Disease-bearing black-legged ticks have spread across the country, including a fivefold increase in the number of counties in Minnesota reporting established tick populations. That compares to a 45 percent increase in the number of counties nationally reporting infestations.

“It’s important for Minnesotans to know the area where the black-legged ticks … are found has been expanded,” said Dave Neitzel, a supervisor of tick-borne disease studies at the Minnesota Department of Health. Neitzel commented Thursday on the federal study.

Black-legged ticks, formerly called deer ticks, carry disease, most notably Lyme, which is marked by a variety of symptoms from fatigue and fever to muscle aches. A bull’s-eye rash at the spot of the bite is often the indicator of transmission.

Since 1991, when standardized tick surveillance and reporting began, the number of Lyme disease cases has steadily risen both in number and geographic location.

Minnesota has seen the number of Lyme cases skyrocket in recent years, though numbers can fluctuate year to year based on the weather. In dry years, ticks burrow into the dirt. In wet years, they’re out and about looking for hosts.

The median number of reported Lyme disease cases annually in Minnesota from 2006 through 2014 was 1,065, according to the Health Department. From 1996 to 2005, the median was 464. Lyme case reporting for 2015 is not yet complete.

The study by scientists at the Centers for Disease Control (CDC) compared tick infestation data collected in 1996 and 2015 and was published Monday online in the Journal of Medical Entomology.

Nationwide last year, black-legged ticks were found in 1,531, or 49 percent, of counties in 43 states. In the initial study, 1,058 counties in 41 states reported the presence of the ticks.

Neitzel pointed out that the numbers in the study were self-reported by the state agencies investigating the ticks and that their search efforts vary. “What it is is a good documentation of what’s going on nationally,” he said.

In Minnesota, nine of 87 counties reported established black-legged tick populations for the earlier study. In 2015, that number rose to 45 counties.

Neitzel, who was at the state agency during the previous federal survey, said the first study reflects the early days of black-legged tick identification. State scientists knew the ticks likely were in other counties, but had yet to formally identify them through field studies, Neitzel said, adding that since then Minnesota’s tick-identification effort has intensified.

“We’ve been putting a lot of effort into researching these ticks. We’re looking a little bit harder” than other states, he said.

The spread of ticks, however, has been steady — not increasing in rate, he said. Ticks are not fast and they can’t fly like mosquitoes, so transmission occurs by hosts wandering through their habitat. They live in densely wooded areas and are not prevalent in more open, agricultural or residential areas.

Said Neitzel, “Minnesotans will be at risk if they go out and play in the woods.”