Scientists fear bird flu has spread from mammals to humans for the first time, marking another step in the evolution of the deadly virus.
A new analysis concludes there is “strong evidence” that a Texas farm worker who tested positive for the H5N1 virus he contracted it from sick dairy cattle.
Although there have been other people infected with the virus in recent years, including some who have died, they all acquired it from birds.
There is growing concern about the failure of US authorities to contain the spread of the virus.
So far 36 stoves in nine states have tested positive. But tests on milk have suggested the virus has spread much more widely.
The longer it spreads unchecked in a domesticated mammal, the greater the chance it will adapt to a new species and spread to humans in close contact.
According to the report published in The New England Journal of Medicine, the unidentified man’s symptoms were mild. He developed conjunctivitis from an eye infection, but had no fever or trouble breathing. His lungs were also clear.
He and the people he lived with were given antiviral medication as a precaution. Nobody else got sick.
Use the Chrome browser for a more accessible video player
How does bird flu spread?
The team, which included researchers from the Centers for Disease Control, said: “Since the infected human was a dairy farm worker with exposure to sick and presumed infected cows in Texas and no exposure to other mammals or birds, we believe that genetics and epidemiological data are strong evidence of human infection following exposure to cows presumed to be infected with the virus.
Some infected mammals
The highly pathogenic strain of H5N1 has spread throughout the world through wild birds.
At first it was mainly poultry that became infected, and millions were culled to prevent further spread.
But there have been some infected mammals, including a small number of foxes, seals and dolphins in the UK.
There have also been suspicions that the virus has spread among farmed mink and within sea lion colonies.
But there was surprise when it appeared in the US in dairy cows, which were suffering a sharp reduction in milk production.
The virus has been spreading for months
Evidence suggests the virus has been spreading undetected since the beginning of the year. Some cows are asymptomatic.
More from Sky News:
First wild animal seen using plants as medicine
They reconstruct the face of a 75,000-year-old Neanderthal woman
Avian flu has been found in seals in the United States and the United Kingdom. Photo: AP
Scientists believe the man could have been infected by a virus transmitted through the air in the milking parlor that landed in his eyes, or by a virus on his hands or gloves that transferred to his eyes.
Although the farm worker only had a mild infection, H5N1 can be serious and even fatal to humans.
According to the World Health Organization, 888 people have been infected worldwide between 2003 and the end of March this year, with 463 deaths.
Beef and dairy products are safe
US authorities have detected viral fragments – but not active virus – in pasteurized milk. They say dairy and beef are safe.
Stay up to date with the latest news from the UK and around the world by following Sky News
In the United Kingdom, a risk assessment by government scientists has concluded that the threat of a similar outbreak in dairy herds is very small.
They say the animals are kept in different conditions and there is little chance of wild birds transmitting bovine flu across the Atlantic.