SEQUIM — A whooping cough outbreak that has infected people in Jefferson County has reached Clallam County, with five confirmed cases in Sequim.

Dr. Jeanette Stehr-Green, Clallam County interim health officer, announced the news to the county Board of Health on Tuesday.

Neighboring Jefferson County, which has had an ongoing outbreak since March, has had 27 confirmed cases of whooping cough, also known as pertussis.

Whooping cough is a serious respiratory infection caused by the pertussis bacteria, which begins with cold-like symptoms and develops into violent coughing that can cause pneumonia, seizures and brain damage, and can be fatal for infants younger than 1 year.

Diagnoses have been confirmed this summer in five people, all from one family in two households in Sequim, Stehr-Green said.

The illness first appeared in a 55-year-old Sequim resident June 1, followed by members of the same household, who became sick later that month, she said.

The other members of the household were a 39-year-old adult who began showing symptoms June 18, a 12-year-old who became sick June 28 and a 10-year-old whose symptoms began June 29.

Stehr-Green said they remained at home self-quarantined and did not immediately seek medical attention.

The four younger members of the family went to a Kitsap County clinic this week, where they were diagnosed and the infections were reported to the Clallam County Health Department, Stehr-Green said.

A 62-year-old woman, grandmother of the children, began having symptoms July 9 and saw a Clallam County doctor, who diagnosed her with whooping cough, Stehr-Green said.

She said the two children were not vaccinated by parental choice and the two adults in the household did not know their vaccination history.

The 64-year-old grandmother did not know she should be vaccinated, Stehr-Green said.

Time from exposure to onset of infection can be from five to 21 days, and debilitating symptoms can last more than three months.

One of the primary symptoms of pertussis is extreme coughing, leading to nausea or “turning blue” from oxygen depravation.

“It’s also known as ‘the hundred days’ cough,’” she said.

Lisa McKenzie, communicable disease nurse for Jefferson County Public Health, said the disease can be treated with antibiotics in early stages for those who know they have been exposed.

Early treatment can reduce or eliminate the long, lingering cough, McKenzie said.

There have been 37 total cases in Jefferson County since March, according to laboratory testing, but early treatment stopped the cough in 11 people before their illness reached the Centers for Disease Control and Prevention requirement of two weeks of coughing to be classified as a confirmed case, she said.

McKenzie said the rate of new infections has slowed since school was let out for summer.

Only one new case was reported this month, she said.

The CDC recommends diphtheria and tetanus DTaP (diphtheria, tetanus and pertussis) vaccinations at age 2 months, 4 months, 6 months and 18 months, and a booster between the ages of 4 and 6 years old.

Stehr-Green said pregnant women also should be vaccinated because the illness can last three months or more, and a pregnant mother may give her child pertussis immediately after childbirth.

Adult members of a family who will be exposed to an infant should also be vaccinated, and older children should get a booster, she said.

The vaccine is most effective in the first year after vaccination; after a year, its effectiveness drops to about 50 percent, Stehr-Green said.

After five years, the pertussis vaccine continues to reduce the severity of a pertussis infection but is not as effective at preventing infection, she said.

Stehr-Green said boosters are recommended after five years to renew immunity.

The state outbreak has reached 897 cases in Washington state, compared with194 cases during the same time period in 2014.

Statewide, the outbreak is most common among teens ages 14-18, 323 teens representing 36 percent of the confirmed cases, followed by 101 early adolescents age 10-13, with 20.2 percent, and 132 children age 5 to 9, representing 14.7 percent of those who have become ill.

A total of 55 babies 1 year or younger have been infected, representing 6.1 percent, and 11 were hospitalized, according to the health report.

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Reporter Arwyn Rice can be reached at 360-452-2345, ext. 5070, or at arice@peninsuladailynews.com.