It’s been nearly two decades since the last publication of a nationwide survey on the distribution of blacklegged ticks — the primary transmitters of Lyme disease. That survey, released in 1998, reported the tick in 30 percent of U.S. counties. Today, a new study using similar surveillance methods has found the tick in more than 45 percent of counties.

“These kinds of changes tend to be gradual,” study co-author Rebecca Eisen, a research biologist in the Centers for Disease Control and Prevention’s Division of Vector-Borne Diseases, told me. “But when you see the cumulative change over 20 years, it is surprising.”

The study, published earlier this week in the Journal of Medical Entomology, found that the blacklegged tick — scientifically known as Ixodes scapularis and a primary Lyme disease vector in the eastern U.S. — has now been documented in 1,420 of the 3,110 counties within the continental U.S. Its relative in the western U.S., known as Ixodes pacificus, is now in 111 counties. Together, the ticks have now been documented in nearly 50 percent of counties across 43 states. And the new numbers mark a more than 44 percent increase in the number of counties with a documented presence of such ticks since 1998. The study also noted that Lyme disease case counts have also increased steadily, from 10,000 in 1991 to more than 30,000 in 2008, though the true disease burden is likely much greater.

Eisen said her main message to people is to be aware that these vectors are expanding their range and to take precautionary steps to reduce contact with ticks. But before getting too much deeper into the study and its findings, it’s probably a good idea to answer two questions right away.

First, does the expansion of blacklegged tick territory translate into an increased risk of Lyme disease exposure? Not necessarily. Eisen said while blacklegged ticks may now be established in more counties, their host-seeking behavior and the particular animal hosts they seek out still differ according to region. For example, ticks in the South tend to stay immersed in the leaf litter, so they’re less likely to come into contact with people. While ticks in the Northeast are more likely to be on top of leaf litter. Of course, the very different climates contribute to host-seeking and survival behaviors that impact the chance of human encounters as well. As of 2014, the great majority of confirmed Lyme disease cases — 96 percent — were in just 14 states, all of which are in the Northeast, North Central and Mid-Atlantic regions.

The second question: Is this study the first that my local or state health department knows of this trend? Most definitely not, Eisen said. Just about every year, most state health departments post updates on vectors and vector-borne diseases, and local health departments are continuously working to keep their residents aware of risks. However, the updated survey that Eisen co-authored provides public health practitioners with a national overview — a chance to view overall trends and examine distribution changes across state borders. Eisen and study co-authors Lars Eisen and Charles Beard write:

Data on the current geographic distributions of medically important tick vectors, such as I. scapularis and I. pacificus, provide information complementary to epidemiological data on geographic disease case occurrence to inform the medical community and the public of where risk for exposure to tick-borne disease agents may occur. The lack of routine systematic surveillance across the continental United States of ticks of public health importance hampers our ability to define their current geographic distributions and to monitor changes in their ranges and densities over time. Although we are able to report in this paper where I. scapularis and I. pacificus are now known to be present at the county level, our certainty in where the tick is absent is low, especially at the edges of their ranges and in regions where they can be assumed to occur only at low densities.

They go on to write:

Given the lack of systematic surveillance for I. scapularis, one might ask if the range expansion suggested by our data is real or merely an artifact of enhanced tick surveillance and research activities in some areas. A true range expansion of I. scapularis in northern states, as described in this report, is supported by the largely concordant changes in the distribution of human Lyme disease cases captured through mandatory reporting of the disease since 1991.

Overall, the study found that most of the blacklegged tick’s expansion has occurred in the northern U.S., while populations in the southern states have remained pretty stable. The tick’s range in the western U.S. increased only slightly, from 3.4 percent to 3.6 percent of counties. More specifically, I. scapularis has expanded its range northward into upstate New York, Vermont, New Hampshire and northern Maine; westward across Pennsylvania, eastern Ohio and New York; and south- and southwestward into West Virginia, Virginia and North Carolina. For example, in West Virginia, the blacklegged tick was reported in just four counties in 1998, whereas the updated survey found it in 43 counties. On the other side of the country, I. pacificus is now established in 95 counties, the majority of which are located in California, Oregon and Washington (that’s a fairly small increase from the 1998 survey in which that tick was established in 90 counties).

While Eisen’s study doesn’t definitively pinpoint the factors behind expanding tick territories, she said one of the long-term drivers is an expanding deer population. Ticks, she said, like to feed on deer, and deer tend to follow forested areas. So as forests and deer populations expand, so too do ticks. Other drivers include changes in climate conditions that enable ticks to survive and reproduce in new regions, among many other variables, she said. (Click here for more on how public health scientists predict climate change will impact the distribution of vector-borne disease.)

Luckily, there a number of ways to reduce one’s risk of exposure to blacklegged ticks and Lyme disease. For example, Eisen said, avoid areas with high grass and leaf litter; hike in the center of a trail and not on the edges; use repellents containing at least 20 to 30 percent DEET; and shower as soon as possible when returning indoors. All the tips you need can be found on the CDC site.

Next, Eisen and her research colleagues hope to develop a statistical model to help predict where tick expansion might happen in the future. While she said it wouldn’t be surprising to see some additional expansion, she predicts there is an “edge” to the tick’s potential range. Because blacklegged ticks need forests to thrive, that edge is likely where forests runs into prairie lands, high alpine areas and agricultural regions.

“One of the main things I hope people take away (from this study) is the recognition that tick populations change over time, so we need to continually monitor that distribution,” Eisen said. “We need to continue to focus on ways to prevent tick-borne diseases and reduce human encounters. And there’s lots of options on the table.”

To download a full copy of the new study, visit the Journal of Medical Entomology.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for nearly 15 years.