Cases of whooping cough are increasing dramatically in Washington state this year and the trend could bring a spread of the disease not seen since the 1940s, the state's health secretary said Tuesday.

As of March 31, officials had already recorded 640 cases, compared with 94 cases reported by this time last year. If the pattern continues, the Health Department expects numbers well into the 2,000s, according to Tim Church, head of communications for the department.

Cases have been reported in 23 counties, including King, Snohomish and Pierce. The rate of infection in Skagit County is the highest in the state.

Health officials say the increase is due to a cyclical pattern, much like the ebb and flow of flu outbreaks.

Secretary of Health Mary Selecky urged vaccinations for teens and adults. Officials noted that vaccination is especially important for those in close contact with babies less than 1 year old.

"It can really wreak havoc on families," said state health officer Dr. Maxine Hayes, emphasizing that younger, unvaccinated children are at the most risk.

Also known as pertussis, whooping cough is a contagious bacterial infection that causes a severe, long-lasting cough. For adults, pertussis can seem like a mild cold with occasional coughing; for young children, especially infants, it can be severe and life-threatening.

No deaths related to the infection have been recorded this year; four infants have died since 2010.

Infants cannot receive the vaccination until they are 4 to 6 weeks old, and they are most likely to be infected by close family members who do not know they are sick.

For children who receive the vaccine when they are little, the shot becomes less effective over time. Teens and adults often don't know they should receive a booster shot to keep their immunity to pertussis strong, a factor the Health Department believes is contributing to the surge in cases.

"A lot of it is a lack of awareness," said Michele Roberts, communication manager for the department's immunization office. "There is a lot of assumption that their vaccination is up to date or they just don't even know they need it at all."

Pertussis vaccines are about 85 percent effective overall, so some vaccinated people will still get sick.

Heidi Bruch, a mother of two in the Seattle area, passed whooping cough to her infant daughter about two years ago.

Bruch, who spoke at a Tuesday news conference, said she approached her doctor about a persistent cough before giving birth, but her doctor said it was seasonal asthma. Bruch had no idea she had pertussis and was not vaccinated.

About a month after her daughter Caroline was born, the baby began gagging and choking at feeding time. Then she turned blue.

Caroline, now nearly 2 and healthy, was hospitalized for more than a month, spending a week in intensive care at Seattle Children's hospital.

When the doctor asked Bruch "who in the family has been coughing," Bruch said she knew she had inadvertently given her child a life-threatening illness.

During the news conference, Health Department officials said they want health-care providers to encourage their patients to get vaccinated.

"We are taking measures in this state for an epidemic that is emerging," Selecky said. The vaccine is "the best shot at protecting our community."

Mary Jean Spadafora: 206-464-2168 or mjspadafora@seattletimes.com