The impact of acid and pH on avian influenza virus persistence in raw milk

Authors: Harrell, Telvin; SHWANI, ABDUL - Orise Fellow; Suarez, David

Submitted to: Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/10/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Highly pathogenic avian influenza virus is an important disease in both poultry and mammals. In March 2024, highly pathogenic avian influenza virus (AIV) H5N1 was detected in dairy cattle. Since its initial detection, the virus has spread across 17 states infecting more than 1000 dairy herds causing concern for the dairy industry regarding the effects on dairy cattle, the risk associated with milk quality and production, and ultimately its risk to humans. Fortunately commercial pasteurization methods effectively kills the virus in milk. However, there remains a risk of human exposure in raw (unpasteurized) milk or milk products. Milk is used for many fermented dairy products including cheese, yogurt and kefir. For fermented dairy products, there will be a drop in pH of the product, and influenza is known to be inactivated at lower pH levels. This study examined if and how quickly avian influenza virus is inactivated at different pH levels at different incubation times. At pH 6, close to the pH found in milk, the virus was stable at 24 hours at room temperature. At pH 4 and 5, the virus was inactivated with all or almost all virus inactivated by 24 hours. This data supports that avian influenza is likely inactivated in raw milk when the pH drops in fermented milk products. However, not all cheeses may have a large enough pH drop to inactivate all virus, and more work needs to be done to evaluate these raw milk cheeses.

Technical Abstract: In March 2024, highly pathogenic avian influenza virus (AIV) clade 2.3.4.4.b H5N1 was detected in dairy cattle. Since its initial detection, the virus has spread across 17 states infecting more than 1000 dairy herds causing concern for the dairy industry regarding the effects on dairy cattle, the risk associated with milk quality and production, and ultimately its risk to humans. It has been shown that pasteurization is sufficient to inactivate AIV if present in milk making it safe for human consumption. However, unpasteurized raw milk that is routinely consumed and is also used to make cheese, yogurt, and kefir is a public health concern. We acidified raw milk, synonymous with making fermented milk products, with acetic acid, propionic acid, lactic acid, or citric acid to pH 5; or acidified with acetic acid to pH 6, 5, or 4. Each sample was subsequently spiked with AIV and incubated at room temperature for up to 24h before being inoculated into specific-pathogen free embryonating chicken eggs. Embryos were assessed daily for viability and hemagglutination assays were used to confirm the presence of viable AIV. The effect of time and pH, but not the individual acids, significantly correlated with a reduction of AIV titers. Each individual acid treatment at pH 5 showed a reduction in AIV with increasing time and a complete inactivation by 24 hours. Samples at pH 6 had little reduction in virus viability, but samples at pH 5, and 4 had a more pronounced effect on AIV reduction with complete inactivation not achieved until 24 hours of incubation.