If you don't live in the northeastern United States, you may not think much about diseases transmitted by ticks. If you do live there, or spend part of the summer along the coastal arc that stretches from Virginia and Maryland up through southern Maine, consciousness of these sneaky, potentially disabling illnesses—Lyme disease and its lesser-known brethren, including erlichiosis, anaplasmosis, babesiosis and the rest—is hard to escape.
That very regionalized awareness occurs both because Lyme can be a dreadful illness, causing arthritis and neurological problems years after the tick bite that transmitted it, and also because Lyme is oddly geographically limited. It is named for the town where cases in kids were first identified and linked, Lyme, Conn., and still is most likely to occur in New England and nearby states.
If you are a resident or visitor to that area, though, it has been hard to understand just how much at risk you are. To date, the Lyme disease maps published by the Centers for Disease Control and Prevention have pinpointed Lyme disease clusters with literal pinpoints—one point per reported case, as in the map below from 2013.
Now, though, the CDC has re-analyzed its Lyme data to produce a new map, which displays the areas of highest risk of catching the disease by county. Compiling the map, which appears in a paper published online last week in the CDC's journal Emerging Infectious Diseases, made two things clear. First, where the highest risks are, and second, that the areas of highest risk are expanding.
The map displays counties where cases of Lyme disease are at least twice as high as epidemiologists would predict based on the size of the county's population. To make that determination, it uses data collected in four 5-year periods: 1993-97, 1998-2002, 2003-07, and 2008-12. Over that 20-year period, only a few counties fell off the high-risk list. Many more were added.
Here's what the map and the data behind it show:
In 1993, 69 counties in the US had enough cases to qualify as high-risk areas. In 2012, the number was 260, almost 4 times as many.
In 1993, the high-risk counties were concentrated in Connecticut, New York and New Jersey, and a few counties in western Wisconsin. The most recent data shows high-risk areas in 17 states. Lyme is expanding "in all accessible directions," the paper says.
In the Midwest, the most high-risk areas have remained the same over 20 years. On the East Coast, though, they have moved, away from the coast and into eastern Pennsylvania.
A little more on which counties, exactly. Many people can probably look at those maps and get a good sense of where their houses or relatives are. I've asked the CDC whether there's a version of the map that includes county names. In the meantime, an Excel spreadsheet that the CDC separately makes available on its website lists counties and reported cases since 1992. When I sort it by by most recent data, the top counties come up, in order, as: Chester County, Penn.; Bucks County, Penn.; Middlesex County, Mass.; New Castle County, Del.; and Morris County, NJ.
Why is Lyme expanding? The CDC paper, written by Kiersten J. Kugeler, PhD and colleagues, doesn't speculate. Other research, though—especially at the Cary Institute of Ecosystem Studies in upstate New York—has documented that Lyme is more likely to occur where suburban development breaks up existing forest. Their work has found that, the smaller the chunks an ecosystem is carved into, the greater the chance that the most important animal in the Lyme-disease transmission cycle, the white-footed mouse, can flourish without pressure from predators.
What that means is this: One way to gauge your risk from Lyme is to study this new map and pinpoint your location. But another way, if you live in the risky areas, is to look around you for mega-mansions and new subdivisions forcing clearing of formerly undeveloped land. When you see that—and with the resurgence of the economy, you will—take an extra look for ticks after you go outside.
