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Lyme disease-carrying ticks spread to half of U.S. counties

Ticks that transmit Lyme disease have significantly spread across the United States over the past 20 years, according to new research.

The report from the Centers for Disease Control and Prevention, published in the Journal of Medical Entomology, found that the parasites that carry the disease -- known as blacklegged ticks -- are now found in nearly half of all U.S. counties. This was the first study since 1998 to examine where these ticks live.

Each year, approximately 30,000 cases of Lyme disease are reported to the CDC, though the number is estimated to be much higher, as only a fraction of cases are actually reported. In fact, according to an analysis published last summer, health officials report the number of actual cases is closer to 329,000.

The disease is transmitted to humans through tick bites, and typical symptoms include fever, headache, fatigue, and a rash that often looks like a "bulls-eye" at the site of the tick bite. Most cases are easily treated with antibiotics, but in some patients, if left untreated, Lyme disease can lead to chronic joint inflammation and neurological problems weeks, months, or even years after infection.

For the current study, Dr. Rebecca Eisen, a research biologist at the CDC, and her team looked at data from published reports of state and county tick surveillance data going back to 1996. They analyzed numbers from all 3,110 counties in the U.S. to determine where the ticks were "established" -- meaning there were sightings of at least six individual ticks or at least two of the three host-seeking life stages had been identified in a single year. The researchers also noted which counties had one or more reports of blacklegged ticks and which ones had none.

The results showed that the blacklegged tick has been reported in more than 45 percent of U.S. counties, compared to 30 percent of counties in 1998. What's more, these ticks are now considered established in twice the number of counties as in 1998.

The most dramatic geographic expansion was seen in the northeast and northern U.S. states, including large areas of New York, Vermont, Wisconsin, Ohio, and Minnesota.

The map on top is from 1998, and the one below it is from 2015. Red indicates a county where I. scapularis (blacklegged tick) is established, and blue indicates that it has been reported. Green indicates a county where I. pacificus (western blacklegged tick) is established, and yellow indicates that is has been reported. Entomological Society of America

Dr. John Aucott, director of the Johns Hopkins Lyme Disease Clinical Research Center, said that the report correlates to what clinical researchers have observed in human cases.

"The nice thing about this data is that it shows the vector -- the ticks that transmit Lyme disease --spreading in the same ways that we've been seeing the human cases spreading," he told CBS News. "So, the take home message is that Lyme disease is a geographically expanding infectious disease and so the areas of risk have expanded dramatically over the last two decades."

Aucott explained that reforestation and an increased deer population have contributed to the tick expansion.

"It's really a reconstitution of where the ticks were originally. In the early 1900's, North America was deforested for agriculture and deer were hunted almost to extinction, and so the ticks lost their normal habitat," he said. "Now as we allow the United States to reforest and have a booming deer population, the ticks are reclaiming their habitat."

Climate change may also play a role, he said, as ticks thrive in warm, damp environments.

Experts say we will continue to see the number of ticks expand -- along with a rise in cases of Lyme disease -- in areas not previously affected.

Aucott said it's important for people to keep in mind that they can get bitten by a tick anywhere there's green space.

"To get Lyme disease, you don't have to live out in the country," he said. "There are many green belts and streams that come down into cities and those habitats support deer and ticks really well. The ticks are really everywhere unless you live in an incredible urban center where it's all asphalt and concrete."

As such, people should take precautions to avoid ticks. The CDC recommends the following:

  • Avoid wooded and brushy areas with high grass and leaf litter and walk in the center of trails.
  • Use repellents that contain 20 to 30 percent DEET on exposed skin and clothing for protection that lasts up to several hours.
  • Use products that contain permethrin to treat clothing and gear, such as boots, pants, socks and tents.
  • Bathe or shower as soon as possible after coming indoors (preferably within two hours) to wash off and more easily find ticks that are crawling on you.
  • Conduct a full-body tick check using a hand-held or full-length mirror to view all parts of your body upon return from tick-infested areas.
  • Examine clothing, gear, and pets for ticks.
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