Two more cases of pertussis have been reported at Freeport High School, where school officials have been dealing with an outbreak of the highly contagious preventable disease.

The Maine Center for Disease Control and Prevention this week confirmed an outbreak at the school after three cases had been reported. On Friday, Principal Jen Gulko notified parents that two more cases of pertussis – also known as whooping cough – have been confirmed at the high school.

“At Freeport High School, we will continue to be vigilant about any students showing signs or symptoms of pertussis and we will contact families immediately if your child shows symptoms,” Gulko wrote in a letter to parents.

An outbreak is defined as three or more cases of an infectious disease at one location in a close time frame. Maine has the highest rate of pertussis in the country, and is reporting 337 cases through October of this year.

The state recorded 446 cases in 2018, and Maine’s pertussis rate of 33.16 cases per 100,000 was more than eight times the national average.

So far in 2019, 48 percent of all pertussis cases occurred in children ages 11-18, the Maine CDC said.

Pertussis is a bacterial infection that causes a violent cough that can trigger vomiting and exhaustion. The cough – known commonly as whooping cough – can last up to 10 weeks and can be treated with antibiotics. Elderly people and babies too young to be vaccinated are especially vulnerable to serious cases, which may require hospitalization and can lead to death.

Other pertussis outbreaks in 2019 occurred in Portland, Falmouth and Saco schools.