Minnesota health officials have confirmed a ninth case of measles in the Hennepin County outbreak that began last week, and they expect the count to rise as additional lab specimens are tested.

The patients, all children, were not vaccinated. Most of the cases have occurred in the Twin Cities Somali-American community, where vaccination rates have been relatively low.

“Whenever you have an outbreak in unvaccinated kids, measles just has a field day,” said Kris Ehresmann, infectious disease division director at the Minnesota Health Department. “That is what is happening here,”

The state’s health laboratory is testing additional samples, and Ehresmann said she expects the case count to rise. The outbreak is the state’s largest since 2011, when 26 cases were recorded.

In the meantime, public health investigators are interviewing families of the affected children, trying to pin down where they might have been exposed.

State and local health officials are contacting schools, mosques, clinics and child care centers that cater to the Somali-American population to educate them about measles symptoms and the importance of the MMR vaccine, which provides protection against measles, mumps and rubella.

Ehresmann said the latest estimates show that only 42 percent of 2-year old Somali-American children had received their first MMR shot, which most health experts recommend at age 1. A second shot is administered when a child enters kindergarten, and produces highly effective immunity. Statewide, the vaccination rate is 88.5 percent.

Low vaccination rates were a factor in a 2011 measles outbreak, including many Somali-American children. At the time, an anti-vaccine advocate was telling community members that the MMR vaccine caused autism — a view that has been discredited by extensive scientific research.

“A lot of the anti-vaccine groups targeted the Somali community,” Ehresmann said.

The source of the current outbreak has not been identified yet, but it most likely involved a traveler to a foreign country who came back with a case of measles, which no longer occurs naturally in the United States.

Several child care centers have been identified as likely transmission points, according to Ehresmann, but it is also likely that the infection was passed on in other settings as the children moved from place to place. So far, all of those infected have been between 1 and 4 years of age.

In the 2011 measles outbreak, exposure occurred in day cares, homeless shelters, medical clinics and in households. That outbreak was Minnesota’s largest over the past 20 years.

It can take up to three weeks for symptoms to develop after a patient is infected, meaning that part of the investigation involves waiting to see if all of the children that were potentially exposed show signs of sickness.

Tests for the measles are usually not reliable until a person develops the telltale rash that starts at the head and spreads to the rest of the body. Before then, measles symptoms almost mimic those of a cold, including runny nose, cough, watery eyes and a fever.

Children hospitalized

One of the first measles cases detected in the current outbreak began at Children’s Hospitals and Clinics of Minnesota, where clinicians initially thought a child was having a skin reaction to an antibiotic, according to Patsy Stinchfield, a nurse practitioner and senior director of infection control.

So far Children’s has hospitalized and discharged five of the known cases, Stinchfield said, and another three possible cases are hospitalized and awaiting test results.

None of the cases so far has required hospital intensive care. Among other complications, measles can cause serious damage to the lungs and brain.

In 2015, 134,000 measles-related deaths were reported worldwide. The last death in the U.S., where vaccination rates are generally high, was in 2015.

Stinchfield said health officials stress vaccination because measles can be a very serious disease and “because there is no medicine.” Because there is no treatment, hospitals try to stabilize patients by giving them fluids to counter dehydration — most are usually too sick to eat or drink. They are kept in special hospital rooms that prevent the virus from infecting others in the hospital.

Children’s is also following up with those who might have been exposed on campus to make sure none are vulnerable to contracting the disease.

“The measles virus is one of the most highly contagious viruses that we have,” Stinchfield said. “It can be passed just by breathing so it is easily spread.”