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State health officials confirmed two additional measles cases on Wednesday, bringing the total to 11 in an outbreak first detected last week.

With the case count still rising, public health officials have asked more than 200 people to voluntarily quarantine themselves if they might have been exposed to the highly contagious virus. And Twin Cities medical clinics said they are seeing a growing number of patients who want advice about how to protect their children.

So far, all 11 cases involved children ages 1 through 5, and nine were children known to have been unvaccinated. All the cases have occurred in Hennepin County, and nine occurred in Somali-American families.

The outbreak, the largest to hit the state since 2011, was first identified early last week and has sent several children to the hospital.

“People are asking about the measles vaccine, both from the Minnesota Somali community [and] from other families ... that are concerned about the outbreak,” said Dr. Kristina Whitesell, a pediatrician at Fairview University Children’s Clinic in Minneapolis.

“People forget that measles is a deadly disease,” she added. “We were lucky in 2011 that there were no reported deaths.”

State and county public health investigators are now contacting people who might have been exposed to the 11 stricken children. Those with exposure but no immunity protection are being asked to stay at home for three weeks, the length of time it can take for measles symptoms to develop.

“We really need people to adhere to those guidelines to protect the people around them,” said Dave Johnson, manager of epidemiology for Hennepin County Public Health.

Johnson said the county’s chief concern is gathering places such as child care providers, schools, workplaces and places of worship. The measles virus is easily spread. Studies have shown that an unvaccinated person in proximity to someone with measles stands a 90 percent chance of getting infected.

Because of that, some of those quarantined include entire families. The county then follows up over the next 21 days to see if they have developed symptoms, Johnson said.

Until these quarantines play out, the number of measles cases could continue to grow.

Solving a puzzle

Health investigators haven’t identified the source of the outbreak, which probably was imported by an infected person who had traveled to a foreign country. Measles was declared eradicated in the United States in 2000, and it no longer occurs naturally here.

Some of the children who developed measles had been in child care centers before they were diagnosed, but health officials are investigating other sites as well, and interviewing the new cases to track their movements and potential places of exposure.

“Most of the cases were exposed through our earliest identified case,” said Kris Ehresmann, infectious disease division director at the Minnesota Health Department. Investigators believe that person was infected around March 30. But because that individual has not been linked to foreign travel, the original source of the outbreak remains unclear.

Investigators have also examined the genetic makeup of the virus causing the local outbreak, but the results could not conclusively link it to a strain in a particular country or region as a possible source.

Although measles is very rare in the United States, public health officials take any outbreak seriously because the virus is extremely contagious among people without vaccinations and, in severe cases, can cause lasting lung and brain damage.

Measles symptoms include high fever, cough, runny nose and watery eyes and a rash that spreads from the head to the rest of the body. There is no medical cure for infection, and severe cases can lead to death or permanent side effects, which is why public health officials recommend vaccination.

“This isn’t the first time that we’ve had measles spreading in an unvaccinated population,” said Johnson. “We want to remind people that the measles vaccine is very effective and important to the health of people around them.”

Glenn Howatt • 612-673-7192