Public health officials in Cascade County said Wednesday they are investigating one case of measles in a local adult resident who spent time at a local business when they were likely contagious.

As of publication time, the county website listed several locations for public exposure, including the Great Falls Walmart at 5320 10th Ave. S, the Rib & Chop House at 21 Third St. N, and the Great Falls Clinic Walk-in at 3000 15th Ave. S.  Any individual who frequented those locations during the listed times “is considered to have been exposed to measles,” the website said.

The incident is the county’s first confirmed case of measles, a highly contagious and deadly disease that resurfaced in Montana this year for the first time in three decades amid a nationwide outbreak. 

The state has reported 26 total cases since April across six counties: Gallatin, Flathead, Yellowstone, Lewis and Clark, Hill and now Cascade. Two people have been hospitalized for the disease and no deaths have been reported.

A person who has been exposed to measles can spread the virus four days before they develop a rash — one of the first physical signs of infection — and can remain contagious for several weeks. Symptoms may not appear for up to 21 days after exposure, giving a person’s transmissibility a long tail. 

Health experts in Montana and elsewhere in the U.S. have stressed that the best protection against the measles virus for adults and children is the MMR vaccine, a long-lasting immunization that is typically recommended for infants between 12 and 15 months of age. In some circumstances of community transmission, health officials in Montana have recommended an earlier vaccination schedule for young infants.

In a Wednesday press release, the Cascade City-County Health Department said the newly confirmed case is an adult who is reportedly vaccinated, but that their immunization history had not been confirmed.

The health department reiterated that vaccination or other forms of protection are critical for preventing illness. 

“Individuals who are immune to measles through vaccination, laboratory evidence of immunity, laboratory confirmation of disease, or were born before 1957 are not considered at risk for infection. Individuals without prior measles infection or vaccination have a 90% likelihood of contracting the disease if exposed,” the release said.

If a person is not sure about their MMR vaccination status, the health department suggested requesting vaccination records from their former high schools, colleges, universities, doctor’s offices, pediatricians, or the health departments for the towns in which they grew up.

Montana is the only state in the nation that does not publicly share data about county-by-county vaccination levels, particularly among young children who are most at risk for adverse effects from measles. An existing data collection program was scrapped after the state legislators passed a law striking that language from code in 2021.

A recent federal survey of 127 Montana children born in 2021 put the state’s estimated measles vaccination rate among toddlers at 85.5%, one of the lowest figures in the country.