BETHLEHEM -- Seven cases of pertussis - commonly known as whooping cough - have been reported in Bethlehem schools.

There are three cases at the high school, two at the middle school and one each at Eagle and Slingerlands elementary schools, county spokeswoman Mary Rozak said.

The outbreak is the first to occur statewide this year, according to the state Health Department.

Only four of the Bethlehem cases have been confirmed by laboratory reports. Three students have symptoms fitting the definition of pertussis and also had contact with students whose cases were laboratory-confirmed, according to a note to parents from Jody Monroe, superintendent of the Bethlehem Central School District.

Pertussis is highly contagious, and spreads through air droplets after an infected person sneezes or coughs. Infected people can spread the disease up to three weeks after they develop a cough from the illness.

The illness is usually not serious, except for infants, said Dr. Eric Schnakenberg of Community Care Family Medicine in Clifton Park.

"Typically, this is sort of a lingering cold, it will be kicking around for six weeks," Schnakenberg said.

Treatment is recommended, however, to keep the illness from spreading, he said. Serious complications include pneumonia, impaired brain function and even death, according to the state Health Department.

Pertussis is a bacterial infection in the lungs that can be treated with antibiotics. It starts with cold symptoms and a cough that worsens during the first week or two, but can last for months. The cough is sometimes followed by a "whoop," giving it its name.

Most schoolchildren in New York must be vaccinated for pertussis. The vaccine is effective, but wanes over time, requiring booster shots. Because the protection decreases over time, even people who have been vaccinated can catch pertussis.

"The immunity tends to wane, so at age 11 we recommend all the kids get a booster," Schnakenberg said.

Booster vaccinations are also recommended for anyone who expects to be in close contact with babies, he said, including pregnant women, child-care or health-care workers, or parents and grandparents of infants.

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There have been no other reported cases of pertussis in the county this year. Last year, there were 10 cases countywide, and in 2015, there were five.

Statewide, so far in 2017, there have been 119 cases of pertussis, compared to 220 cases at this time last year. There have been isolated clusters and sporadic cases around the state this year, but no outbreaks outside Bethlehem, according to the state Health Department.

Schnakenberg said outbreaks are infrequent.

"Since the pertussis vaccine has been available since 1990, there's been a dramatic decrease in whooping cough in this country," he said.

Pertussis does tend to spike every three to five years, however, according to state health officials. The last such increase was in 2012, when more than 3,000 New Yorkers contracted the illness. More than 500 of those patients were in Suffolk County, according to state Health Department data. There were 58 cases in Albany County that year, and 168 in the four-county Capital Region.