As studies continue amidst the H5N1 highly pathogenic avian flu (HPAI) outbreak in dairy cattle, researchers are finding that infected cows are producing less milk even before the diagnosis, and the 60-day required aging process for raw milk cheese is not inactivating the virus.
In research led by Cornell University, milk production in an Ohio herd studied was found to decline about five days before H5N1 clinical diagnosis and last 60 days. Cows that had lactated multiple times were found to be at higher risk than those lactating for the first time, which the researchers said, “may suggest a link between cumulative exposure to the milking process and the risk of clinical disease.” Although the route of transmission between cattle is not yet determined, the high rate of infection at the farm, with findings positive for nearly 90% of the animals on the farm, show that the virus can quickly spread through susceptible mammalian populations.
Additional research from Cornell has demonstrated that HPAI can survive the federally minimum required aging period of 60 days for raw milk cheeses (which are made from unpasteurized milk). Using raw milk spiked with H5N1 virus, the researchers found that the infectious virus survived for at least 60 days of aging in cheese groups with pH of 6.6 and 5.8. However, in a cheese at pH 5.0, the virus did not survive the cheese-making process. The results were validated in a commercial raw milk cheese at a dairy that inadvertently used HPAI H5N1 contaminated raw milk following an H5N1 influenza virus outbreak in the farm to make cheddar cheese. The presence of high viral RNA loads in this cheese (with a pH of 5.34) provide compelling evidence that the HPAI H5N1 virus survives throughout the minimum 60-day aging period.
An ongoing FDA study, however, has not shown contaminated raw milk cheeses to have reached the market. In the study, FDA is collecting 60-day aged raw milk cheese samples from warehouses and distribution hubs throughout the US, which it is testing for H5N1 genetic material (either RNA or DNA) using a PCR test. Any samples that indicate the presence of H5N1 will undergo viability testing through an egg inoculation test developed by USDA. If a viable virus is detected, FDA will take follow-up action as warranted. As of March 10, 2025, 110 samples of the planned 299 had been collected. Of those, 96 were negative and 14 are still in progress. Final results are expected later this spring.
While the FDA market sampling has not, as of yet, detected any raw milk cheese with viable HPAI, nor is the agency aware of any H5N1 illnesses from raw milk cheese consumption, the Cornell study has shown the potential for the cheese to not only retain the infectious virus, but to inadvertently make it to market. Additionally, consumption of raw milk cheeses is not without risk. Some studies have shown that the 60-day aging requirement may not fully inactivate all pathogens. From 2014-2016, FDA evaluated 1,600 samples of aged raw milk cheese and found very low levels of pathogens, with no evidence of any illnesses that tracked to these cheeses. Work at the University of Vermont also confirmed that, if present, pathogen population levels in raw milk are extremely low. However, comprehensive risk assessments considering the breadth of potential hazards need to be conducted.
Given all this, TAG advises that the industry ensure that any raw milk you use in cheeses, or any ready-to-eat/drink product is free of HPAI and other pathogens that may pass through milk.
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