The Texas Department of State Health Services has issued guidance to schools as the measles outbreak in Gaines County rises to 15 cases, mostly in children.

Two dozen cases of measles have been reported in a small West Texas county that has one of the state’s highest vaccine exemption rates. In Gaines County, which has a population of about 22,000, 24 cases of the airborne infectious disease have been reported since late January. Most of them are unvaccinated, school-aged children.

Public health authorities said Tuesday that an outbreak of measles in western Texas has expanded, while a new case was confirmed nearby across state lines in New Mexico.