Three veterinarians had been infected with H5N1 and none of them knew. That’s a small number — and the reason it matters has nothing to do with how small it is.
The CDC tested 150 bovine veterinary practitioners with cattle exposure in the previous three months, in a serosurvey conducted on 12–13 September 2024 and published in MMWR on 13 February 2025. Three (2%; 95% CI 0.7%–5.7%) had antibodies to highly pathogenic avian influenza A(H5) suggestive of recent infection.
The finding that matters: none of the three reported respiratory or flu-like symptoms — including conjunctivitis — and none had been tested for influenza since January 2024. They were infected, and nothing in the surveillance system would ever have caught it.
And it isn’t only cattle. A subsequent CDC report documented serologic evidence of H5N1 infection in a veterinary professional exposed to an infected domestic cat in Los Angeles County. 🦠
- What the study was
- Why “silent” is the important word
- What this means for the case numbers
- The conjunctivitis detail
- The cat case
- What this is not
- Why virologists watch this anyway
- Who is actually at risk
- Protection that matters, and the eyes
- What the general public should do
- Raw milk and raw pet food
- Frequently asked questions
- Your checklist
What the study was
| Element | Detail |
|---|---|
| Type | Seroprevalence survey (antibody testing) |
| Conducted | 12–13 September 2024 |
| Published | MMWR, 13 February 2025 |
| Participants | 150 bovine veterinary practitioners |
| Eligibility | Cattle exposure in the previous 3 months |
| Positive for A(H5) antibodies | 3 (2%; 95% CI 0.7%–5.7%) |
| Reported symptoms | None — including no conjunctivitis |
| Previously tested for influenza | None since January 2024 |
A serosurvey looks for antibodies — the immune system’s record that it has met a virus. It doesn’t find active infection; it finds evidence that infection happened at some point. That’s exactly the right tool for answering “are we missing cases?”
Why “silent” is the important word
Three out of 150 is a small number, and the headline could reasonably be read as reassuring. The significant part is how those three were found.
They weren’t identified because they got sick and were tested. They were identified because researchers went looking in people who had no idea anything had happened. None reported symptoms. None had been tested for influenza in the preceding eight months.
Surveillance for this virus has largely depended on people noticing they’re unwell and presenting for testing. If infection can occur without symptoms, that system has a blind spot — and this study measured it.
That’s the finding. Not “bird flu is spreading dangerously,” but “our ability to see it spreading is worse than we assumed.”
What this means for the case numbers
Every reported human H5N1 case figure you’ve seen is a count of infections that were detected. This study is direct evidence that detected infections and total infections are not the same number.
Three things follow, and it’s worth being precise about each:
- The true number of infections is higher than the confirmed count. That’s what the serosurvey demonstrates.
- It does not follow that the virus is more dangerous than thought. If anything, undetected mild or asymptomatic infections mean the proportion of infections that become severe is lower than a count based only on people sick enough to seek testing would suggest.
- It does mean more human exposure than recorded — and human exposure is the thing that matters most for the risk everyone is actually watching for.
“More people were infected than we knew” sounds worse. “Those people had no symptoms” sounds better. Both are accurate, and they point in different directions on different questions.
On severity, undetected mild infections are reassuring — the denominator is bigger than we thought. On surveillance, they’re concerning, because they mean the system misses infections. Good reporting of this study holds both, and a lot of coverage picked whichever half fitted the headline.
The conjunctivitis detail
The study specifically noted that none of the three reported conjunctivitis, and that detail carries more weight than it first appears.
Conjunctivitis — red, irritated eyes — has been the hallmark presentation of H5N1 in dairy workers in this outbreak, rather than the respiratory illness people expect from flu. It’s the symptom clinicians were told to look for.
So the three infected vets didn’t just lack dramatic illness. They lacked the specific sign the entire case-finding effort was oriented around. Someone with mild conjunctivitis and cattle exposure might get tested; someone with nothing would not.
The cat case
A subsequent CDC report extends the picture beyond cattle in a way worth knowing if you work with animals.
It documented serologic evidence of H5N1 infection in a veterinary professional exposed to an infected domestic cat in Los Angeles County, California, over December 2024 to January 2025.
That matters because it widens the occupational exposure route. Cats are susceptible to H5N1 and can become severely unwell, and companion-animal practice is a different setting from dairy work — different people, different premises, often less awareness that avian influenza is even on the differential.
It’s a single case with serologic evidence, so don’t over-read it. What it establishes is that the exposure route exists.
What this is not
Being clear about the limits of this finding matters as much as reporting it.
- It is not evidence of human-to-human transmission. These were people with occupational animal exposure. Nothing here shows the virus passing between people, which is the threshold that would change everything.
- It is not evidence the virus has become more dangerous. Asymptomatic infection is, on its face, the opposite.
- It is not a general-population finding. The participants were selected specifically for recent cattle exposure — a group with far higher exposure than anyone else.
- Three cases is a small number with a wide confidence interval (0.7%–5.7%), which is honest to state.
- It is not a reason to change what you eat, beyond the raw-product advice below.
Why virologists watch this anyway
Given all those caveats, why does anyone care about three asymptomatic infections?
Because every human infection is an opportunity for the virus to adapt. Influenza viruses change through mutation and through reassortment — swapping gene segments when two flu viruses infect the same host at the same time. A person infected simultaneously with H5N1 and a seasonal human flu virus is precisely the setting in which reassortment could occur.
So the number of human infections is the variable worth tracking, and undetected infections are ones nobody can count, monitor or study. The concern isn’t these three people. It’s that the system that’s supposed to see this is demonstrably porous.
It also has a practical consequence: seasonal flu vaccination for people with animal exposure isn’t about protecting them from H5N1 — it doesn’t — but about reducing the chance of simultaneous infection with two flu viruses in one person.
Who is actually at risk
| Group | Exposure level |
|---|---|
| Dairy and poultry workers | Highest — direct, repeated animal contact |
| Veterinarians and vet technicians | High, and now documented for both cattle and cats |
| Cullers and response workers | High during outbreaks |
| Backyard flock keepers | Moderate |
| People handling wild birds | Moderate — including dead birds |
| Raw milk consumers | Avoidable exposure |
| General public | Low |
Protection that matters, and the eyes
For anyone in the occupational groups above, the practical lesson from this outbreak is that eye protection has been under-prioritised.
Standard respiratory precautions focus on the nose and mouth. But the conjunctiva — the surface of the eye — is a genuine entry route, and the dominance of conjunctivitis as a presentation in dairy workers points directly at it. Milking parlours involve splashes and aerosols at face height.
Sealed Safety Goggles
Conjunctivitis has been the hallmark presentation in dairy workers — which points at the eyes as a real route of exposure
If you work with dairy cattle, poultry or potentially infected animals, this is the item most often skipped. Choose sealed goggles rather than safety glasses — open-sided glasses don’t stop splashes arriving from below or the side, which is exactly what happens in a milking parlour. Indirectly vented goggles resist splash while staying wearable. Wear them alongside the rest of the recommended protection: respiratory protection, gloves, coveralls and boot covers — eye protection alone is not a substitute for a full set. Clean and disinfect them between uses, and don’t touch your face while working. If you develop red or irritated eyes after animal contact, that is the symptom to report and get tested for — not to wait out.
- Sealed goggles, not open safety glasses — splashes come from below
- Not a substitute for respiratory protection, gloves and coveralls
- Clean and disinfect between uses; avoid touching your face
- Follow your employer’s and public health guidance on full PPE
- Report red eyes or flu symptoms after animal contact
What the general public should do
Very little, and that’s the honest answer:
- Don’t touch dead or visibly sick wild birds. Report them to the relevant authority rather than handling them.
- Keep pets away from dead birds and from raw milk.
- Get your seasonal flu vaccine — especially with any animal exposure. It doesn’t protect against H5N1, but it reduces the chance of being infected with two flu viruses at once.
- Cook poultry and eggs properly. Standard cooking inactivates influenza viruses.
- Tell a doctor about animal exposure if you develop flu symptoms or conjunctivitis — that context changes what they test for.
- Follow official guidance rather than social media, and check it directly if you’re in an affected area.
Raw milk and raw pet food
H5N1 has been found in the milk of infected dairy cattle. Pasteurisation inactivates the virus; raw milk does not have that step. Drinking raw milk in the middle of an outbreak affecting dairy herds is an avoidable exposure, and the same applies to raw milk given to cats and dogs.
Cats are highly susceptible to H5N1 and can become severely unwell. There have been illnesses and deaths in cats linked to raw milk and raw pet food products. If you feed a raw diet, this is worth reconsidering while the outbreak continues — and worth discussing with your vet.
Frequently asked questions
What did the CDC study actually find?
The CDC tested 150 bovine veterinary practitioners who had cattle exposure in the previous three months, in a serosurvey conducted on 12–13 September 2024 and published in MMWR on 13 February 2025. Three (2%; 95% CI 0.7%–5.7%) had antibodies indicating recent infection with highly pathogenic avian influenza A(H5). Crucially, none of the three reported respiratory or flu-like symptoms, including conjunctivitis, and none had been tested for influenza since January 2024.
Does this mean bird flu is more dangerous than we thought?
No — arguably the opposite on that specific question. Finding infections that produced no symptoms means the total number of infections is larger than the confirmed count, which makes the proportion becoming severe lower than a count based only on people sick enough to seek testing suggests. What the study does show is that surveillance has a blind spot: if infection can happen without symptoms, a system relying on people noticing they’re ill will miss cases.
Is it spreading between people?
Nothing in this study shows that. All the participants were people with occupational animal exposure, and the finding is about infections acquired from animals going undetected — not about transmission between humans. Sustained human-to-human transmission is the threshold that would change the situation fundamentally, and this isn’t evidence of it. It’s evidence that more animal-to-human infections are happening than get recorded.
Why does conjunctivitis keep coming up?
Because red, irritated eyes — rather than respiratory illness — has been the hallmark presentation of H5N1 in dairy workers during this outbreak, and it’s the sign clinicians were told to look for. That’s what makes the study’s detail striking: the three infected vets didn’t just lack dramatic illness, they lacked the specific symptom the entire case-finding effort was built around. It also points at the eyes as a real route of exposure, which is why sealed eye protection matters for people working with affected animals.
Should I worry about my cat?
Cats are genuinely susceptible to H5N1 and can become severely unwell, and a CDC report documented serologic evidence of infection in a veterinary professional exposed to an infected domestic cat in Los Angeles County. The practical steps: don’t give cats raw milk or raw pet food while the outbreak continues, keep them away from dead or sick birds, and speak to your vet if your cat becomes unwell after any such exposure. For most indoor cats with no access to birds or raw products, the risk is low.
Should I get a flu vaccine because of this?
Seasonal flu vaccine does not protect against H5N1 — that’s worth being clear about. The reason it’s recommended for people with animal exposure is different: it reduces the chance of being infected with a human flu virus and H5N1 at the same time, which is the situation in which influenza viruses can swap gene segments and change. So it’s a population-level precaution about viral evolution rather than personal protection against bird flu.
Your checklist
- Know the finding: 3 of 150 vets, 2%, no symptoms, never tested
- Understand it’s a surveillance finding, not a severity finding
- It is not evidence of human-to-human transmission
- Note the wide confidence interval (0.7%–5.7%) on three cases
- Remember conjunctivitis is the hallmark sign — and they didn’t have it
- Exposure isn’t only cattle — a cat-linked case is documented
- If exposed occupationally, wear sealed eye protection
- Eye protection is additional to, not instead of, full PPE
- Report red eyes or flu symptoms after animal contact
- Get seasonal flu vaccine if you have animal exposure — for reassortment risk
- Don’t handle dead wild birds; report them
- No raw milk — for you or your pets
- Tell a doctor about animal exposure if you become unwell
Medical disclaimer. This article is general information, not medical or occupational-health advice. If you work with cattle, poultry or other potentially infected animals, follow your employer’s and your public health authority’s current guidance on personal protective equipment — eye protection is one component and does not replace respiratory protection, gloves and coveralls. Tell a doctor about any animal exposure if you develop flu-like symptoms or red, irritated eyes, as that context changes what they test for. Do not handle dead or visibly sick wild birds; report them to the appropriate authority. Do not consume raw (unpasteurised) milk or give it to pets, and speak to a vet about raw pet food during an ongoing outbreak.
Affiliate disclosure. Some links on this page are affiliate links. As an Amazon Associate we earn from qualifying purchases. The item recommended here is one component of occupational protection and is not sufficient on its own.
