Sewage Tracked Ontario's Measles Outbreak but Sounded No Early Alarm

Wastewater surveillance run alongside clinical testing through Ontario's 2025 measles outbreak followed the epidemic closely but did not warn of it in advance, a team from the University of Windsor, the University of Ottawa and Canada's National Microbiology Laboratory reports in the CDC journal Emerging Infectious Diseases.

In the paper, the authors describe testing sewage as a complement to counting cases, valuable above all for finding measles among people who are undervaccinated or unlikely to seek care. Case reports alone can miss those infections.

Ryland Corchis-Scott and colleagues sampled the Leamington Pollution Control Centre in the Windsor-Essex region, which serves about 30,000 people, three times a week from February to November 2025. The measles signal in the sewage rose and fell with confirmed cases in the town. Compared week by week, though, it trailed the clinics by roughly a week rather than leading them, and it gave no early alert of a change in incidence. The authors add that they began sampling in reaction to a clinical case, so the study draws no conclusion about whether sewage can flag an outbreak's onset: a later search of frozen archived samples turned up no measles RNA from before the first case was identified.

The paper's account of the outbreak puts Ontario's 2025 total at about 2,400 confirmed cases and records Canada's loss of measles elimination status in November 2025. The authors name reduced vaccination coverage and vaccine misinformation among the likely causes.

Two method results came out of the work. The team adapted a test that distinguishes the vaccine strain from the virus circulating in the outbreak and detected the vaccine strain in Leamington's wastewater about six weeks after a vaccination campaign began among agri-food workers housed in bunkhouses. It also found measles RNA spread mainly through the liquid part of wastewater rather than the settled solids, which the authors say makes the liquid the more productive fraction to concentrate when testing for the virus.