Three hundred and fifty-five cases have been reported in the ongoing measles outbreak in Texas, New Mexico and Oklahoma, but local officials say that it’s an undercount and that the virus could continue to spread spread for months.

Measles cases in West Texas are still rising two months after the outbreak began. Local public health officials say they expect the outbreak to last for at least several more months and the case number is likely an undercount.

The Texas measles outbreak has grown to 309 confirmed cases, not counting dozens of additional cases in neighboring states. Cases have been identified in three additional counties, according to the Texas Department of State Health Services' Friday update.