Local, state and federal health experts are urgently trying to contain a large mumps outbreak raging across Iowa that now has spread to at least eight other Midwestern states.

At least 605 mumps cases have been reported in Iowa this year, far outpacing the five cases the state usually sees in a year and the 200 to 300 that typically occur nationally. The epidemic, the largest since a 1988 outbreak in Kansas, shows no sign of slowing, with at least 100 more cases in Nebraska, Kansas, Minnesota, Wisconsin, Indiana, Michigan, Missouri and Illinois.

Among other steps, the effort to stem the outbreak has led investigators to hunt down passengers on a March flight to Washington, D.C., that carried a woman who later developed mumps, and to vaccinate more than a dozen people she visited on Capitol Hill.

A public health official in Maine said Friday that an advisory has been sent to health care providers throughout the state. Dr. Andrew Pelletier, an epidemiologist with the Maine Centers for Disease Control and Prevention, said no suspected cases of mumps have been reported in Maine since last summer, when two unrelated cases were confirmed at residential summer camps. Both were “imported” from other countries. Before that, he said, the last known case here was in 1999.

Officials from the federal Centers for Disease Control and Prevention were in Iowa this week, helping to plan prevention and control strategies, but state health officials Thursday acknowledged the outbreak is still out of control. No deaths from the viral outbreak have been reported.

As health officials work to break the chain of transmission of the viral infection, disease detectives are trying to puzzle out what is causing it. Cases appear concentrated among young, otherwise healthy adults.

“Why Iowa and why now? We really don’t know,” said William Bellini of the federal Centers for Disease Control and Prevention in Atlanta, which has sent investigators to help control and investigate it. “There are a lot of unknowns.”

Among them: Does the mumps vaccine fail to “take” in more people than had been thought? Does its protection wane? Is the virus causing this outbreak less susceptible to the vaccine for some reason, or more infectious?

The outbreak has underscored that old infectious diseases that most people rarely think about – and doctors don’t often encounter – can suddenly re-emerge.

“This shows us once again that the world is a very small place,” said William Schaffner, an infectious disease expert at Vanderbilt University. “We have to remain on guard, even about diseases we usually rarely see anymore in this country.”

To try to keep the outbreak from widening further, the CDC announced Wednesday that officials were trying to track down 222 airline passengers who sat near two people from Iowa who developed the mumps after traveling on nine flights between March 26 and April 2. One flew from Tucson, Ariz., to Cedar Rapids, Iowa, stopping in Arkansas and St. Louis. The second, a 51-year-old woman on a trip to lobby the Iowa congressional delegation, flew from Waterloo, Iowa, to Washington and back, stopping in Detroit and Minneapolis.

About 17 people who came in contact with her on Capitol Hill have been given vaccine shots as a precaution, health officials said.

“The states where these passengers traveled and landed we’ll be watching closely and putting prevention and control activities in place if they start to see some cases,” said Jane Seward of the CDC.

In the states that already have cases, officials are alerting the public to the symptoms in hopes that anyone who has been infected can be identified quickly and isolated. They also are trying to identify and inoculate unvaccinated adults.

“We’re doing everything we can to get out in front of this outbreak and try to get it stopped,” said Patricia Quinlisk, Iowa’s state epidemiologist. “Mumps can produce severe complications, and we’re trying to do whatever we can to prevent them from happening.”

About a dozen patients in Iowa have been hospitalized so far, including one who was treated for encephalitis, but all have recovered, officials said.

Much of the effort has been focused on college campuses, where many of the cases are occurring.

Officials speculate that the epidemic might have been sparked by someone from Britain, which has been experiencing a large mumps outbreak for several years. Last summer, a much smaller outbreak at a summer camp in New York was started by an unvaccinated British counselor.

“We suspect that there was some importation into Iowa, and initially perhaps it wasn’t recognized immediately because physicians don’t see mumps much anymore,” Seward said. “That’s how these things can get going.”

The virus in the Iowa outbreak is the same strain as in the United Kingdom. But officials have not yet identified the first Iowa case.

“We’re not looking back right now,” Quinlisk said. “We’re focused on stopping the transmission.”

Experts hope the relatively high U.S. vaccination rates will contain the outbreak. The tens of thousands of cases in Britain have been blamed on problems with that country’s vaccination program, and concerns among some parents that childhood vaccines may increase the risk of autism, which left a significant proportion of the population unvaccinated.

“The United States, fortunately, generally has good vaccine coverage,” said Stanley Plotkin, an infectious disease expert from the University of Pennsylvania.

Maine has among the highest rates of compliance with recommended vaccine administration in the country, with 93.8 percent of entering kindergartners in 2003 having received two doses of vaccine, which is thought to provide lifetime immunity. Still, state epidemiologist Kathleen Gensheimer said Friday that the current mumps outbreak serves as a reminder to all parents to “take vaccine-preventable diseases seriously” and get their children immunized.

The vaccine is believed to be only about 95 percent effective, meaning that even among 100 fully vaccinated people five would still be vulnerable, enough to allow an outbreak to occur.

But the size of the current episode has experts wondering whether the vaccine is actually less effective than that, and whether the immunity wanes over time. About 68 percent of the cases have occurred among people who received the two recommended doses of vaccine as part of their routine childhood immunizations.

“That’s a very pregnant question in this outbreak because such a large proportion of the individuals affected have a history of having had two immunizations,” Schaffner said.

Investigators are gathering detailed information about how many shots the infected people got and how far apart they got them to figure out why they lacked immunity.

“Does it last a lifetime? As far as we’re aware it does. But that’s something we’re interested in studying,” Seward said.

BDN writer Meg Haskell and The Associated Press contributed to this report.