The first probable case of the mumps has been reported in Island County, said Dr. Brad Thomas, county health officer.

The county’s sole probable mumps patient is a 2-year-old boy who wasn’t in any group environments, he said.

However, the diagnosis is uncertain because the mother did not return with the boy for further testing. “She dropped off the radar,” Thomas said.

“There have been two other folks evaluated that were negative for mumps,” he added.

Statewide, mumps cases are mounting at a record rate, primarily in King, Pierce, Snohomish and Spokane counties. As of Thursday, the Washington Department of Health reported 738 cases.

Most likely, Island County has dodged the outbreak because of its proximity and population.

Washington’s cases are mainly clustered around certain sub-populations, such as college dorms, fraternities, sororities and other places where people live in close contact to one another, Thomas explained.

Where the Island County toddler was exposed isn’t known.

“That’s the mystery,” Thomas said, “and partly the reason it’s not a 100 percent diagnosis.”

Why mumps has taken hold statewide is confounding health experts. Parents choosing not to have their elementary school children vaccinated may be one factor.

Only one confirmed case of the mumps was reported each year in Washington state from 2009 to 2012. Two to six suspect and probable mumps cases were reported each year in more recent years.

Nationally, mumps cases rose to 5,311 cases last year, the second-highest annual case count of mumps in more than a quarter-century, according to the Centers for Disease Control and Prevention.

Mumps protection is provided in the combination MMR vaccine together with measles and rubella.