Could sewage reveal where bowel cancer is clustering before hospitals do?

What if the earliest warning sign of a cancer cluster wasn't a hospital record or a screening result, but something flushed down a toilet?

That question drove a small but suggestive study in Louisville, Kentucky, where researchers collected wastewater samples from four neighborhoods and tested them for a molecular marker linked to bowel cancer. The results, published in the Journal of Epidemiology and Community Health, showed that the marker tracked with known cancer rates - higher in neighborhoods with more diagnosed cases, lower where fewer cases were recorded.

It's a proof-of-concept study, not a finished surveillance tool. But it gestures toward something potentially valuable: a way to detect cancer burden at the community level without relying on individuals to show up for screening.

CDH1 in the sewer lines

The marker in question is CDH1, a gene associated with cell adhesion that's often disrupted in cancerous tissue. When cancer cells shed into the gut, fragments of their RNA can end up in feces and, eventually, wastewater. The researchers also measured a control marker called GAPDH (glyceraldehyde 3-phosphate dehydrogenase), a housekeeping gene present in all human cells, which served as a baseline for comparison.

The study targeted four specific sewersheds - defined drainage areas feeding into particular sewer lines - in Jefferson County, Kentucky. Three of these areas were selected because they contained geographical clusters of bowel cancer cases, defined as zones with more than four cases within a half-mile radius. The fourth area, where no patients from either the local tertiary care center or state cancer registry lived, served as a comparison site.

Wastewater samples of 175 milliliters were collected three times from each sewershed on a single day in July 2023, at 7:00, 10:00, and 13:00 hours. The timing aimed to capture variation across the morning and early afternoon, though a single day of sampling limits what can be concluded about consistency over time.

The numbers tracked, with caveats

All samples from all four sites showed detectable levels of both CDH1 and GAPDH. The average CDH1-to-GAPDH ratio - used to normalize for population differences - was highest in Cluster 1, at 20, compared to 4 in Cluster 3, 2.2 in Cluster 2, and 2.6 in the comparison area.

That Cluster 1 figure stands out. That neighborhood also had more than double the number of known bowel cancer patients per 100 residents compared to the other two cancer clusters, which helps explain the large gap. Still, the correlation between known case density and wastewater biomarker levels held across the four sites in a broadly consistent direction.

The comparison site's ratio of 2.6 wasn't zero, which the researchers acknowledge. Some residents in the control area may have been treated for cancer at facilities not captured by the tertiary care data. The CDH1 marker isn't exclusively a cancer signal, either - it's associated with cancer but not unique to it. Healthy tissue also sheds some CDH1-containing RNA, though presumably at lower levels.

Why wastewater, and why now?

The appeal of wastewater-based epidemiology is that it doesn't depend on individual compliance. Colonoscopies are effective at catching bowel cancer early, and stool-based screening tests have improved considerably. But participation rates remain stubbornly uneven. Communities facing economic barriers, limited healthcare access, or distrust of medical systems often have lower screening uptake - precisely the populations where early detection matters most.

Wastewater surveillance sidesteps that problem. Sewage collects biological data from everyone in a drainage area, regardless of whether they have a primary care physician or health insurance. The approach gained prominence during COVID-19, when wastewater monitoring for SARS-CoV-2 became a standard tool for tracking infection trends. Extending it to chronic diseases like cancer is a logical next step, though a considerably harder one.

Bowel cancer is the third most common cancer in the United States and the second leading cause of cancer-related death, with an estimated 154,000 new cases annually. Rates among younger adults have been rising for reasons that remain poorly understood, adding urgency to finding surveillance approaches that can complement existing screening methods.

The gap between signal and screening

The researchers are careful to frame this as early-stage work, and the limitations are substantial. A single day of sampling from four neighborhoods in one city does not establish that wastewater biomarkers can reliably predict cancer burden. The sample size is small. The geographical scope is narrow. The relationship between the CDH1-to-GAPDH ratio and actual new cancer cases - as opposed to prevalent cases already diagnosed - remains unclear.

There's also the question of specificity. Elevated CDH1 levels might reflect cancers other than bowel cancer, inflammatory conditions, or simply variation in who happened to use the bathroom before the sample was collected. Wastewater is a blunt instrument - it captures everything that enters the sewer system, and teasing apart the signal from the biological noise requires validation in much larger and more diverse populations.

The researchers did not attempt to identify individuals or track outcomes. The method, by design, operates at the community level. It can't tell you who has cancer. It can only suggest that a given area may have a higher burden than expected, which could then trigger targeted outreach or mobile screening efforts.

Seasonal variation, rainfall dilution, and differences in sewer infrastructure could all affect results. The study doesn't address these variables. A sewershed serving 5,000 people will produce different concentrations than one serving 50,000, and normalization methods for population are still being developed in the wastewater epidemiology field.

What would make this useful?

For wastewater cancer surveillance to move beyond proof of concept, several things would need to happen. Longitudinal studies - sampling the same sewersheds repeatedly over months or years - would be necessary to establish whether biomarker trends correlate with changes in diagnosed cancer rates. Larger studies across multiple cities and demographic contexts would test generalizability. And researchers would need to determine threshold levels: at what CDH1-to-GAPDH ratio should a public health response be triggered?

The researchers suggest that detecting elevated colorectal cancer markers across community zones, before high incidence appears in hospital or registry data, could help target areas for practical and cost-effective screening interventions. That's a reasonable hypothesis. Whether it pans out will depend on whether the signal holds up under more rigorous testing.

The researchers emphasized that wastewater surveillance could be particularly relevant given the recent trend of increasing colorectal cancer incidence in younger populations, a shift that existing screening guidelines - which typically begin at age 45 - are still catching up to. Community-level monitoring could flag emerging hotspots in age groups that aren't yet routinely screened, providing a signal that individual-based approaches would miss entirely.

The concept itself, though, is worth watching. Wastewater already tells public health officials about drug use patterns, antimicrobial resistance, and viral circulation in a community. Adding cancer biomarkers to that list is ambitious but not implausible. The infrastructure exists. The analytical techniques are maturing. The question is whether the biology cooperates - whether cancer biomarkers in sewage are stable enough, specific enough, and informative enough to be actionable.

So can your city's sewage reveal a cancer cluster? Not yet. But after this study, the idea is no longer purely theoretical.