantumQualifax 11:15 am 02/2/2012

    Richard Pollack, you state the obvious several times.

    I live within the range of Lyme-infected ticks, have been bitten dozens of times, have not panicked nor have I reported such ticks.

    I do not have Lyme disease.

    People tend to get worked up over stuff nowadays.

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  3. 3. twiggy042999 2:51 pm 02/3/2012

    Dear Quantum,

    I beg to differ with your comment that people tend to get worked up over stuff.

    As a child I had an engorged tick removed from my armpit. I developed a rash and was sent away because Lyme disease did not “exist” in my area of Illinois. Approximately 6 months later I developed neurological symptoms that could not be explained to my parents. For 16 years I suffered with headaches, optic neuritis, fatigue and severe joint pain (amongst other things) before I took it upon myself as an adult to seek other opinions. 7 years ago and then again a year ago I asked my primary doctor for a Lyme test. Again, due to lack of knowledge and ignorance in the matter, I was told it was rare in my area and was not given the test with either request. Through the past 7 years I underwent test after test to rule out things like MS, Lupus, RA, etc. Finally an eye specialist asked me if I had ever been checked for Lyme. An ELISA and Western Blot were done and both were positive. I am finally and thankfully making slow progress with treatment.

    So needless to say….this is a topic people should get worked up over. Nobody should have to endure the hell others and I have gone through because this topic is taken so lightly.

    A friend once said, “You should not judge Lyme unless you or someone you love has it.”

    And thank you Richard for your acknowledgement that education of Lyme and ticks is key to preventing it!

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  4. 4. ErnestPayne 4:51 pm 02/6/2012

    I notice a red X over what appears to be Erie, Pennsylvania. Would it be possible to get an explanation?

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