Metro

Mass. counties have some of the highest rates of Lyme disease in the US

Counties in Massachusetts have some of the highest rates of Lyme disease in the country.

During the five-year stretch ending in 2015 — the most recent years of available data — Nantucket County residents reported the single highest average annual per capita rate of confirmed and probable Lyme disease cases among all 3,142 counties in the United States, federal figures show.

The fifth-highest such rate nationwide was next door, in Dukes County, which consists of Martha’s Vineyard and the Elizabeth Islands. Nearby Plymouth and Barnstable counties ranked 57th and 81st, respectively.

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Massachusetts has historically been a hotbed for Lyme disease, a tick-borne illness concentrated primarily in the Northeast and upper Midwest.

And the Cape and islands were the first places where Lyme emerged in Massachusetts decades ago, state health officials have said.

While the recent rates are considerably higher in some parts of the state, health officials say Lyme disease “is endemic across the Commonwealth.”

“Infected ticks are found in every municipality, including heavily urbanized ones,” Dr. Catherine M. Brown, Massachusetts public health veterinarian, said via e-mail.

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The county-by-county statistics come from the US Centers for Disease Control and Prevention and the rates reflect the hometown of people who contract Lyme disease, not where they were infected.

It’s possible that cases of Lyme disease in places like Cape Cod and the islands, for example, could include cases of people only living there for the summer.

Officials caution the numbers are believed to only account for a fraction of the true total number of cases because many go unreported. Brown said that based on previous estimates by federal health officials, “it is likely that there are 10 times as many cases as have ever been reported.”

There are other potential limitations to the data, including false positives, slight changes in data collection methods, and states’ abilities to capture and submit data in a timely manner, which depend on funding and staffing.

Below is a list of each Massachusetts county, its average annual rate of confirmed and probable Lyme disease cases per 100,000 residents for the 5-year period ending in 2015, and a rundown of how that rate ranks nationally:

County Lyme cases per 100,000 residents National rank
Nantucket 501 1st
Dukes 342 5th
Plymouth 127 57th
Barnstable 108 81st
Franklin 89 105th
Bristol 88 109th
Berkshire 78 126th
Norfolk 72.7 139th
Hampshire 72.6 141st
Worcester 58 185th
Essex 53 201st
Middlesex 49 212th
Hampden 36 269th
Suffolk 15 364th

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Matt Rocheleau can be reached at matthew.rocheleau@globe.com. Follow him on Twitter @mrochele