May 28, 2024

Bird flu continues to be a real public health threat, but apparently not to our food supply at least. The Food and Drug Administration and U.S. Department of Agriculture separately announced late Wednesday that their tests of pasteurized milk and ground beef failed to find any live H5N1 in samples at risk of being contaminated. The findings appear to confirm that pasteurization can effectively neuter the virus if it ends up in milk, but tainted raw milk might be still dangerous to consume.

Outbreaks of highly pathogenic H5N1 in cows have occurred recently in dozens of dairy farms across nine states, with cases dating as far as back as late 2023. These strains are considered highly pathogenic due to the massive illness and deaths that they can cause among wild and domestic birds. So far, infected cows have typically avoided severe illness, but many have experienced symptoms such as reduced or discolored milk production and low appetite. There have also been several fatal cases of H5N1 in cats tied to these farm outbreaks, along with at least one nonfatal human case.

In late April, the FDA reported that it had detected genetic material from strains of H5N1 in samples of store-bought pasteurized milk, results that were later replicated by other researchers. Officials noted at the time that the method used to find H5N1 in milk—highly sensitive quantitative polymerase chain reaction (qPCR) testing—could not confirm the presence of live virus. And the FDA stated that it would follow up with “gold standard” testing, which could include trying to inoculate (grow) samples of the virus detected via qPCR inside chicken eggs.

Last Friday, the FDA announced that an initial round of egg inoculation testing failed to find any live virus; yesterday, it reported that a larger round of testing turned up negative as well. All told, the agency has tested nearly 300 samples of dairy products, including sour cream and cottage cheese, collected from 38 states.

“[T]hese results reaffirm our assessment that the commercial milk supply is safe,” the FDA said in its latest update.

Elsewhere, a USDA report released on Wednesday assessing the meat supply should reassure the public. Officials collected 30 samples of ground beef from stores in states where dairy farm outbreaks had occurred and sent them off to the National Veterinary Services Laboratories for PCR testing. All of these tests have returned negative. Additionally, PCR testing of retail powdered infant formula and powdered milk products marketed as toddler formula by the FDA has been negative as well.

Experts did expect that pasteurization would render H5N1 inert, as it has for many other infectious germs. So the results aren’t too surprising. But given the unprecedented spread of bird flu in dairy cows, it was nonetheless important to know for sure. And we certainly aren’t out of the woods yet. The mere discovery of H5N1 in store-bought milk, coupled with genetic evidence of its arrival in cows months before anyone knew, strongly suggests that these outbreaks are far larger than our current data shows.

Bird flu is always a potentially serious threat in part because flu strains native to birds are less familiar to the immune system of humans and other mammals. Right now, these viruses can’t transmit well between people. But the longer that H5N1 is able to linger in cows, the greater the likelihood that some strains will adapt and become better at spreading between mammals, humans included. And the right assortment of mutations could turn a bird flu virus into a deadly and fast-spreading pandemic germ.

While pasteurized milk might be safe to drink, the same isn’t necessarily true for raw milk. Raw milk is regularly a vector for many infectious diseases. And some cases of H5N1 in cats are already suspected to have been caused by the cats drinking contaminated raw milk. The FDA announced Wednesday that it would next extend its testing to raw milk to better assess the threat posed by these products.

A version of this article originally appeared on Gizmodo.

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