Overview
Recent events have shown that highly pathogenic avian influenza H5N1 has reached dairy cattle, with detectable levels of virus in milk. This raises questions about the safety of unpasteurized dairy products, especially raw-milk cheeses. A recent study evaluated how long H5N1 can remain infectious in cheeses made from raw milk, across a range of pH conditions that mirror those found in artisanal cheeses sold to consumers.
Key findings on pH-dependent virus survival
The researchers prepared cheeses using raw milk and adjusted the milk to three pH levels before cheese making: 6.6, 5.8, and 5.0. They then tested virus persistence in the finished cheeses, as well as during a 120-day aging period. The results showed a clear pH effect on virus survival.
- At pH 6.6 and 5.8, infectious H5N1 persisted in the cheese and remained detectable for up to 120 days in the aged products tested (n = 3 cheeses per condition).
- At pH 5.0, the virus did not survive the cheese-making process, suggesting high acidity can inactivate the virus during production.
Transmission observations in animal models
In experiments using ferrets, animals fed contaminated raw milk (n = 4) became infected, while those fed raw-milk cheese (n = 4) or a cheese suspension (n = 4) did not show infection. These findings imply that while the virus can persist in certain cheese matrices, the form in which humans are exposed (for example, cheese) may reduce the immediate risk of transmission compared with raw milk consumption. However, the potential for viral persistence in raw-milk cheeses under specific production conditions remains a public health concern.
Public health implications
The study underscores several important considerations for food safety and animal health policy. First, there is a demonstrated risk that H5N1 can survive in raw-milk cheeses under certain pH conditions, meaning that unpasteurized products could, in theory, transmit the virus if contaminated milk is used. Second, fermentation and aging processes that lower pH or increase acidity may mitigate this risk, but the effectiveness depends on the cheese type and production parameters. Finally, the interspecies transmission observed in ferrets highlights the ongoing need for vigilance as avian influenza viruses circulate among poultry, wild birds, and livestock.
Mitigation strategies and recommendations
To reduce potential exposure to H5N1 from dairy products, authorities and producers should consider the following high-level measures:
- Promote pasteurization of milk used for cheese-making, especially when introducing milk from potentially exposed herds into the supply chain.
- Monitor pH profiles in raw-milk cheese production and understand how acidity affects virus stability in different cheese formats.
- Enhance surveillance of dairy cattle for avian influenza and implement clear traceability for milk from herds with confirmed infections.
- Communicate risks to consumers of raw-milk cheeses and encourage informed choices, especially for vulnerable populations.
Conclusions
The research confirms that H5N1 can remain infectious in raw-milk cheeses under certain pH conditions and aging protocols, though exposure via cheese may carry a different risk profile than exposure to raw milk. These findings reinforce the need for robust mitigation measures in dairy production and ongoing monitoring to protect public health while preserving safe dairy-food traditions.
Notes for readers
This article summarizes a laboratory study on virus persistence in cheese matrices. It should not be used to guide DIY cheese production. Always follow local public health guidance and industry best practices for handling milk and dairy products.
