Reports of Lyme disease in the US have historically been concentrated in two regions of the US: the upper Midwest and the Northeast, including Massachusetts, which during 2014 had the third-highest infection rate trailing only Maine and Vermont, data from the Centers for Disease Control and Prevention shows.
Within Massachusetts, rates of reported infection vary significantly from one area to another.
Between 2010 and 2014, the municipality with the highest proportion of reported confirmed and probable cases was the small Martha’s Vineyard town of Chilmark, with 1,316 cases per 100,000 residents, according to data from the Massachusetts Department of Public Health.
The island of Nantucket had the next highest rate at 598 cases per 100,000 residents.
Other areas of the Cape and the islands also had high rates of Lyme disease during those years.
“The Cape and the islands were actually the first places where Lyme emerged in Massachusetts, and they’ve been experiencing increasing rates of the diseases since the 1980s,” said Dr. Catherine M. Brown, Massachusetts public health veterinarian.
While the area “continues to be a hot spot for all of our tick-borne diseases,” she said the rate of infection of the Cape and islands appears to have stabilized in recent years.
The state said it calculates the rates based on the hometown of people who contract Lyme disease, not where they were infected, so the data is not, for example, skewed by people who are infected while just visiting a town.
The rates are also based on cases that are reported. Officials believe that many Lyme disease cases go unreported.
The southwest corner of the state — including the towns of Egremont, Great Barrington, Monterey, New Marlborough, Sheffield, and Stockbridge — also has had noticeably higher rates, ranging from 261 cases to 356 cases per 100,000 residents.
Brown said that that section of Berkshire County is “another hotspot for Lyme disease,” and that rates are similar in nearby areas of New York and Connecticut.
Two other parts of the state — Worcester and Middlesex counties — have seen sharp increases in Lyme disease rates in recent years, she said.
Still, Brown stressed that all residents and visitors to the state should take precautions, because the risk of contracting the disease exists statewide.
“We have infected ticks everywhere in the state. No matter where you are in Massachusetts, there is a possibility that a tick that gets on you is carrying at least one and up to three illnesses,” said Brown, referring to Lyme and two other less-common, but increasingly prevalent illnesses, anaplasmosis and babesiosis.
Tick-borne illnesses in Massachusetts come from two species of the arachnid: dog ticks, which can carry some illnesses but are not known to carry Lyme disease; and deer ticks, also called black-legged ticks, which are known to carry Lyme disease.
Ticks can be common not only in wooded areas, but also in rural and suburban locales including parks and yards.
Health experts say people should:
— Avoid areas where ticks are found;
— Wear tick repellent; and
— Check themselves and pets for ticks after being outdoors
Lyme disease, which is the most common tick-borne illness locally, often causes flu-like symptoms and a rash that can take a bull’s-eye shape. If untreated, it can cause long-term problems, including cognitive issues and arthritis. The disease rarely causes death.
Lyme disease rates in Massachusetts have risen in most years since the early 1990s, when there were only a couple of hundred reported cases, according to figures collected by the state Department of Public Health. In 2014, the most recent year for which data are available, the state saw 3,830 confirmed cases and another 1,770 probable cases.
Increases in numbers here and elsewhere in the country have been attributed to factors including residential development expanding into wooded areas and better awareness and reporting of the disease.
Matt Rocheleau can be reached at firstname.lastname@example.org. Follow him on Twitter @mrochele