Avian Influenza's Jump to Cattle and the Unseen Risk in Cheese
A virus on the move is rewriting the rules of cross-species infection
For decades, the term "bird flu" meant exactly that—a devastating virus primarily affecting poultry and wild birds. But in 2025, the narrative shifted. The highly pathogenic H5N1 avian influenza virus, already responsible for unprecedented global outbreaks in birds, crossed a new frontier: the American dairy herd. This development has sent ripples through the scientific community, raising critical questions about pandemic preparedness and revealing unexpected transmission risks.
The story of H5N1 is no longer just about dead birds; it's about the virus's remarkable ability to adapt and find new hosts. From its initial detection in dairy cows to the discovery that it can survive for months in cheese made from contaminated raw milk, the unfolding saga of avian influenza challenges our understanding of viral resilience and exposes new vulnerabilities in our food systems. This article explores the latest chapter in this evolving story, examining how a simple dairy product became the focus of urgent scientific inquiry.
Avian influenza, commonly called "bird flu," is an infection caused by Type A influenza viruses that naturally spread among wild aquatic birds worldwide 9 .
Between June and September 2025, 19 human cases of avian influenza virus infection, including three deaths, were reported across four countries 1 .
In early 2025, the H5N1 story took an unexpected turn with its detection in U.S. dairy cattle. This marked the first time the virus had widely infected cattle, creating a new dimension to the outbreak with implications for both animal and human health 5 .
The virus demonstrated a particular tropism for mammary gland cells, leading to severe viral mastitis in infected cows 6 . This mammary tissue targeting resulted in extremely high viral loads in milk—as much as 8.8 log10 TCID50 per milliliter—creating a potent source of infection 6 .
The discovery of high levels of infectious H5N1 virus in raw milk immediately raised public health concerns. Studies confirmed that the virus could persist in refrigerated raw milk for at least 8 weeks, challenging conventional food safety assumptions 6 .
A U.S. Food and Drug Administration survey showed that 4.4% of U.S. adults reported consuming raw milk at least once a year, with 1.6% consuming it frequently 6 .
California, one of the states with the largest number of affected herds, issued voluntary recalls of raw milk dairy products after retail samples tested positive for H5N1 6 .
When H5N1 was detected in dairy cattle, scientists immediately recognized the potential risk in raw milk products. However, an important question remained unanswered: could the virus survive the cheese-making process, particularly the 60-day aging period required by U.S. regulations for raw-milk cheeses? To address this, researchers at Cornell University designed a comprehensive study to evaluate H5N1 stability in raw-milk cheeses under various conditions 6 .
The research team employed two parallel approaches to ensure robust results:
| pH Level | Infectious Virus After Processing | Infectious Virus After 60 Days | Infectious Virus After 120 Days |
|---|---|---|---|
| 6.6 | Yes | Yes | Yes |
| 5.8 | Yes | Yes | Yes |
| 5.0 | No | No | No |
The implications were clear: the mandatory 60-day aging period for raw-milk cheeses provided insufficient protection against H5N1. As lead researcher Diego G. Diel stated, "The current regulation requiring 60-day aging of raw-milk cheese before marketing proves insufficient to achieve HPAI H5N1 virus inactivation and guarantee cheese safety" .
Ferrets that drank H5N1-contaminated raw milk became infected, showing fever and viral shedding 6 .
Ferrets that ate contaminated raw-milk cheese or cheese suspension did not become infected 6 .
This suggested that while the virus remains infectious in cheese, actually contracting the infection through eating cheese might be more difficult than through drinking raw milk. The researchers hypothesized that differences in how liquids versus solids spread in the mouth and throat might affect the infectious dose reaching susceptible tissues .
Understanding and tracking the H5N1 virus requires sophisticated diagnostic tools and research methods. Scientists and veterinary diagnosticians rely on a range of specialized reagents and solutions to detect outbreaks, conduct research, and monitor the virus's evolution.
| Tool/Reagent | Function | Application Example |
|---|---|---|
| VetMAX-Gold AIV Detection Kit | USDA-licensed PCR test detecting 3 regions of influenza A genome with high specificity/sensitivity 4 | First-line diagnostic testing in veterinary labs |
| Avian Influenza Virus Subtype-Specific Kits | Target specific H (hemagglutinin) and N (neuraminidase) subtypes (e.g., H5N1, H7N9) 9 | Precise identification of circulating virus subtypes |
| Embryonated Chicken Eggs (ECEs) | Traditional gold standard for virus propagation and infectivity titrations 6 | Determining if virus in a sample is infectious |
| Real-time Reverse-Transcriptase PCR (rRT-PCR) | Detects and quantifies viral RNA loads; indicates presence of virus 6 | Measuring viral load in milk, cheese, or tissue samples |
| Cell Culture Systems | In vitro method for virus isolation and growth 6 | Studying viral behavior without animal models |
These tools have been crucial in responding to the H5N1 outbreak in dairy cattle. For instance, without the USDA-licensed detection kits, health authorities would struggle to quickly identify infected herds and implement control measures 4 . Similarly, the use of embryonated chicken eggs was essential in the cheese study to confirm that the virus detected in aged cheese remained infectious, not just present as genetic fragments 6 .
The expanding host range of H5N1 demands enhanced surveillance and control strategies. Experts from the Global Virus Network are calling for:
Testing not just symptomatic individuals but farmworkers, their families, and nearby communities, alongside wastewater and environmental sampling 5 .
Implementing personal protective equipment (PPE) and sanitation standards as routine practice in farm and dairy facilities 5 .
Evaluating existing H5N1 vaccines, testing candidates against current strains, and prioritizing high-risk populations like farmworkers 5 .
Helping the public understand risks and protective actions without causing unnecessary alarm 5 .
Scientists are increasingly using mathematical models to understand avian influenza transmission and evaluate control measures. A recent systematic review identified 46 articles using mechanistic models to study field outbreaks. These models have revealed that optimal control strategies vary depending on the virus subtype and local conditions 2 .
A consistent finding across studies is that early implementation dramatically improves the effectiveness of any intervention 2
The discovery of H5N1 in dairy cattle and its persistence in raw-milk cheese represents more than just a scientific curiosity—it signals an evolving threat that demands our attention. The virus has demonstrated remarkable flexibility in finding new hosts and transmission routes, challenging our existing food safety regulations and pandemic preparedness plans.
Yet within this challenge lies opportunity. The same research that identified the risk in raw-milk cheese also pointed to practical solutions: pre-cheese making acidification, bulk tank testing, and thermization (a gentle heat treatment) can significantly reduce the risk while preserving traditional cheese-making practices .
Pasteurized milk and cheeses are the safer choice.
Implement additional mitigation measures and strengthen surveillance systems.
As virologists continue to track the mutations and reassortments of H5N1, one lesson stands clear: in our interconnected world, a virus that can jump species barriers demands a response that crosses disciplinary boundaries. The story of H5N1 is still being written, and its next chapters will depend on how effectively we apply the lessons we're learning today.