Overview
The recent spread of highly pathogenic avian influenza (HPAI) H5N1 to dairy cattle has raised urgent questions about the safety of unpasteurized dairy products. In a focused study, researchers examined how long the H5N1 virus remains infectious in raw-milk cheeses made from milk with varying pH levels. The findings illuminate how cheese-making conditions can influence viral persistence and potential human exposure risk.
Experimental setup and key findings
Researchers evaluated H5N1 stability in raw-milk cheeses under three pH conditions reflecting typical marketplace variations: pH 6.6, 5.8, and 5.0. For each pH level, cheese batches were prepared from raw milk, with three cheeses per condition. They also validated results using cheeses inadvertently produced from naturally contaminated raw milk (four samples).
The central observation was clear: the virus’s survival was strongly pH-dependent.
pH-dependent persistence
At pH 6.6 and 5.8, infectious H5N1 persisted through the cheese-making process and remained detectable for up to 120 days of aging. In contrast, at the more acidic pH of 5.0, the virus did not survive the cheese-making process. This demonstrates that even within raw-milk cheeses, the acidity of the product can influence viral stability over time.
Transmission observation in animal models
As part of assessing real-world risk, ferrets were used as a model to simulate human exposure scenarios. Ferrets fed H5N1-contaminated raw milk (n = 4) became infected, indicating a clear transmission route from contaminated milk to a mammalian host. Conversely, ferrets fed raw-milk cheese (n = 4) or cheese suspension (n = 4) did not become infected in these experiments, suggesting that the cheese matrix and aging process can reduce or prevent transmission under the tested conditions.
Public health implications
The study’s outcomes hint at two intertwined realities for public health and dairy safety. First, raw-milk cheeses produced from contaminated milk can harbor infectious H5N1 for extended periods, particularly when the product’s pH is near neutral to mildly acidic (pH 6.6–5.8). Second, the risk of transmission to humans varies with the food form: liquid milk may present greater exposure potential than aged, properly handled cheese, though cheese can still pose a risk if conditions are favorable to viral survival.
Mitigation strategies for cheese producers and consumers
To mitigate risks associated with H5N1 in raw-milk products, authorities and producers can consider several measures. Key strategies include:
- Screening and surveillance of dairy herds for avian influenza to prevent contaminated milk entering the production stream.
- Maintaining acidic environments in cheese production (lower pH targets) where feasible, to reduce viral stability.
- Implementing robust pasteurization or post-pasteurization controls for dairy products derived from potentially exposed sources.
- Clear consumer guidance about the risks of consuming raw-milk dairy products during avian influenza outbreaks.
- Enhanced hygiene and sanitation across the dairy supply chain, including testing of cheeses at various aging stages in outbreak scenarios.
These steps, combined with continued research, can help reduce the likelihood of human exposure to H5N1 through dairy products while preserving safe culinary options for consumers.
Conclusion
The H5N1 stability observed in raw-milk cheeses depends on pH and aging conditions, with a notable persistence at higher pH levels and in cheese products. The translation of these findings to human risk highlights the importance of vigilant dairy safety practices, transparent risk communication, and proactive mitigation to prevent foodborne transmission during avian influenza outbreaks.
