<i>Notes from the Field:</i> Wastewater Surveillance for Measles Virus During a Measles Outbreak — Colorado, August 2025

Measles, a highly transmissible vaccine-preventable respiratory virus, can cause severe illness and result in hospitalization or death.*During March-July 2025, Colorado reported 16 confirmed measles cases while measles outbreaks were occurring in neighboring New Mexico, Texas, and Utah (1); the first five Colorado cases were confirmed by late April.Measles virus RNA shed in feces and urine can be detected in wastewater (2), and sequencing of the nucleoprotein gene can identify the wild-type measles lineage genotype.Detection of measles virus in wastewater can precede clinical case reporting (3), and evidence of the value of supplementing clinical case reporting with wastewater surveillance is growing (4).On May 1, wastewater surveillance testing for measles virus was implemented in Colorado.In early August, the Colorado Department of Public Health and Environment (CDPHE) identified measles virus in a wastewater sample from a Mesa County wastewater treatment plant, providing local public health agency authorities with an early indicator of possible community transmission.During the next 4 days, two measles cases were reported among residents served by the same wastewater treatment plant where measles virus detection had occurred.This report describes the detection of measles virus through wastewater surveillance in Mesa County and its contribution to the subsequent outbreak response. Investigation and Outcomes Colorado Wastewater Surveillance ProgramThe Colorado wastewater surveillance program was established in 2020 in response to the COVID-19 pandemic.The wastewater surveillance program monitors wastewater statewide for respiratory viruses and emerging pathogens using digital polymerase chain reaction (dPCR). On May 1, 2025, after the identification of five measles cases in the state, the wastewater surveillance program initiated a measles surveillance pilot project to supplement statewide clinical measles surveillance.Sampling occurs twice weekly at 21 sentinel wastewater treatment facilities across the state.