A case of measles in Crow Wing County has grown to a cluster of four as members of the same family were infected, state officials said Friday.

That brings the total case count in Minnesota's outbreak to 54, an increase of 10 since last Friday.

Minnesota Health Department officials said the increase was expected, as all of the Crow Wing cases involve children who were not vaccinated against the measles.

The first Crow Wing County case was announced last week and is linked to the ongoing outbreak because that child had visited places in Hennepin County where other children had been exposed.

Health officials are watching for other connected cases to appear because the family involved had also traveled to other parts of the state and may have exposed other unvaccinated children, according to Kris Ehresmann, the Health Department's infectious disease control director.

The current outbreak was first detected April 11 and has grown into the state's largest since 1990.

Measles is a highly contagious disease that spreads easily within groups of people who haven't been immunized against it. Measles symptoms include a cough, runny nose, watery eyes, fever and a body rash. Serious cases can lead to pneumonia or swelling of the brain and cause lasting health damage. In some outbreaks, infections have also lead to death. The 1990 outbreak caused three deaths.

The current outbreak started in child care centers in Hennepin County frequented by Somali-American families, whose members have low immunization rates for the measles vaccine. State health officials believe it originated with someone who had traveled abroad, because measles no longer occurs naturally in the United States.

Hennepin County now has 48 cases, with another two in Ramsey County.

Health officials urge anyone in the affected counties who lacks immunity to get the MMR vaccine, which protects against the measles, mumps and rubella. That recommendation extends to adults, too, as three adults have been sickened in the outbreak.

Officials also recommend that children in affected counties, as well as all Somali-American children, who have had just one of the two recommended MMR shots to get the second shot on an accelerated schedule. Typically the shots are given around age 1 and then around age 5.