Bird flu virus found in US milk supply

Remnants of bird flu have been identified in pasteurised milk in the United States, where H5N1 appears to be spreading widely in cattle. 

On Tuesday, the US Food and Drug Administration (FDA) said it has found genetic traces of the virus in commercially bought milk, though the agency stressed that they have “seen nothing that would change our assessment” that the supply is safe. 

This is because PCR tests, used to analyse the milk, can detect both live and dead virus fragments - and cannot distinguish between the two. 

While the lengthy FDA statement did not explicitly 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 infectious or intact virus, said Dr Angela Rasmussen, a virologist at the Vaccine and Infectious Disease Organization at the University of Saskatchewan. 

“I do agree the milk supply is safe to consume,” she wrote on X, formerly Twitter. “Viral RNA is not infectious for influenza [and] it persists after heat inactivation at pasteurisation temperatures. There’s no reason to think pasteurisation doesn’t magically work for H5N1 only when it does for other viruses.”

The FDA’s statement comes amid mounting criticism of its response since H5N1 was first spotted in cows last month. The virus, which has killed tens of millions of birds worldwide since 2020, has subsequently been identified in 36 dairy herds across eight US states, raising concerns about its ability to infect mammals.

Race to understand the outbreak

There has also been one human case, announced in early April - a dairy worker who developed conjunctivitis, or ‘pink eye’, after close contact with a cattle herd. But according to Science Magazine, US vets have heard anecdotes about more workers with these and other symptoms - such as fever, cough and lethargy - who do not want to be tested.

“I too have heard that there are other suspected human cases,” Dr Gerald Parker, an associate dean for Global One Health College of Veterinary Medicine and Biomedical Sciences in Texas, told the Telegraph. “I think this is becoming a rapidly evolving situation.” 

Dr James Lowe, a researcher who specialises in pig viruses, told Science he believes “there are probably lots of human cases”, though the majority are likely to be asymptomatic and US agencies are ramping up surveillance of dairy workers. 

But so far, the US Centre for Disease Control has said that surveillance, such as including to emergency departments, have not flagged any unusual trends in flu-like illnesses, according to the Washington Post. 

It is highly unlikely that any of these cases are linked to the milk supply, as the pasteurisation process involves heating milk at high temperatures to kill problematic microorganisms. 

As this is the first time that avian flu has been detected in cattle, studies about whether it survives the process are limited. But one 2022 paper, by researchers in Germany, found that the presence of influenza 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.

While milk is a very different substance, most scientists the Telegraph has contacted said they would be surprised if live H5N1 virus survives pasteurisation. The FDA said additional laboratory testing is underway to grow the virus in cells and in fertilised eggs which is the “gold standard” to determine whether live virus persists. 

‘Pathetic lack of transparency’

Still, experts have warned that the FDA and US Department of Agriculture should be transparent about what is and isn’t known definitively, and share details about how investigations have unfolded to maintain public trust.

“I’m not worried about H5N1 transmission to humans (yet) but the USDA’s pathetic lack of transparency, how long it took for the genomes to be released, lack of testing asymptomatic cattle,” Dr Eric Topol, director and founder of the Scripps Research Translational Institute, wrote on X. 

While the FDA said investigations are ongoing and they are “committed to sharing results as soon as possible”, the latest statement on milk had glaring holes. It did not, for instance, say how many commercial samples were tested, where they were purchased, the percentage PCR-positive for H5N1, or the levels of genetic material found in samples. 

“This information was shared with no supporting data, characteristic of the current response,” said Dr Rasmussen. “It’s very unclear why all the relevant data has not been shared rapidly. 

“I’m not attributing this to malice, incompetence, territorialism, bureaucracy, or anything else, but it’s very clear that this apparent lack of transparency and urgency to share these relevant data are greatly harming both the US and global capacity to respond.

“It’s a matter of time before we find infected cows elsewhere,” she added. “Cattle - both dairy and beef production - is big business around the world. There are billions of cows. Failing to share information rapidly not only hurts food security, the economy, and human/animal health in the US, it’s preventing a global response to a major global threat.” 

Signs of asymptomatic spread

Dr Rasmussen added that the discovery of H5N1 traces in milk is most significant because it is further evidence that the virus is spreading widely in cattle. It may also signal broad asymptomatic spread, which is “very bad news, that makes surveillance and ultimately containment much harder”. 

One source also told Science that seemingly healthy cattle had been transported from Texas to North Carolina, bringing the virus with them. This also suggests that herds can be infectious without showing symptoms, which complicates the response. 

This comes as scientists pore over 239 genetic sequences of the H5N1 flu from poultry, wild birds, and dairy cows, which were finally released by the USDA on Sunday evening. 

Professor Michael Worobey, a biologist at the University of Arizona, said analysis he has helped pull together based on these sequences suggests the virus has been “circulating in cattle for months under our noses”. 

This is because it looks like the virus has evolved from a single spillover event late last year - rather than repeated exposures through contact with wild birds or through animal feed. 

On Tuesday, Prof Worobey, Dr Rasmussen and their collaborators released further genetic analysis which confirms the USDA’s finding that H5N1 is also jumping back into birds from cattle.

“We’re close to decisive evidence that US bovine H5N1 had a single origin from birds, and that when related viruses from birds have been found, they are jumps from cattle back into birds,” he said, adding that mammalian adaptations have been identified in grackles, blackbirds and chickens. 

“So, are the birds on these farms giving this virus to the cattle, or are the cattle giving it to the birds? It is cattle to birds, very likely. There is just no good reason to think there’s an epizootic of mammalian adapted H5N1 in birds,” Prof Worobey said on X. 

“Still, the caveat here is that there are still some uncertainties about the metadata here. Also, this might merely be the major bovine H5N1 lineage.”

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