H5N1 has killed tens of millions of birds and thousands of mammals worldwide since 2020 in an unprecedented outbreak.
It is still unclear how the virus first jumped to cattle, but many think the most likely route of infection is via wild birds, which have been found dead on some US farms.
Its apparent ability to spread within cows is of particular concern because, with every new mammal H5N1 jumps to, it has more opportunities for the virus to evolve – and, potentially, to better infect humans.
So far this year, there have been two confirmed cases of H5N1 in humans.
In Vietnam, a man died in March after direct contact with an infected bird, whilst in Texas a farm worker caught the virus from sick cattle – although his symptoms were mild.
A spokesperson for the Centre for Disease Control said last week they are monitoring a further 44 people who have been exposed to the virus via cattle for signs of infection.
The WHO considers the risk to humans low, but urged countries to rapidly share information to enable real-time monitoring of the situation to ensure preparedness as the virus continues to spread.
It also warned on Tuesday that there is a risk of H5N1 jumping to cattle in countries beyond the US via migratory birds.
“With the virus carried around the world by migratory birds, certainly there is a risk for cows in other countries to be getting infected,” Wenqing Zhang, head of the agency’s Global Influenza Programme, told reporters in Geneva.
A Government spokesperson said: “These reports do not change the risk level for animals for the UK, which is currently ‘low’ and we have no reason to suspect the virus is circulating in our cattle and nor is this virus strain circulating in Europe.
“Food Standards Agency advice remains that avian influenza poses a very low food safety risk for UK consumers.”
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