Bird Flu Was in Your City’s Sewers First — But Was It Humans, Cows, or Wild Birds? The $12B Detection Mistake Public Health Can’t Afford in 2026
Wastewater can catch H5 early—but it can’t label the source. A sewer signal might mean human infection, or it might be milk, animal waste, or birds.
By TheMurrow Editorial
March 25, 2026
Key Points
- 1Recognize the limit: CDC says H5 wastewater detections can’t identify whether RNA comes from humans, animals, or animal products like milk.
- 2Interpret carefully: NWSS reports “influenza A(H5) viral RNA,” and H5 alone doesn’t automatically confirm H5N1; subtyping display began August 2024.
- 3Investigate locally: CDC MMWR found non-human contributors, including eight milk-processing inputs—plus wildlife pathways that can “spoof” sewer signals.
Sewers have a talent for telling the truth before anyone is ready to hear it.
During COVID, that often meant a clean, intuitive story: rising viral RNA in wastewater, followed by rising cases in clinics. With influenza A(H5)—the subtype that includes the widely watched H5N1 bird flu—the story is murkier. A positive sewer signal can precede clinical reporting, yes. But it can also come from places we don’t usually picture when we think about “infection”: a milk-processing facility, animal waste, or even wild birds roosting near water and storm runoff.
The CDC has been unusually blunt about this. In its guidance for the National Wastewater Surveillance System (NWSS), the agency warns that wastewater testing cannot determine whether the virus originates from humans vs animals vs animal products (like milk)—and that a detection does not necessarily mean people in the community are infected.
That nuance is not a footnote. It is the whole story.
“A positive H5 signal in wastewater can be an early warning—or a red herring. The difference lies in the source.”
— — TheMurrow (Pullquote)
Wastewater can spot H5 early—but it can’t tell you “who”
Wastewater surveillance is powerful because it aggregates what a community sheds, flushes, and drains. For respiratory viruses, that can offer a quicker, less biased signal than clinic-based testing, which depends on who seeks care and what gets tested.
The CDC acknowledges that wastewater can detect influenza A/H5 genetic material before clinical reporting. Yet the agency also emphasizes that interpreting H5 detections differs sharply from COVID-era habits. The core problem is attribution: H5 RNA in sewage can come from humans, animals, or animal products—and wastewater methods alone cannot separate those possibilities. The NWSS page states this directly: a detection does not automatically imply community human transmission.
The CDC acknowledges that wastewater can detect influenza A/H5 genetic material before clinical reporting. Yet the agency also emphasizes that interpreting H5 detections differs sharply from COVID-era habits. The core problem is attribution: H5 RNA in sewage can come from humans, animals, or animal products—and wastewater methods alone cannot separate those possibilities. The NWSS page states this directly: a detection does not automatically imply community human transmission.
What NWSS is actually reporting
NWSS publicly displays “influenza A(H5) viral RNA detections.” That phrasing matters. H5 is a subtype, and H5 detection alone does not automatically confirm H5N1, because more than one H5 virus exists. The CDC notes that subtyping results began being publicly displayed in August 2024, reflecting a system still expanding what it can confidently distinguish.
The practical consequence is that wastewater is best understood as a signal, not a diagnosis. It can tell public health officials that something carrying H5 genetic material is entering the sewer system. It cannot, by itself, tell you whether that “something” came from infected people, from a dairy pipeline, or from birds overhead.
The practical consequence is that wastewater is best understood as a signal, not a diagnosis. It can tell public health officials that something carrying H5 genetic material is entering the sewer system. It cannot, by itself, tell you whether that “something” came from infected people, from a dairy pipeline, or from birds overhead.
Why COVID comparisons mislead
Many readers remember wastewater as a clean proxy for human infection. That mental model becomes risky with H5. COVID wastewater trends largely reflected human fecal shedding in a human-only pandemic. H5 is different because the U.S. is tracking a virus with major animal reservoirs, plus industrial pathways that can move viral RNA into wastewater without any person being sick.
“Wastewater doesn’t lie. But it also doesn’t label its sources.”
— — TheMurrow (Pullquote)
What the CDC found in 2024: H5 in sewers often had non-human explanations
The most revealing federal evidence arrived in a CDC MMWR analysis covering May 12–July 13, 2024, during the early phase of the U.S. dairy-cattle event. Investigators looked at influenza A and H5 signals in wastewater and then did what many dashboards can’t: they tried to explain them.
The key finding was not a dramatic surge of hidden human infections. Instead, among wastewater sites with H5 detections, investigators frequently identified plausible non-human contributors within or near the sewershed. The MMWR specifically reports that identified animal sources included “eight milk-processing inputs.” That is a striking data point because it anchors a theoretical concern—industrial dairy pathways—into documented field investigation.
The key finding was not a dramatic surge of hidden human infections. Instead, among wastewater sites with H5 detections, investigators frequently identified plausible non-human contributors within or near the sewershed. The MMWR specifically reports that identified animal sources included “eight milk-processing inputs.” That is a striking data point because it anchors a theoretical concern—industrial dairy pathways—into documented field investigation.
May 12–July 13, 2024
The CDC MMWR surveillance window that closely examined influenza A and H5 wastewater detections during the early U.S. dairy-cattle event.
8
CDC MMWR-reported “milk-processing inputs” identified as plausible non-human contributors at wastewater sites with H5 detections.
The scale of surveillance—and why that matters
The same MMWR makes clear the system’s reach: wastewater influenza A testing was being conducted at scale, with hundreds of sites across many states, and H5 subtyping submissions ramped up in mid-2024. Scale cuts both ways. A broad network catches more signals early, but it also captures more “noise” from unexpected inputs—especially when a virus is circulating widely in animals and animal products.
A different standard of proof
The MMWR’s interpretive posture is worth underlining. H5 detection in wastewater is treated as a prompt for contextual investigation—local animal outbreaks, industrial discharges, relevant facilities—not as a stand-alone indicator of a human outbreak. That is a subtle but profound shift from how many people learned to read wastewater charts in 2020 and 2021.
For readers, the lesson is simple: a wastewater alert is not a verdict. It is a starting question.
For readers, the lesson is simple: a wastewater alert is not a verdict. It is a starting question.
Why dairy changed the sewage signal: milk can be “louder” than people
If the H5 wastewater story feels confusing, dairy provides the missing clarity. Multiple authoritative sources converge on one operational reality: infected lactating dairy cows shed high concentrations of H5N1 viral RNA in milk, making milk an unusually strong source of detectable genetic material.
A CDC Emerging Infectious Diseases report describes milk as an “ideal sample source” because virus is shed in high concentrations in milk, and it highlights bulk milk tank sampling as an efficient approach for herd-level surveillance. That sentence carries a secondary implication: if milk is ideal for surveillance because it is rich in viral RNA, it is also ideal for seeding wastewater detections when milk enters disposal or processing streams.
A CDC Emerging Infectious Diseases report describes milk as an “ideal sample source” because virus is shed in high concentrations in milk, and it highlights bulk milk tank sampling as an efficient approach for herd-level surveillance. That sentence carries a secondary implication: if milk is ideal for surveillance because it is rich in viral RNA, it is also ideal for seeding wastewater detections when milk enters disposal or processing streams.
How milk ends up in wastewater
Wastewater systems don’t only collect toilet flushes. They also collect industrial effluent and facility discharges. The CDC’s MMWR finding of eight milk-processing inputs among potential contributors illustrates one plausible pathway: milk handling and processing may introduce viral RNA into sewers even when no one in the community is infected.
A real-world public health example echoes that mechanism. Southern Nevada’s 2024–25 influenza wrap-up report documents H5 detections in wastewater where investigation identified a milk processing facility within the sampling boundaries that was receiving milk—an obvious candidate source.
A real-world public health example echoes that mechanism. Southern Nevada’s 2024–25 influenza wrap-up report documents H5 detections in wastewater where investigation identified a milk processing facility within the sampling boundaries that was receiving milk—an obvious candidate source.
Why this matters for public interpretation
Milk’s outsized viral RNA loads skew intuition. Human infection is the highest-concern scenario, but humans may not be the most efficient contributors of detectable RNA in a mixed sewer system. A relatively small industrial stream can produce a strong wastewater signal, potentially dominating what a dashboard shows.
“When milk carries high viral RNA, a single facility can shape a city’s wastewater chart.”
— — TheMurrow (Pullquote)
Wild birds can “spoof” wastewater signals—especially where water and infrastructure meet
The third major contributor is the one most people never picture: birds. Oregon-focused research, covered by Oregon State University, argues that wild birds can account for much of the avian influenza evidence found in wastewater. In other words, wastewater detections do not automatically indicate human cases, poultry outbreaks, or dairy-cattle infections.
That claim is not mere speculation. A peer-reviewed Oregon surveillance paper in JAMA Network Open (covering a surveillance window spanning July 2024–February 2025) underscores the interpretive challenge: wastewater methods do not distinguish human vs animal contributors. Oregon observed H5 positives in wastewater even though there were no dairy-cattle outbreaks in the state during that surveillance window.
That claim is not mere speculation. A peer-reviewed Oregon surveillance paper in JAMA Network Open (covering a surveillance window spanning July 2024–February 2025) underscores the interpretive challenge: wastewater methods do not distinguish human vs animal contributors. Oregon observed H5 positives in wastewater even though there were no dairy-cattle outbreaks in the state during that surveillance window.
July 2024–February 2025
The Oregon wastewater surveillance window reported in JAMA Network Open that highlighted the challenge of distinguishing human vs animal contributors.
How birds plausibly enter the sewer story
Wild bird contribution is plausible in communities where birds congregate around waterways, wetlands, and areas that interface with stormwater and wastewater infrastructure. Feces can enter runoff, runoff can enter collection systems, and detection can follow.
None of that makes wastewater useless. It makes it more honest: wastewater tells you what’s present in the environment feeding into the system, not what’s happening in a clinic exam room.
None of that makes wastewater useless. It makes it more honest: wastewater tells you what’s present in the environment feeding into the system, not what’s happening in a clinic exam room.
A fair reading of the bird hypothesis
Skeptics sometimes bristle at the idea that wild birds could account for “much” of the signal, worrying it downplays legitimate risk. The stronger interpretation is more balanced: bird inputs expand the range of plausible explanations, and they make context indispensable. A detection in a state with no dairy event is not meaningless; it is a prompt to ask whether wildlife could be the source.
Reading an H5 wastewater detection like a professional, not a panicked citizen
Public health agencies have learned—sometimes the hard way—that dashboards can cause as much confusion as clarity when readers assume every spike means human spread. The CDC’s NWSS language is careful for a reason: H5 is not a single-source signal.
A responsible reading starts with a sequence of questions, not a conclusion.
A responsible reading starts with a sequence of questions, not a conclusion.
The three-source framework: humans, animals, products
Wastewater H5 can plausibly originate from:
- Humans (the most concerning possibility)
- Animals (including livestock and wildlife)
- Animal products (notably milk and dairy waste streams)
The CDC states clearly that wastewater testing cannot determine which of those sources is responsible. That means the next step is investigation, not inference.
- Humans (the most concerning possibility)
- Animals (including livestock and wildlife)
- Animal products (notably milk and dairy waste streams)
The CDC states clearly that wastewater testing cannot determine which of those sources is responsible. That means the next step is investigation, not inference.
What to look for when H5 appears on a dashboard
A practical way to interpret detections is to ask what else is known locally:
- Is there a milk-processing facility or industrial discharge in the sewershed?
- Are there nearby animal operations or known animal outbreaks?
- Are there wetlands, roosting sites, or environmental conditions consistent with wild bird input?
- Are clinicians reporting unusual respiratory illness patterns, or are there relevant lab confirmations?
Wastewater should be treated as early situational awareness, not case confirmation. Readers deserve that clarity, especially after years of being trained to see sewers as a near-direct proxy for human infection.
- Is there a milk-processing facility or industrial discharge in the sewershed?
- Are there nearby animal operations or known animal outbreaks?
- Are there wetlands, roosting sites, or environmental conditions consistent with wild bird input?
- Are clinicians reporting unusual respiratory illness patterns, or are there relevant lab confirmations?
Wastewater should be treated as early situational awareness, not case confirmation. Readers deserve that clarity, especially after years of being trained to see sewers as a near-direct proxy for human infection.
Dashboard reality-check: questions to ask first
- ✓Is there a milk-processing facility or industrial discharge in the sewershed?
- ✓Are there nearby animal operations or known animal outbreaks?
- ✓Are there wetlands/roosting sites consistent with wild bird input?
- ✓Are clinicians seeing unusual respiratory illness patterns or lab confirmations?
The policy argument: wastewater is still worth funding—but messaging must mature
There is a temptation, when nuance grows, to dismiss the tool. That would be a mistake. Wastewater remains one of the most cost-efficient ways to monitor pathogens across large populations. The issue is not whether it works; it is what “works” means in a world where human and animal health intertwine.
The CDC’s 2024 MMWR offers a blueprint for mature use: pair wastewater detections with on-the-ground context and be candid about uncertainty. Oregon’s experience adds a second lesson: surveillance can produce positives even without local dairy outbreaks, which means environmental pathways can dominate.
The CDC’s 2024 MMWR offers a blueprint for mature use: pair wastewater detections with on-the-ground context and be candid about uncertainty. Oregon’s experience adds a second lesson: surveillance can produce positives even without local dairy outbreaks, which means environmental pathways can dominate.
Multiple perspectives: what advocates and skeptics get right
Wastewater advocates argue—correctly—that early detection can buy time, especially when clinical testing lags. Skeptics argue—also correctly—that without source attribution, detections can trigger unnecessary alarm. Both are responding to real features of the data.
The synthesis is straightforward: wastewater is a screening signal. It should trigger targeted follow-up: veterinary intelligence, industrial facility checks, and—when warranted—human testing strategies. The CDC’s own public language points in that direction by emphasizing limits, not certainties.
The synthesis is straightforward: wastewater is a screening signal. It should trigger targeted follow-up: veterinary intelligence, industrial facility checks, and—when warranted—human testing strategies. The CDC’s own public language points in that direction by emphasizing limits, not certainties.
The communications standard readers should demand
A credible public-facing report should clearly separate:
- Detection (RNA found)
- Subtype detail (H5 vs a specific strain such as H5N1)
- Likely source hypotheses (human vs animal vs product)
- Follow-up actions (environmental investigation, facility review, clinical surveillance)
The pandemic taught institutions to publish data quickly. The H5 era demands they publish it carefully.
- Detection (RNA found)
- Subtype detail (H5 vs a specific strain such as H5N1)
- Likely source hypotheses (human vs animal vs product)
- Follow-up actions (environmental investigation, facility review, clinical surveillance)
The pandemic taught institutions to publish data quickly. The H5 era demands they publish it carefully.
Key Insight: What “detection” should (and shouldn’t) imply
Detection means RNA was found entering the system. It does not prove human infection, confirm H5N1, or identify the source without follow-up.
What this means for readers: practical takeaways without false reassurance
Most readers are not trying to become epidemiologists. They want to know what a detection means for their family and their community. The honest answer is: it depends on source—and wastewater alone cannot tell you.
That is frustrating. It is also manageable, if you know how to interpret the signal responsibly.
That is frustrating. It is also manageable, if you know how to interpret the signal responsibly.
Practical takeaways
- Don’t equate a wastewater H5 detection with human cases. The CDC explicitly warns against that interpretation.
- Ask what’s in your sewershed. Milk processing inputs were identified by investigators in CDC’s MMWR; that kind of local industrial context can matter.
- Look for corroboration. The strongest concern arises when wastewater detections align with clinical findings or other evidence—something wastewater cannot provide by itself.
- Track how reporting has evolved. CDC notes subtyping results began being publicly displayed in August 2024, which affects what you can infer from dashboards across time.
The bottom line is not complacency. It is precision. Wastewater is a useful alarm system, but H5 is an alarm with multiple possible triggers.
- Ask what’s in your sewershed. Milk processing inputs were identified by investigators in CDC’s MMWR; that kind of local industrial context can matter.
- Look for corroboration. The strongest concern arises when wastewater detections align with clinical findings or other evidence—something wastewater cannot provide by itself.
- Track how reporting has evolved. CDC notes subtyping results began being publicly displayed in August 2024, which affects what you can infer from dashboards across time.
The bottom line is not complacency. It is precision. Wastewater is a useful alarm system, but H5 is an alarm with multiple possible triggers.
A final thought worth sitting with
The most valuable lesson of H5 wastewater surveillance may be cultural rather than technical: public health data is rarely self-interpreting. The sewer can tell you what’s arriving. It can’t tell you who sent it.
Strong surveillance is not just measurement. It is interpretation—careful, contextual, and humble about what isn’t known.
Strong surveillance is not just measurement. It is interpretation—careful, contextual, and humble about what isn’t known.
1) Does H5 in wastewater mean people in my community are infected?
Not necessarily. The CDC states that wastewater testing cannot determine whether H5 originates from humans, animals, or animal products (like milk), and detections do not necessarily mean people are infected. A positive signal is best treated as an alert that requires local context and follow-up, not proof of community transmission.
2) Is “H5 detected” the same as “H5N1 detected”?
No. H5 is a subtype, and multiple H5 viruses exist. The CDC notes that NWSS publicly displays influenza A(H5) viral RNA detections, and that H5 detection alone does not automatically confirm H5N1. Subtyping detail has expanded over time; CDC notes subtyping results began being publicly displayed in August 2024.
3) Why would milk affect wastewater bird flu readings?
CDC-linked research indicates infected lactating dairy cows can shed high concentrations of H5N1 viral RNA in milk, making milk a strong source of detectable RNA. If milk or dairy waste enters industrial discharges, it can drive wastewater detections. CDC’s 2024 MMWR investigations identified eight milk-processing inputs among plausible contributors at H5-positive sites.
4) Can wild birds really cause positive H5 wastewater results?
Yes, it’s plausible. Oregon State University coverage and a JAMA Network Open surveillance study report that wild birds may account for much of the avian influenza evidence in wastewater, and that methods do not distinguish human vs animal contributors. Oregon observed H5 positives during a period with no dairy-cattle outbreaks in the state, supporting non-dairy explanations such as wildlife input.
5) If wastewater can’t tell the source, what’s the point?
Wastewater still provides broad, early situational awareness—especially when clinical testing is limited. Its value increases when paired with context: knowledge of local animal activity, industrial facilities, and public health follow-up. CDC’s MMWR approach treats wastewater as a trigger for investigation, not a stand-alone indicator of human outbreak status.
6) What should local officials do after an H5 wastewater detection?
The evidence suggests a layered response: assess the sewershed for milk-processing inputs or other industrial discharges, review local animal outbreak information, consider wildlife pathways, and increase targeted human and veterinary surveillance as appropriate. CDC emphasizes that interpretation requires contextual investigation, because wastewater alone cannot attribute the source.
7) How should I read a wastewater dashboard without overreacting?
Treat it like a smoke detector in a building with multiple kitchens. A detection means H5 RNA is present in inputs to the system—but not where it came from. Look for updates on subtyping (CDC began publicly displaying subtyping results in August 2024), local explanations (industry or wildlife), and any clinical corroboration before drawing conclusions about community human infection.
Editor's Note
Throughout this piece, “H5 detection” refers to influenza A(H5) viral RNA detections as publicly displayed by CDC NWSS, which do not inherently attribute source or confirm H5N1.
T
About the Author
TheMurrow Editorial is a writer for TheMurrow covering science.
Frequently Asked Questions
Does H5 in wastewater mean people in my community are infected?
Not necessarily. The CDC states that wastewater testing cannot determine whether H5 originates from humans, animals, or animal products (like milk), and detections do not necessarily mean people are infected. A positive signal is best treated as an alert that requires local context and follow-up, not proof of community transmission.
Is “H5 detected” the same as “H5N1 detected”?
No. H5 is a subtype, and multiple H5 viruses exist. The CDC notes that NWSS publicly displays influenza A(H5) viral RNA detections, and that H5 detection alone does not automatically confirm H5N1. Subtyping detail has expanded over time; CDC notes subtyping results began being publicly displayed in August 2024.
Why would milk affect wastewater bird flu readings?
CDC-linked research indicates infected lactating dairy cows can shed high concentrations of H5N1 viral RNA in milk, making milk a strong source of detectable RNA. If milk or dairy waste enters industrial discharges, it can drive wastewater detections. CDC’s 2024 MMWR investigations identified eight milk-processing inputs among plausible contributors at H5-positive sites.
Can wild birds really cause positive H5 wastewater results?
Yes, it’s plausible. Oregon State University coverage and a JAMA Network Open surveillance study report that wild birds may account for much of the avian influenza evidence in wastewater, and that methods do not distinguish human vs animal contributors. Oregon observed H5 positives during a period with no dairy-cattle outbreaks in the state, supporting non-dairy explanations such as wildlife input.
If wastewater can’t tell the source, what’s the point?
Wastewater still provides broad, early situational awareness—especially when clinical testing is limited. Its value increases when paired with context: knowledge of local animal activity, industrial facilities, and public health follow-up. CDC’s MMWR approach treats wastewater as a trigger for investigation, not a stand-alone indicator of human outbreak status.
What should local officials do after an H5 wastewater detection?
The evidence suggests a layered response: assess the sewershed for milk-processing inputs or other industrial discharges, review local animal outbreak information, consider wildlife pathways, and increase targeted human and veterinary surveillance as appropriate. CDC emphasizes that interpretation requires contextual investigation, because wastewater alone cannot attribute the source.
