Lyme disease is a mysterious, chronic condition that can lead to devastating symptoms, and a new study suggests the ticks that spread it may be increasing in numbers.

The report, published Wednesday in the Journal of Medical Entomology, found that the blacklegged tick and the western backlegged ticks, two vectors of Lyme, have now been reported in nearly half of all U.S. counties. The last comprehensive survey of the ticks’ presence was published in 1998, Science Daily reported.

By using a similar method to the one used in 1998, researchers at the Centers for Disease Control and Prevention (CDC) observed that the backlegged tick has been reported in more than 45 percent of counties compared to 30 percent in 1998. Compared to 1998, the backlegged tick is considered established in twice the number of counties today, Science Daily reported. The western backlegged tick, meanwhile, only increased from 3.4 to 3.6 percent of counties.

The Northeast has seen the biggest spread of the backlegged tick, while the nusiance’s presence in the South has stayed stable.

"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," said Dr. Rebecca Eisen, a research biologist at the CDC, according to Science Daily. "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."

The symptoms of Lyme can often be mistaken as the flu, but prompt treatment with antibiotics can aid treatment. The CDC recommends using insect repellent, reducing the tick habitat, applying pesticides and removing ticks quickly to help prevent Lyme.