Overview: H5N1 in dairy contexts raises food-safety questions
Recent research into highly pathogenic avian influenza H5N1 has drawn attention to dairy products sourced from infected animals. In outbreaks where dairy cattle test positive for H5N1 and milk is contaminated, the critical question becomes whether unpasteurized dairy products, especially raw-milk cheeses, can harbor infectious virus and potentially pose a human health risk. A study designed to mimic real-world cheese-making conditions evaluated how long the virus remains infectious in raw-milk cheese and how acidity (pH) affects its survival.
Experimental design and key findings
The investigators prepared raw-milk cheese under three pH scenarios that reflect pH values observed in raw-milk cheeses at the marketplace: 6.6, 5.8, and 5.0. For each pH, cheese was produced and aged, with three samples per condition. A separate validation phase used four cheese samples produced from naturally contaminated raw milk. Across these sets, the researchers tracked the persistence of infectious H5N1 virus.
In cheeses made from milk acidified to pH 6.6 and 5.8, infectious virus persisted through the cheese-making process and remained detectable for up to 120 days of aging. By contrast, when acidification reached pH 5.0, the virus did not survive the cheese-making process, suggesting that greater acidity may disrupt viral stability in the cheese matrix.
To gauge actual transmission potential, the study also used a ferret model, a common proxy for human influenza transmission. Ferrets fed H5N1-contaminated raw milk became infected, indicating that exposure to contaminated milk represents a clear risk. In contrast, ferrets consuming raw-milk cheese or a cheese suspension derived from contaminated milk did not become infected within the study period, implying that the cheese matrix and aging processes may reduce, but not necessarily eliminate, transmission risk under certain conditions.
Implications for public health and food safety
These findings add nuance to the risk assessment for raw dairy products in H5N1-endemic regions. Key implications include:
- Risk varies with cheese acidity. Lower pH (more acidic conditions) can limit or prevent virus survival, while pH closer to neutral may permit longer persistence.
- Raw-milk exposure matters. Direct ingestion of contaminated raw milk presents a more immediate transmission risk than consumption of some raw-milk cheeses, depending on processing and aging conditions.
- Cheese production processes are not uniformly protective. While certain cheeses may suppress viral viability, others—particularly those aged at higher pH—could retain infectious virus for extended periods.
From a public-health perspective, these results reinforce the importance of robust mitigation strategies in dairy production, especially during animal outbreaks. Pasteurization of milk before cheese production remains a proven barrier to viral transmission. While some raw-milk cheeses may exhibit reduced viral survival under specific acidity profiles, reliance on cheese acidity alone is not a fail-safe approach to ensuring safety for consumers.
Practical recommendations for consumers and producers
Given the potential for H5N1 persistence in certain raw-milk cheeses, stakeholders should consider prudent precautions:
- Prioritize pasteurization for dairy ingredients used in cheese making when H5N1 exposure risk is elevated in cattle populations.
- Maintain strict hygiene and testing protocols in dairy facilities to prevent contaminated milk from entering the cheese chain.
- Public health communications should clearly distinguish between pasteurized dairy products and raw-m milk products, emphasizing the increased risk associated with the latter during avian flu outbreaks.
- Consumers at higher risk or in outbreak areas may opt for pasteurized dairy products or seek cheeses produced under stringent surveillance from reputable producers.
In summary, the H5N1 virus can remain infectious in raw-milk cheeses under certain pH and aging conditions, which underscores the need for careful management of dairy supply chains during avian influenza events. While the cheese matrix and acidity can influence viral stability, pasteurization and vigilant monitoring remain essential tools to minimize human exposure risks.
