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Ticks that Transmit Lyme Disease Reported in 48.6% of U.S. Counties

A blacklegged tick (Ixodes scapularis). Photo by Gary Alpert, Harvard University,

By Hannah Foster

Lyme disease is a potentially debilitating disease caused by the bacterium Borrelia burgdorferi. The disease is spread to humans by two tick varieties: the blacklegged tick (Ixodes scapularis), which is responsible for most cases of Lyme disease in the United States, and the western blacklegged tick (Ixodes pacificus), the culprit for cases in the western U.S. The number of Lyme disease cases reported to the Centers for Disease Control and Prevention (CDC) has tripled over the last 20 years — from 10,000 to 30,000 per year — making it the most commonly-reported vector-borne disease in the U.S. In fact, researchers estimate that in reality closer to 300,000 people are infected each year. Lyme disease was previously contained mainly in the Northeast and Upper Midwest, but research suggests that its range is expanding.

Hannah Foster

One of the first symptoms of Lyme disease that occurs in some people is a rash shaped like a bulls-eye that forms around the point of the bite. However, this rash, called erythema migrans, is not present in all cases and is sometimes overlooked. Other early symptoms of Lyme disease are nonspecific and may include fever, headache, and fatigue — all of which can be mistaken for the common flu. As many as half of the people diagnosed with Lyme disease do not even recall a tick bite. Because of this and its lack of unique symptoms, the diagnosis of Lyme disease can be very challenging. Consequently, one of the key factors doctors consider when diagnosing a patient with Lyme disease is whether or not the patient may have come in contact with ticks. Unfortunately, until researchers at the CDC tackled it, the range of blacklegged ticks had not been re-evaluated in nearly two decades.

Dr. Rebecca Eisen noticed this paucity of information immediately when she began her research on Lyme disease. Eisen is a research biologist at the CDC, where she studies vector-borne diseases, including plague and Lyme disease. Eisen observed that the last comprehensive survey of blacklegged tick distribution was published in 1998, even though the areas of the U.S. affected by Lyme disease are steadily expanding. This means that doctors might overlook Lyme disease as a possible diagnosis because they are unaware that their patients could have been bitten by blacklegged ticks. In addition, people living in and around these newly-affected areas may not realize the importance of protecting themselves against ticks, since previously they had no cause to worry.

To remedy the situation, Eisen and colleagues performed a new survey to establish the current geographic distribution of the blacklegged tick and western blacklegged tick in the U.S. The team used surveillance methods similar to those used in 1998 so that they would be able to accurately judge the degree to which the distribution of these ticks had changed. The researchers searched scientific literature for county-specific reports of tick presence. They also contacted other Lyme disease researchers and visited the websites of state health departments, which often report tick surveillance data. Using the data they gathered, they labeled counties as “established,” “reported,” or “no records.” Counties reporting six or more individual ticks or ticks of at least two of the three host-seeking life stages (larva, nymph, or adult) were classified as “established.” Counties that did not qualify as “established” but had reported at least one tick were classified as “reported.” A county labeled “no records” does not necessarily mean that there aren’t any ticks in the county, but merely indicates that there were no records of tick reports.

The map on top is from 1998, and the one below it is from 2015. Red indicates a county where I. scapularis is established, and blue indicates that it has been reported. Green indicates a county where I. pacificus is established, and yellow indicates that is has been reported.

Eisen’s results were published in the Journal of Medical Entomology. In brief, Eisen and colleagues found that the blacklegged tick has been reported in more then 45% of U.S. counties, compared to 30% of counties in 1998. Even more alarming, the blacklegged tick is now considered “established” in twice the number of counties as in 1998. Interestingly, more of the geographic expansion of the blacklegged tick appears to be in the North, with populations in southern states remaining relatively stable. In contrast, the range of the western blacklegged tick only increased from 3.4% to 3.6% of counties. According to Eisen, “This study shows that the distribution of Lyme disease vectors has changed substantially over the last nearly two decades and highlights areas where risk for human exposure to ticks has changed during that time.”

You might be wondering if this apparent expansion of the blacklegged tick distribution is simply due to improvements in tick surveillance. However, the CDC’s new data correlate well with the geographical spread of reported Lyme disease cases. Moreover, Eisen’s conclusions are supported by data from areas with very rigorous surveillance methods, both in the 1990’s and today. Eisen’s study did not look into biological or ecological explanations for the observed tick distribution changes. According to one theory, the blacklegged tick was once prevalent across much of the Eastern U.S., and the current trends may be due to the blacklegged tick recolonizing its previous territory. This recolonization may be facilitated by reforestation efforts and the increase in the number of white-tailed deer (the primary host for the blacklegged tick).

Eisen is quick to acknowledge that much work remains to be done to truly establish epidemiological trends.

“The observed range expansion of the ticks highlights a need for continuing and enhancing vector surveillance efforts, particularly along the leading edges of range expansion,” she said.

One major hindrance to better understanding tick distribution is the fact that tick surveillance in counties and states is profoundly variable. State health departments have no standardized method of acquiring tick distribution data; some states have routine sites for drag sampling, while others gather information from reports from veterinarians or ordinary citizens. Thus, information is very accurate in some areas, while in others it is spotty or lacking completely. More comprehensive surveillance is necessary to accurately determine the distribution of ticks and to predict the areas they may inhabit in the future. Importantly, with better surveillance, researchers can also assess the distribution, abundance, and prevalence of B. burgdorferi in ticks. Eisen’s new survey is a start towards providing valuable, current information for doctors who treat patients with tick bites.

If you find that you are now in a region inhabited by the blacklegged tick, the best way to prevent Lyme disease it to protect yourself from tick bites. When you are heading out to prime tick habitat (such as woods or fields), here are a few simple precautions you can take:

  • Wear long sleeves and long pants.
  • Use an insect repellent with 20-30% DEET.
  • Bathe or shower shortly after you come inside.
  • Check yourself, pets, and gear that traveled with you for hitchhiking ticks.

For more information on Lyme disease and how best to protect yourself, check out the CDC’s website.

Read more at:

County-Scale Distribution of Ixodes scapularis and Ixodes pacificus (Acari: Ixodidae) in the Continental United States

Hannah Foster is a PhD student in molecular and cellular biology at Harvard University and a freelance writer. She studies protein biochemistry in microbes, and enjoys writing about science and non-science alike. You can follow her on Twitter at @Foster_HR and read her blog about boxing as it pertains to life at She is also a frequent contributor to Harvard Science in the News Flash and to The Bitter Empire.


  1. eastern county size seems different from western county size – is this a factor in the percent affected counties data?

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