New research has examined how long influenza viruses, including the highly pathogenic H5N1 strain, remain viable in unpasteurized milk. The study, which has not yet been peer-reviewed, provides insights into potential transmission risks associated with contaminated dairy products.
The research follows the March 2024 identification of H5N1 highly pathogenic avian influenza virus in U.S. dairy cattle. Scientists discovered this virus after investigating significantly reduced milk production across affected farms. Genetic analysis indicated the virus was first introduced to cattle populations in late 2023.
Unlike typical respiratory influenza infections in mammals, H5N1 in cattle produces extremely high levels of viral shedding in milk. The virus has spread to over a thousand cattle herds and infected dozens of dairy workers across the United States. While pasteurized milk contains no viable H5N1 virus, raw dairy products carry infection risks through consumption or occupational exposure.
Several dairy workers developed conjunctivitis after milk accidentally splashed into their eyes while milking without proper protective equipment. These individuals were subsequently diagnosed with H5N1 infections from cattle exposure. Additional cases occurred without direct animal contact.
Researchers have detected H5N1 genetic material in consumer dairy products throughout the U.S. Previous studies demonstrated that virus material remains stable in milk and cheese products for extended periods. On one Texas dairy farm, over half the cats contracted infections after consuming H5N1-contaminated cow milk.
The current study tested various influenza viruses, including H5N1, for stability in cow and sheep milk under different storage conditions. Researchers used wild-type H5N1 strain under high-containment laboratory conditions and evaluated other influenza types including low-pathogenic avian strains and influenza D viruses.
Test viruses were mixed with unpasteurized milk at specific ratios and stored in sealed containers to prevent evaporation and ultraviolet light exposure. Samples were incubated at room temperature (approximately 20°C) and refrigerated conditions (4°C) for up to 12 days. Researchers assessed viral infectivity using standard laboratory assays.
Results showed H5N1 virus gradually lost infectivity over time in milk under both storage conditions, but infectious levels remained detectable in raw milk for over one week. Substantial variation occurred in virus stability, particularly at room temperature. Some viruses remained viable for more than eleven days, while others reached detection limits within two days.
Under refrigeration, most viruses stayed infectious for over seven days, with some remaining viable longer. In sheep milk, H5N1 remained infectious for more than seven days at 4°C and up to four days at room temperature.
Laboratory conditions excluded many environmental factors that typically reduce virus survival, suggesting real-world persistence may be shorter. Variation in stability depended on specific virus strains and milk types, indicating biological and chemical milk differences affect viral longevity.
The study confirms influenza viruses remain infectious in milk for extended periods, though the infectious dose required for human illness through consumption remains unknown.
