Cheeses made with raw milk are popular in the United States and the rest of the world, and the products are required to be aged for at least 60 days to inactivate bacterial pathogens. The fairly recent detection of H5N1 avian flu in dairy cattle, however, raises questions about whether the same process can inactive the virus.
The answer appears to be no, in most instances, scientists from Cornell University reported today on bioRxiv, a preprint server.
The US Food and Drug Administration (FDA) and the state of New York funded the study as part of efforts to assess the threat of H5N1 avian flu from potentially contaminated raw milk cheese. Separately, the FDA in December 2024 launched an effort to test nearly 300 60-day aged raw milk cheese samples, and today it provided an update.
In an update today, the FDA said that, of 110 samples that have been collected so far, 96 were negative on polymerase chain reaction testing. Tests on 14 others are still in progress. No samples have yet to test positive for H5N1.
Study findings raise food safety concerns
In the Cornell study, which has not yet been peer-reviewed, researchers made raw-milk cheese from raw milk spiked with H5N1 under different pH levels, ranging from 6.6 to 5.0—all in the acidic range. They also tested samples of raw milk cheddar cheese that was inadvertently produced with H5N1-contaminated raw milk after an outbreak in dairy cows on a farm.
In cheese made with the spiked milk, tests show that infectious virus remained throughout the cheese-making process and for up to 60 days of aging at the 6.6- and 5.8-pH levels. The virus didn't survive at the 5.0-pH level, which is the most acidic, supporting earlier findings that influenza A viruses are sensitive to acidic environments.
Ingestion of contaminated raw dairy products repeatedly may increase the probability of infections.
The investigators said their findings raise food safety concerns, not just for aged raw-milk cheese, but for other raw-milk products, especially given that the virus can persist for up to 56 days under refrigeration.
"Although the infectious dose of the virus to humans is not known, ingestion of contaminated raw dairy products repeatedly may increase the probability of infections," the team wrote.
NYC probes infections in cats; USDA notes more poultry detections
In other avian flu developments, the New York City Health Department today said it is investigating two cats in separate households that have H5 avian flu infections.
In a statement, officials didn't say how the cats might have been exposed. They urged people to avoid feeding pets raw food or raw milk and to prevent animals from roaming outdoors, where they may come into contact with wild birds or other animals.
Officials added that the risk to people remains low.
Also today, the US Department of Agriculture (USDA) Animal and Plant Health Inspection Service (APHIS) reported more H5N1 detections in poultry in two states. They include two backyard flocks in different counties in Oklahoma and a live-poultry market in Philadelphia.
