Bird flu in cows: 1 in 5 pasteurized milk samples had traces of virus, FDA says

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FILE - Dairy products inside a grocery store. (Photo by: Jeffrey Greenberg/Universal Images Group via Getty Images)

One in five samples of pasteurized milk tested across the U.S. contained remnants of the bird flu virus that has infected dairy cows, U.S. public health officials said. 

The U.S. Food and Drug Administration said the results came from a "nationally representative commercial milk sampling," and a greater proportion of positive results came from milk in areas with infected herds.

The announcement comes nearly a month after an avian influenza virus that has sickened millions of wild and commercial birds in recent years was detected in dairy cows.

So far, bird flu has been detected in 33 dairy herds in at least eight states: Idaho, Kansas, Michigan, New Mexico, North Carolina, Ohio, South Dakota, and Texas, according to the FDA.

The updated results shared Thursday follow an announcement made earlier this week from the FDA, which first said Tuesday that it found viral fragments in commercially-sold milk. However, the FDA and other health officials have maintained that it’s still safe to drink.

Pasteurization kills pathogens, scientists say

Milk sold at the grocery store is pasteurized, a process that kills harmful bacteria and viruses by heating milk to a specific temperature for a set period of time. The FDA noted the viral particles detected by highly-sensitive qPCR tests were likely to have been remnants of viruses killed during the pasteurization process.

"To date, we have seen nothing that would change our assessment that the commercial milk supply is safe," the FDA said.

Although the findings may be concerning to consumers, it doesn’t necessarily mean the milk contains a live virus that could cause an infection, according to Nam Tran, a professor and senior director of clinical pathology at UC Davis Health.

"With qPCR tests, the genetic material, not necessarily the whole active or infectious virus, is what is detected," Train said. "In the case of food, the genetic material, the RNA found in the grocery store milk samples, may not be the infectious H5N1 virus, but fragments from it."

To determine if any active, infectious virus remains in the milk samples, the agency is performing egg inoculation tests, which are considered "the gold standard" for determining viability, Tran said.

"With these tests, they inject the sample into the egg to see if the virus replicates or not," Tran explained, adding that this process takes longer to complete than other, less-sensitive methods of testing.

FDA officials didn’t indicate how many samples were tested or where they were obtained. Results of additional tests are expected in "the next few days to weeks."

Can you catch bird flu from eggs or meat? 

Only dairy cows, not beef cattle, have been infected or shown signs of illness to date, agriculture officials said.

The largest egg producer in the U.S. temporarily halted operations on April 2 after finding bird flu in its chickens. Cal-Maine Foods culled about 1.6 million laying hens and another 337,000 pullets, or young hens, after the detection.

The company said that there was no risk to eggs in the market and that no eggs had been recalled.

For its part, the U.S. Department of Agriculture says that poultry and eggs that are properly prepared and cooked are safe to eat. 

"Proper food safety practices are important every day. In addition to proper processing, proper handling and cooking of poultry provides protection from viruses and bacteria, including avian influenza," the USDA states.

The Associated Press contributed to this report. It was reported from Cincinnati.