Notes from the Field: Retrospective Analysis of Wild-Type Measles Virus in Wastewater During a Measles Outbreak — Oregon, March 24–September 22, 2024

Investigation and Outcomes

Data Collection

Oregon began wastewater surveillance for SARS-CoV-2 (the virus that causes COVID-19) in 2020. Routine wastewater surveillance now includes SARS-CoV-2, influenza, respiratory syncytial virus, influenza A(H5), and, since October 2025, wild-type measles virus. Unlike vaccine-derived measles virus, wild-type measles virus is transmitted from person to person and can cause outbreaks. Wastewater surveillance activities include collecting and archiving 24-hour composite samples from up to 40 wastewater treatment facility influents statewide once or twice weekly (1). To ascertain the presence of wild-type measles virus in wastewater in the outbreak-affected area, archived specimens collected during March 19–September 26, 2024, were retrospectively tested during July and August 2025 from four communities in the two counties with outbreak cases. The study period initially ranged from April 30 through September 26, which was two incubation periods (42 days) before illness onset for the first case through two incubation periods after onset for the last case. When wild-type measles virus was detected in samples from the first week of the study period (on May 5, 2024), the beginning of the study period was extended to include four incubation periods before illness onset for the first case (i.e., March 19–September 26) (2).

Processing of Samples

Filtered, preserved 24-hour composite wastewater influent samples were homogenized using bead-beating with 0.7-mm garnet beads to lyse the cells. Nucleic acids were extracted from 200–400 μl of the homogenate and analyzed for a wild-type–specific measles virus target using reverse transcription–digital polymerase chain reaction (RT-dPCR) (1,3). Detections were defined as samples with a viral concentration above the assay limit of detection, which was calculated based on the assay limit of blank.^† Data were analyzed in RStudio (version 4.3.1; RStudio, Inc.). This activity was reviewed by the Oregon Health Authority, deemed not research, and was conducted consistent with federal law.^§

Detections of Wild-Type Measles Virus in Wastewater

Among 94 analyzed samples collected during March 19–September 26, 2024, a total of 20 (21.3%) tested positive for wild-type measles virus (Figure). The first detection of measles virus in wastewater was in a sample collected on April 3. Wastewater detections preceded reported cases by approximately 10 weeks. After scattered detections of measles virus at low concentrations (i.e., above the limit of detection but fewer than three positive dPCR partitions) in both counties, a period during which measles virus was detected at higher concentrations (i.e., above the limit of detection and three or more positive dPCR partitions) occurred during June 12–July 23, corresponding with the first reported clinical cases. The last sample in which measles virus was detected was collected on July 24. Overall, 11 (55%) of the 20 measles virus detections were in samples collected during the outbreak period. No virus was detected in wastewater during the last 9 weeks of the outbreak. After the last detection of wild-type measles virus in a wastewater sample, an additional eight cases were reported.

Preliminary Conclusions and Actions

Wastewater surveillance can provide an early warning signal for emerging pathogens, including measles, independent of health care–seeking behavior and access to testing (1). In this retrospective study, wastewater detection of wild-type measles virus preceded the first reported case by 10 weeks. A 6-week period of higher concentrations of measles virus in samples corresponded to the outbreak peak.

The findings in this study are subject to at least two limitations. First, because the measles viruses detected in the wastewater samples were not sequenced, whether all detections were epidemiologically linked to the outbreak is unknown. Second, the absence of measles virus detections in wastewater samples does not rule out the presence of measles in a community, as evidenced by identification of eight cases after the last wastewater sample tested positive for measles virus.

During 2025, the United States experienced the highest number of measles cases since elimination was declared in 2000. To prevent transmission, systems to rapidly identify, isolate, and investigate suspected measles cases, as well as high population immunity, are needed (4). This study highlights the usefulness of wastewater surveillance as an early warning of measles in a community, with the potential to detect community transmission before the first cases have been identified. Wastewater surveillance can alert clinicians and the public to a current measles risk in the community, guide health care system screening procedures and testing practices, and direct important individual-level protective behaviors, including vaccination.