A recent study conducted by Cornell University, funded by the U.S. Food and Drug Administration (FDA) and New York State, has demonstrated that aging raw milk cheese for 60 days or longer is not on its own effective at eliminating the Highly Pathogenic Avian Influenza H5N1 (HPAI H5N1) virus. However, the study does suggest that that either heat-treating raw milk to adequate parameters, or manufacturing cheese at or below a pH of 5.0, could be effective at inactivating HPAI H5N1.

The U.S. Code of Federal Regulations requires that cheeses produced from unpasteurized (raw) milk must undergo a curing process for a minimum of 60 days at temperatures no lower than 1.67 °C (35 °F) to inactivate bacterial pathogens. However, research has not yet determined whether the HPAI H5N1 virus is inactivated through the cheesemaking process and the mandatory aging period. In light of the ongoing HPAI H5N1 outbreak infecting North American dairy cattle, the present study sought to fill this knowledge gap.

A pre-print version of the study is available. It has not yet been peer-reviewed, but has been disseminated by FDA. Authors on the study include Mohammed Nooruzzaman, D.V.M, Ph.D.; Pablo Sebastian Britto de Oliveira, Ph.D. candidate; and Diego G. Diel, D.V.M., Ph.D. from Cornell's College of Veterinary Medicine; as well as Nicole Martin, Ph.D. and Samuel D. Alcaine, Ph.D. from Cornell's College of Agriculture and Life Sciences. 

The researchers first evaluated the stability of HPAI H5N1 in raw milk cheeses during production and aging under varying pH conditions (6.6, 5.8, and 5.0), using a mini-cheese model prepared with virus-spiked raw milk. During the mini-cheese-making process, milk samples were collected immediately after HPAI-spiking, after pH adjustment, and after heating at 34 °C for one hour. Additionally, whey and curd samples were collected at different steps of coagulation and separation, including after heating at 40 °C for 20 or 30 minutes.

Throughout production, lower viral RNA loads were detected in samples from the pH 5.0 cheese group when compared to corresponding samples from the pH 6.6 and 5.8 groups. Viral titers were slightly lower in whey and curd samples from the pH 5.8 compared to the pH 6.6 group, and no infectious virus was detected in whey and curd samples collected from pH 5.0 cheese group, indicating a pH-dependent stability of HPAI H5N1 virus during raw milk cheese production.

Next, the researchers validated their findings by assessing the stability of HPAI H5N1 during cheese aging or curing at 4 °C over a 60-day period, using cheeses produced with commercial raw milk sourced from a farm that experienced an HPAI outbreak in its dairy herd. Cheese samples were collected daily between days 1 and 7, and then periodically on days 14, 21, 35, 42, 49, 56, and 60 of aging. The HPAI H5N1 viral RNA remained stable in raw milk cheeses throughout the entire 60-day aging period.

However, a pH-dependent survival of the HPAI H5N1 was observed. Infectious virus persisted throughout the cheesemaking process and for up to 60 days of aging in the pH 6.6 and 5.8 cheese groups, but at pH 5.0, the virus did not survive the cheesemaking process.

The researchers state that their findings highlight the potential public health risks of consuming raw milk cheese, underscoring the need for additional mitigation steps in cheese production to prevent human exposure to infectious virus.

The study was supported by FDA as part of its broader dairy food safety research agenda for HPAI H5N1, which includes other initiatives, like sampling and testing pasteurized dairy products at retail and aged raw cow’s milk cheese at distribution. So far, FDA has found that pasteurization is effective at inactivating HPAI H5N1 in dairy products, based on its retail sampling assignment.

Additionally, alongside the pre-print publication of the Cornell study, FDA shared interim results from its ongoing sampling of 60-day aged raw milk cheese at distribution. As of March 10, 2025, 110 samples of the planned 299 have been collected. Of the 110 samples, 96 were negative for HPAI H5N1 per polymerase chain reaction (PCR) analysis, and 14 are still in progress. Final results are expected later in the spring.