by Remnants of bird flu have been identified in pasteurized milk in the United States, where H5N1 appears to be circulating widely in cattle.
On Tuesday, the US Food and Drug Administration (FDA) said it has found genetic traces of the virus in commercially purchased milk, although the agency stressed that it had not seen “anything that would change our assessment” that the supply is safe.
This is because PCR tests, which are used to analyze the milk, can detect both live and dead virus fragments – and cannot distinguish between the two.
Although the FDA’s lengthy statement did not specifically say that no live virus was found in the milk supplies, it is generally much easier to detect harmless traces of H5N1 than it is to find an infectious or intact virus, said Dr. Angela Rasmussen , virologist at the Office. Vaccine and Infectious Disease Organization at the University of Saskatchewan.
“I agree that the milk supply is safe to eat,” she previously wrote on X, Twitter. “Influenza viral RNA is not contagious [and] it persists after heat inactivation at pasteurization temperatures. There is no reason to think that pasteurization doesn’t magically work for H5N1 except when it does for other viruses.”
The FDA’s statement comes amid continued criticism of its response since H5N1 was first detected in cows last month. The virus, which has killed thousands of birds worldwide since 2020, has been identified in 36 dairy herds across eight US states, raising concerns about its ability to infect mammals.
Race to understand the outbreak
There is also one human case, announced in early April – a dairy worker who developed conjunctivitis, or ‘pink eye’, after close contact with a herd of cattle. But according to Science Magazine, US vets have heard stories of more workers with these and other symptoms – such as fever, cough and lethargy – not wanting to be tested.
“I have also heard that there are other suspected human cases,” Dr. Gerald Parker, associate dean for Global One Health College of Veterinary and Biomedical Sciences in Texas, told the Telegraph. “I think this situation is developing quickly.”
Dr James Lowe, a researcher who specializes in swine viruses, told Science that he believes there are “probably many human cases”, although most are likely to be asymptomatic and US agencies are monitoring dairy workers .
But so far, the US Centers for Disease Control has said that surveillance, including emergency departments, has not revealed any unusual trends in flu-like illnesses, according to the Washington Post.
It is unlikely that any of these cases are related to the milk supply, as the pasteurization process involves heating milk at high temperatures to kill problematic microorganisms.
Since this is the first time bird flu has been detected in cattle, studies are limited as to whether or not it will survive the process. But one paper from 2022, by researchers in Germany, found that the presence of flu viruses including H5N1 could be reduced by four orders of magnitude in half an hour if heated at 60 degrees celsius in chicken meat, faeces and eggs.
Although milk is a very different substance, most scientists contacted by the Telegraph said they would be surprised if the live H5N1 virus survived pasteurisation. The FDA said additional laboratory testing is underway to grow the virus in cells and fertilized eggs, which is the “gold standard” to determine whether a virus remains viable.
‘Unfortunate lack of transparency’
Still, experts have warned that the FDA and the US Department of Agriculture should be transparent about what is and isn’t known definitively, and share details about how investigations have progressed to maintain public trust.
“I’m not worried about the transmission of H5N1 to humans (yet) but the lack of transparency of the USDA, how long it took to release the genomes, the lack of testing of asymptomatic cattle,” Dr. Eric Topol, director and founder of Scripps. Research Translational Institute, wrote on X.
Although the FDA said that investigations are underway and that they are “committed to sharing results as soon as possible”, there were glaring holes in the latest statement on milk. It did not say, for example, how many commercial samples were tested, where they were purchased, the percentage PCR-positive for H5N1, or the levels of genetic material found in the samples.
“This information was shared without any supporting data, which is characteristic of the current response,” Dr Rasmussen said. “It is very clear why all the relevant data was not shared quickly.
“I’m not attributing this to malice, incompetence, territorial interests, bureaucracy, or anything else, but it’s very clear that this lack of transparency and apparent urgency to share this relevant data is making a major detriment to both the US and the world’s ability to respond.
“It’s just a matter of time before we find infected cows elsewhere,” she said. “Cattle is big business all over the world – both dairy and beef production. There are billions of cows. Failure to share information quickly not only harms food security, the economy, and human/animal health in the US, but also prevents a global response to a major global threat.”
Symptoms of asymptomatic spread
Dr Rasmussen added that the discovery of traces of H5N1 in milk is most significant as it provides further evidence that the virus is spreading widely in cattle. It could also indicate widespread asymptomatic spread, which is “very bad news, making surveillance and eventual containment much more difficult”.
One source also told Science that seemingly healthy cattle were transported from Texas to North Carolina, bringing the virus with them. This also implies that herds can be infectious without showing symptoms, complicating the answer.
This comes as scientists have completed more than 239 genetic sequences of the H5N1 flu from poultry, wild birds, and dairy cows, which the USDA finally released on Sunday afternoon.
Professor Michael Worobey, a biologist at the University of Arizona, said that an analysis he helped put together based on these sequences suggests that the virus “has been circulating in cattle for months under our noses”.
This is because the virus appears to have emerged from a single episodic event late last year – rather than repeated exposure through contact with wild birds or animal feed.
On Tuesday, Prof. Worobey, Dr. Rasmussen and their colleagues released additional genetic analysis that confirms the USDA’s finding that H5N1 is also jumping back into birds from cattle.
“We are close to definitive evidence that US cattle H5N1 was of purely avian origin, and when avian viruses were found, they jumped from cattle back to birds,” he said, adding that adaptations known mammals. in grackles, black loons and chickens.
“So, do the birds on these farms give this virus to the cattle, or do the cattle give it to the birds? It is cattle for birds, I suppose. There is no good reason to think that epizootics of H5N1 have adapted from mammals in birds,” Professor Worobey said on X.
“Still, the caveat here is that there are still some uncertainties regarding the metadata here. Also, this may just be the primary bovine H5N1 strain.”
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