Black-legged tick inhabits twice as many counties as in 1998, CDC reports, with 320% increase in number of north-eastern counties seen as high risk for disease

Ticks that can carry the debilitating illness Lyme disease have significantly spread across the US over the past 20 years and are now found in nearly half of all American counties, including areas where they have never previously been documented, a new analysis has found.



The black-legged tick is now established in twice the number of counties it inhabited in 1998, according to the US Centers for Disease Control, and has expanded its range in the north-eastern states and the upper midwest. Following its onward march, or hop, across the Ohio river valley, western New York and the shores of Lake Michigan since the 1990s, there has been a tripling in the incidence of Lyme disease in the US.

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The tick, which has a variant species present on the west coast of the US, can burrow into humans and transmit Lyme disease, which causes fever, headache and other flu-like symptoms. Further problems include impaired memory, dizziness, heart palpitation and inflammation of the brain and spinal cord. Around 300,000 people are diagnosed with Lyme disease each year.

The CDC said there has been a 320% increase in the number of counties in the north-east US considered high risk for Lyme disease since the 1990s. Overall, more than 45% of all American counties now host the ticks, up from from 30% in 1998, according to the research published in the Journal of Medical Entomology.

Reforestation and an increased population of deer, which host the ticks, have contributed to the increase in distribution. Temperature and rainfall also influence the fate of the tick, with a warming climate helping their spread.

Dr Rebecca Eisen, a research biologist at the CDC, said the tick expansion has been “substantial” and required mitigating action, especially in areas that have not previously had to deal with the insects.

“A lot of people are seeing ticks where they didn’t see them 20 years ago,” she said. “The observed range expansion documented in our study highlights a need for continuing and enhancing vector surveillance efforts, particularly along the leading edges of range expansion. It’s important to know which ticks are in your area or areas that you visit so that you can take steps to protect yourself.”

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Eisen said the spread of ticks won’t necessarily lead to a uniform increase in Lyme disease cases because of the varying presence of the infected spirochetes that are passed on, as well as uneven numbers of the ticks themselves.

But people in areas at risk of Lyme disease are advised to use repellent, walk in the middle of trails when out hiking and bathe or shower as soon as possible when coming indoors. This will help remove attached ticks.

Thankfully, there is no evidence that Lyme disease is becoming harder to treat. The ticks should also face natural obstacles in their journey across the states.

“It is likely that the ticks will continue to expand into neighboring forested areas and along river corridors,” said Eisen. “However, dry prairies and alpine areas represent natural barriers to the spread of the tick.”