The H5N1 strain of avian flu has been around since 1996, but until now its been largely confined to animals.

But it has now jumped to cattle in America and some think it means we are inching towards eventual human-to-human infection, with potentially serious consequences.

H5N1 first appeared in geese in wet markets in Guangdong, China. Since then there have been various outbreaks around the world where chickens and wild birds have become infected. But as Covid was ravaging the human world, H5N1 began to spread quickly, killing millions of birds – both reared and wild – and affecting seal and sea lion populations around the globe. The virus has also appeared in a mink farm in Spain and polar bears in the Arctic.

In March, farmers in Texas and Kansas started reporting that their cows had low appetites and were producing less milk. Tests came back positive for H5N1. And this was not just individual cases brought on by chance contamination: the cows were infecting each other.

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Since March, H5N1 has been confirmed in dairy cattle in nine US states. Scientists are still trying to establish how the virus is being spread.

“Right now it seems like the milking equipment may be one of the ways,” says Dr Caitlin Rivers, an epidemiologist at the Johns Hopkins School of Public Health

“Scientists are finding very high levels of virus in milk. And so that's why the milking equipment seems like it might be playing a role.”

She adds that the spread of the virus between states seems to be down to the movement of infected cows across state lines.

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Reporting and surveillance of cattle movement in the US is poor compared with the UK and Europe where the BSE outbreaks of the mid-nineties led to all cows having to be tagged.

Science journalist Kai Kupferschmidt says that in an ideal world, we’d know how many cows were asymptomatic carriers and speed up our capacity to test for antibodies in the blood.

“We really should know a little bit more and have a bit more data,” he adds.

Also in March, a dairy farm worker in Texas began displaying symptoms after direct and close contact with sick cows. After treatment he made a full recovery. Tests on the virus showed it had mutated to be better adapted to mammalian cells.

Dr Rivers says that specific mutation has not been detected elsewhere, so it could be that the virus only mutated in that case.

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“As long as that human didn't pass the virus on to other humans, then it's sort of a dead end situation,” she says.

So we aren’t there yet, and perhaps not even close. But Kai Kupferschmidt says that while he doubts we’ll be in a H5N1 pandemic next week or next month, the virus is inching closer to humans.

“The reason we are not in a pandemic has nothing to do with us humans reacting appropriately in this situation,” he says.

“I think it's only because the virus, so far, seems pretty bad at infecting humans. So in some ways we are at the mercy of this virus‘ capabilities to change.”