The ticks that transmit Lyme disease, a debilitating flulike illness caused by Borrelia bacteria, are spreading rapidly across the United States. A new study shows just how rapidly. Over the past 20 years, the two species known to spread the disease to humans have together advanced into half of all the counties in the United States.

Lyme disease cases have tripled in the United States over the last 2 decades, making it the most commonly reported vector-borne disease in the Northern Hemisphere. The disease now affects around 300,000 Americans each year. If diagnosed early—a rash commonly appears around the site of the tick bite—Lyme can be effectively treated with antibiotics, but longer term infections can produce more serious symptoms, including joint stiffness, brain inflammation, and nerve pain.

To get a comprehensive map of where the two species—the blacklegged tick (Ixodes scapularis) and the western blacklegged tick (I. pacificus)—were living, Rebecca Eisen and colleagues from the U.S. Centers for Disease Control and Prevention (CDC) in Fort Collins, Colorado, combined data from published papers with state and county tick surveillance data going back to 1996. They counted reports of tick sightings in each of the 3110 continental U.S. counties to determine whether those counties hosted an established population or just a few individuals. Ticks were considered “established” when sightings of at least six ticks, or two of the three life stages, had been reported in a year.

Their results, published in the Journal of Medical Entomology, show that the blacklegged tick has undergone a population explosion, doubling its established range in less than 2 decades. It is now reported in 45.7% of U.S. counties, up from 30% in 1998. Blacklegged ticks are found in 37 states across the eastern United States. The rarer western blacklegged tick, restricted to just six states, has shown only modest increases in established populations, from 3.4% to 3.6% of counties. Combined, these two Lyme disease vectors are now found in half of all U.S. counties.

“Since the late 1990s, the number of counties in the northeastern United States that are considered high-risk for Lyme disease has increased by more than 320%,” Eisen says. “The tick is now established in areas where it was absent 20 years ago,” she adds.

Perhaps most worrying, the tick-dense northeast is where Lyme disease is most common. Although the blacklegged tick is found from Florida to Minnesota, 95% of confirmed cases come from just 14 states in the northeast and upper Midwest. “Although our map shows a wide distribution … the risk of people getting Lyme disease is not equal across areas of the country,” Eisen says.

A study published in PLOS ONE last year might hold the answer. Parasitologist Isis Arsnoe from Michigan State University in East Lansing and colleagues found that populations of blacklegged ticks behave differently in the north and the south of the United States. Nymphs of the blacklegged tick in the north are bolder and more active in seeking out hosts, a behavior known as questing. Arsnoe and her team found that that tick nymphs originating from Wisconsin and Rhode Island were 20 times more likely to emerge from leaf litter, putting them in the path of passing humans, than nymphs from Tennessee and Florida. "Questing behavior is a key factor affecting the risk of tick bites,” Arsnoe explains. "Ticks that stay buried in the leaves are not likely to have an opportunity to bite passing humans—and unless they bite they cannot transmit disease.” Arsnoe is concerned that the ticks found in the north may also expand into southern states, taking their questing behavior with them.

But despite the wide distribution of the vectors, a tick’s chances of coming into contact with a human are still relatively low. Avoiding areas of thick vegetation, using a strong repellent, and bathing after hiking are usually enough to avoid contact, CDC says. Eisen says that the most important thing now will be to carefully monitor the spread of the blacklegged tick, so that that people can educate themselves about the potential disease vectors in their area and take steps to protect themselves.