Pasteurization inactivates H5N1 bird flu in milk, new FDA and academic studies confirm

The results confirmed findings of earlier testing of a more limited number of samples and add weight to the FDA's conclusion that pasteurized milk products are safe for consumption despite a widespread outbreak of cows infected with H5N1.

"These additional, preliminary results further affirm the safety of the U.S. commercial milk supply," Donald Prater, acting director of the FDA's Center of Food Safety and Applied Nutrition, told reporters.

Last week, the FDA announced it had conducted PCR testing in 96 commercially purchased milk products and found genetic traces of the H5N1 virus in 1 in 5 samples, but that early data showed no live virus. On Wednesday, the agency reported results from testing of a further 201 products, which included cottage cheese and sour cream, in addition to milk. Any PCR-positive samples were then injected into embryonated chicken eggs, to see whether any active virus could be grown — the gold standard test for assessing the viability of an influenza virus. None of the samples produced viable, replicating virus, Prater said.

The 297 samples that the FDA has tested so far represent dairy products from 38 states. In addition, several samples of retail powdered infant formula as well as other powdered milk products were tested. All PCR results from these products were negative. The agency did not disclose when it plans to make its full analysis, including which products were purchased from which states, available to the public.

Bolstering the FDA's data, academic researchers at the Ohio State University and St. Jude Children's Research Hospital told STAT Tuesday night that their own study of 58 PCR-positive milk samples taken from Texas, Kansas, and eight other states in the Midwest also failed to turn up any evidence that H5N1 can survive the pasteurization process.

"It was pretty much what we were expecting," said Richard Webby, an influenza virologist at St. Jude whose laboratory conducted the viral growth experiments — in mammalian cell lines as well as embryonated chicken eggs. The samples they tested had varying levels of viral RNA, from very low to pretty large amounts, Webby said. Although it was nothing like what's been seen coming off the udders of infected cows. "Certainly, the viral load is getting diluted," he said.

But there was still enough RNA in the samples they were testing that if those levels had represented real, viable virus, he would have expected to be find it.

"If these samples had full virus particles capable of replication, we would have expected to see growth in cells and eggs, which we did not see. The conclusion is we simply don't think there's any replicating virus in these samples. What we're finding are probably just remnants of the virus."

Webby and other scientists are more worried about possible health risks from drinking raw milk. Although little is currently known about the potential for humans contracting H5N1 from consuming raw milk from infected cows, researchers are raising the alarm that the consequences could be dire. In the wild, mammals like foxes, bears, and seals that have been sickened with H5N1 — presumably from eating infected birds or drinking water contaminated with their feces — have suffered severe symptoms, including brain damage and death.

"In humans we don't know how much milk you'd have to drink to get infected, or if 10 people drank raw milk from an infected cow how many would come down with it," Webby said. "But it's not an experiment I'd want to participate in."

The FDA is urging consumers not to drink raw milk or eat raw milk cheeses. That is a position the agency has long held, because of the other health risks these products hold, but it has re-emphasized it in the context of the current outbreak. "We continue to strongly advise against the consumption of raw milk," Prater said at the news conference Wednesday.

He did not say whether the FDA has conducted any testing of raw milk products currently on the market. But it is testing samples of pooled raw milk — milk that is trucked in from multiple farms to centralized processing facilities where pasteurization takes place. This will be used to better understand how much virus from asymptomatic or presymptomatic animals is making it into the milk supply, and inform further studies to validate the effectiveness of different methods of pasteurization, Prater said.

FDA has also recommended the dairy industry not "manufacture or sell raw milk or raw milk products, including raw milk cheese, made with milk from cows showing symptoms of illness, including those infected with avian influenza viruses or exposed to those infected with avian influenza viruses." It has also urged producers to discard milk from affected cows and pasteurize raw milk from exposed cattle before feeding it to other animals.

Cats that have been fed raw milk from cows infected with H5N1 have been sickened and some have died, according to a study published Monday. So far, only one person — a farm worker in Texas who developed conjunctivitis — has been confirmed to be infected by dairy cows.

H5N1 bird flu has been circulating in dairy cow herds in parts of the country for months now. So far, the U.S. Department of Agriculture has reported 36 herds in nine states have tested positive for the virus, but the genetic evidence of H5N1 in milk suggests the outbreak is more widespread.