The tick species that spread Lyme disease are now present in half the counties of the United States, according to a new analysis by the Centers for Disease Control and Prevention, and the territory in which they are most firmly established has doubled in the last two decades.
As you can see from this map, the expansion in Minnesota has been especially pronounced:
To arrive at these findings, published Monday in the Journal of Medical Entomology, researchers began with a map of black-legged tick prevalence across the U.S. as it stood in 1998, then updated it with tick counts made through August 2015.
Counties were classed into three tiers based on tick-collection reports gathered from public health or environmental agencies:
- Counties with “established” tick populations were those where at least six individual ticks had been collected in a calendar year, or where fewer ticks representing at least two out of three disease-spreading life stages were found.
- Counties with “reported” or potential populations had tick collection data that fell short of those minimums.
- Counties with “no records” had no ticks officially recorded, which the paper helpfully explains “does not imply that ticks are absent from that county, only that records … are lacking.”
In 1998, 1,058 counties across 41 states were classed as reported or established; by last summer, the number had grown to 1,531 counties in 43 states. That’s an increase of 45 percent.
Two species of black-legged tick are involved here — the so-called deer tick of the central and eastern U.S., and the western blacklegged tick of the Rocky Mountain and Pacific coastal regions. Both are the “primary vectors” bringing Lyme disease to humans; both also transmit other serious but less headline-grabbing diseases, including anaplasmosis, babesiosis and a viral form of encephalitis.
But deer ticks (Ixodes scapularis) are expanding their range much more rapidly: For them, the number of counties in the “established” category more than doubled from 1998 to 2015, from 396 to 842.
The data presented here suggest that I. scapularis over the past two decades has expanded from its northeastern focus 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. A similar geographic expansion … appears to have occurred from the long-established focus in the North-Central states, with notable spread of counties where the tick is now classified as established in all four cardinal directions. The two previously distinct foci in the Northeast and North-Central states appear to be merging in the Ohio River Valley to form a single contiguous focus.
And Minnesota is among the places where the spread has been most pronounced.
The Minnesota picture
The Minnesota figures in the national paper were contributed chiefly by Dave Neitzel, an epidemiologist at the Minnesota Department of Health who works on tick- and mosquito-borne diseases and has been tracking deer ticks since Lyme disease became a health concern here in the 1980s.
Lest there be any doubt that the CDC paper is more than an academic exercise, Neitzel told me on Wednesday that the methods used in the mapping work are a reliable way to gauge the spread of both ticks and the risk of tickborne illness, because the results have been shown to correlate well with statistics on the incidence of Lyme and other infections.
Nationally, about 300,000 Lyme cases are diagnosed each year, with about 1 in 10 confirmed by laboratory tests, which is difficult. In Minnesota, 896 confirmed cases and 520 probables were reported to the health department in 2014.
In fact, Neitzel said, finding two ticks at different life stages may be a more meaningful indicator than finding a bunch of the same age because it shows that the ticks are healthy and maturing in the local conditions.
Just nine Minnesota counties were in the “established” category back in 1998. Clustered mostly in the metro area, and shown in black in the map above, they are Anoka, Carver, Chisago, Dakota, Morrison, Pine, Ramsey, Scott and Washington.
Twelve more, shown in orange, were in the “reported” tier: Aitkin, Beltrami, Carlton, Crow Wing, Douglas, Houston, Isanti, Kanabec, Mille Lacs, Olmsted, Todd and Winona. All are now in the higher tier.
Twenty-seven counties with no ticks counted by 1998 are now mapped in the danger zone. Of these, the 24 shown in red are in the “established” tier (Becker, Benton, Cass, Clearwater, Cook, Fillmore, Goodhue, Hennepin, Hubbard, Itasca, Kandiyohi, Koochiching, Lake, Lake of the Woods, Mahnomen, Otter Tail, Pope, Sherburne, Sibley, St. Louis, Stearns, Wabasha, Wadena and Wright) and the other three, shown in green, are in the “reported” tier: Brown, Nicollet and Rice.
No counties went from a higher tier to a lower one between 1998 and 2015. The 39 counties colored white had no tick reports in either year, but Neitzel said that luck is unlikely to hold everywhere: Ticks need wooded, brushy habitat but they don’t necessarily need a lot, and they do get around.
A mobile disease agent
Under its own power, he said, a black-legged tick may cover only a meter or two of territory in its lifetime. But its horizontal movement is less important than its vertical movement up a deer’s leg, or a hiker’s, and its limited locomotion matters less than its ability to find transport while feeding on mice, chipmunks, birds …
If you go up on the North Shore, ticks are established in parts of those counties, but the distribution is more spotty. Kind of the last frontier was the northern tier of counties right along the Minnesota/Ontario/Manitoba borders, but there are scattered infestations being seen in most of those counties now, too.
I’ve collected one – one lonely – black-legged tick in New Ulm, a few years back, and that’s as far into farm country as we’ve found ’em. So far.
There are areas of interest to us out in the farm lands – probably the best example is, cutting right through the middle of the state, we have the Minnesota River Valley, which in a lot of areas is a half-mile to mile-wide strip of woods on either side of the river, and I think there’s a lot of potential there. I think it’s only a matter of time.
Neitzel said he wasn’t particularly surprised by the national findings, but thought it might offer some clarifying reassurance to certain tick-free regions of the country – particularly in the lower Midwest – that they have little to worry about.
On the other hand, it might have the opposite value in alerting others, on the spreading westward fringe of tick territory, that they’d better be on the lookout.
A while back, he said, North Dakota officials called him to consult on the possible spread of black-legged ticks across the Minnesota border. He gave them some ideas of where to look and now eight counties are on the map, up from zero in 1998.
It remains unclear, he said, how deer tick populations first became established in the upper Midwest in a fairly narrow band along the St. Croix River, whence it spread rapidly in all directions. Until Lyme disease became an issue, there wasn’t much interest in studying ticks; once that interest arose, the populations had already exploded.
How to count ticks
Every wonder how a tick census works?
We’ll have several hundred-meter transects where we’ll do drag sampling – we have a piece of cloth we pull behind us in the woods, and stop every 15-20 meters to pull ticks off of ourselves and the cloth. From that we can get an idea of the density of feeding ticks.
We also keep track of infection prevalence of various disease agents — Lyme disease spirochetes, anaplasmosis bacteria, babesia parasites, and also a few other less common tickborne disease agents.
What we’ve been finding over the years, unfortunately, is you don’t have to go too far before you find infected ticks. Lyme disease, for instance, anywhere from about a third to half of the adult ticks are infected and roughly 20 percent of the nymphs.
So if people are coming into contact with many black-legged ticks at all, they’re going to be coming into contact with infected ticks. And we’re entering into a reality now where it’s hard to find extensive wooded areas that don’t have a population of black-legged ticks.
So I had to ask – has he managed to get through all these years of close-up tick study disease free?
Yes, but I’m extremely aware when I’m in good tick habitat. I get several hundred of these things on me in any given year, but I remove most of them before they even get a chance to feed, and for the ones that do attach I’m generally pretty good at getting them off me within a matter of hours – and that’s the important thing.
With Lyme disease, for instance, the ticks have to be attached for one to two days to effectively transmit the bacteria. So the quicker you get them off, the better.
MinnPost event: On Monday, Feb. 22, MinnPost’s Earth Journal Circle will present its fourth annual event focusing on substantive discussion of critical issues in the environment. This year’s topic is “Land of 10,000 Lakes: Can We Achieve Water Sustainability?” The speaker is Deborah Swackhamer, former director of the University of Minnesota’s Water Resources Center, who will discuss issues threatening regional water quality and quantity. Earth Journal writer Ron Meador will moderate the Q&A session.