Cases of Legionnaires’ disease are on the rise in Maine and other New England states, and it’s not known why.

Maine typically sees 10 cases a year of the bacterial infection that causes Legionnaires’ disease. But it has so far documented 18 cases this year, including 12 in the past three months, said Dr. Stephen Sears, state epidemiologist with the Maine Center for Disease Control and Prevention.

Massachusetts, Connecticut and Quebec have also seen significant unexplained increases in recent months, Sears said.

“It’s pretty usual to see sporadic cases,” Sears said. “What’s unusual is that (the increase) has occurred throughout New England and Canada at about the same time.”

In Connecticut, health officials have documented 72 cases so far this year, compared to 47 at this time last year, according to federal data. Massachusetts has had 211 documented cases this year, compared to 118 last year.

States in other regions of the country are not reporting significant increases in the disease.

Sears said he and infectious disease experts in Massachusetts and Connecticut recently compared their rising numbers, but could not explain the spike.

“Maybe it’s warmer (weather) into the fall,” he said. “Is it because we’ve had so much rain? Those are all speculations.”

Legionnaires’ disease got its name in 1976, when an outbreak struck people attending an American Legion convention in Philadelphia. The Legionella bacteria is commonly present in water, but can cause a severe strain of pneumonia when inhaled in water droplets or vapor. The Philadelphia outbreak, for example, was traced to the air conditioning system.

It cannot be spread from person to person.

Legionnaires’ disease is a form of pneumonia and symptoms include high fever, chills, cough, chest pain and muscle aches. Most patients take antibiotics and recover, although between 5 percent to 30 percent of people diagnosed with Legionnaires’ die. The disease is most severe in patients with underlying medical conditions, such as cancer.

Maine’s CDC issued an advisory Monday to the state’s medical community to watch for new cases, test for possible infections and report them. Legionnaires’ is usually suspected when symptoms are severe, and especially if a patient reports a risk factor such as recent travel, exposure to whirlpool spas or recent plumbing repairs in the home.

Along with causing severe pneumonia, Legionnaires’ disease requires a longer treatment — typically three weeks — with a different antibiotic than the ones often prescribed for more common types of pneumonia.

“If you know its Legionnaires,’ you can target your therapy more effectively,” Sears said.

No Maine fatalies

Most of the 18 Mainers diagnosed with legionellosis this year developed the full-blown Legionnaires’ pneumonia and were hospitalized, Sears said. However, all survived the illness and have recovered or are recovering, he said.

While Legionnaires’ disease is associated with mass outbreaks from contaminated water, doctors in Maine say it is now more common to see scattered, individual cases. Institutions such as hotels and hospitals are more careful about sanitizing water and ventilation systems than they used to be, and more common sources are water in whirlpool spas, showers and faucets, Sears said.

Homeowners can be exposed, for example, when turning on a shower after a plumbing repair or when they do not heat their home’s hot water supply enough to kill the bacteria, said Dr. August Valenti, head of epidemiology and infectious diseases at Maine Medical Center.

Valenti said he has been diagnosing Legionnaires’ cases since the disease was first identified, and believes it is more common than people know.

“Probably there are more coming in (with the infection) than we realize. We do not always test for Legionnaires’ (disease),” he said.

Valenti said he watches for the more severe pneumonia symptoms that can indicate a Legionnaires’ infection and routinely asks pneumonia patients if they have had their plumbing worked on. He also said he treats all pneumonias with antibiotics that cover Legionnaires’ as well as other forms of pneumonia.

Valenti said he also is stumped by the recent increase around the state and region. “I’m really not sure why that is,” he said.

None of the 18 cases diagnosed this year appear to be related to each other, according to the CDC.

“We try to make links to see if there is any water system connecting the cases, and we haven’t made any,” Sears said.

The 18 Maine patients range in age from 26 to 89 years. They live in eight different counties, with nine of the 18 living in Cumberland and York counties.

The CDC does not identify the towns where patients live to make sure residents of small towns cannot be identified, Sears said.

Alfred DeMaria, state epidemiologist in Massachusetts, said the increase seems to have occurred since August throughout the Northeast and in eastern Canada.

“We have investigated the distribution of our cases and found no common exposures except for a couple of circumstances,” DeMaria wrote in an email reply Monday.

Some European experts have linked an increase in the disease to climactic conditions, although that has not been proven, he said. “The idea is that wet and warm but not too hot weather favors the organism and its aerosolization.”