The world is currently in the midst of its largest-ever outbreak of bird flu, and there is emerging evidence that mammals are being infected.
What is the current bird flu situation?
Since October 2021, when the latest outbreak began, the H5N1 subtype of the virus has spread through seabird colonies, farmed poultry and wild flocks across the US, Europe and beyond. Globally around 15 million domestic birds have died from bird flu over the past 15 months, and more than 193 million have been culled to stop the virus spreading to other flocks.
There is little authorities can do to limit the spread, apart from try to keep farmed birds away from wild populations. Since November 2022, all farmed birds in the UK, including those being reared as free-range, have been subject to a mandatory housing order, meaning they must be kept inside until further notice. From this week, 鈥渇ree range鈥� labels will start disappearing from egg cartons across the country.
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Is the outbreak spilling over into mammals?
The virus has been detected in a range of mammal species, from foxes in England to mink in Spain, grizzly bears in the US and seals in the Caspian Sea.
The World Organisation for Animal Health (WOAH) there have been 119 recorded outbreaks of bird flu affecting mammals, with about 200 individual cases recorded.
In the UK, suggest there have been outbreaks in otters and foxes. Some 49 mammal carcasses infected with bird flu were collected in 2021, 119 in 2022 and four so far this year.
Have we got a complete picture of what is happening?
WOAH warns that the number of mammalian infections is likely to be underreported, due to the difficulty of surveilling wild animals across continents. The UK Health Security Agency there is 鈥渧ery limited surveillance鈥� of mammals to monitor the spillover risk across the country.
It is unclear whether mammals always catch the virus directly from birds, or if it is spreading between mammals. Studies by Wageningen Bioveterinary Research in the Netherlands show there is no spread of the H5N1 virus between wild mammals such as foxes, with evidence suggesting those animals became infected independently of each other by eating infected wild birds.
鈥淲hat we don鈥檛 have any evidence of is that it can then go from fox to fox, or otter to otter 鈥� so these are what we call dead end infections,鈥� Ian Brown at APHA told BBC Radio 4 on 2 February.
However, on a mink farm in Spain suggests the animals were infected with a new variant of H5N1 that could spread between mink. Research is also ongoing to establish whether an outbreak that left more than 700 seals dead in the Caspian Sea in December 2022 could provide evidence of mammal-to-mammal transmission, .
More investigation is needed before mammal-to-mammal transmission is confirmed, says Munir Iqbal at the Pirbright Institute, UK. The virus in the mink farm could have spread via a contaminated water source or feed, he says. 鈥淚f one animal is infected and they are all sharing the same water, then probably it is through water,鈥� he says.
Can humans catch bird flu?
Humans can catch bird flu, although it is rare. Over the past 20 years, across multiple outbreaks, there have been almost 870 cases of human infection with H5N1, of which 457 were fatal, according to the World Health Organization.
Symptoms include a high temperature, aching muscles, headache and a cough or shortness of breath. People coming into close contact with infected birds are most at risk, and the public is advised to avoid close contact with wild birds and poultry. Currently there is no evidence the bird flu virus can pass between humans.
Yet researchers are worried that if the H5N1 virus mutates to transmit between mammals, it could soon be able to jump between human hosts. 鈥淚f the virus was able to transmit between wild animals then it would mean that the virus could sustain in those animals,鈥� says Iqbal. 鈥淚f the virus adapts to mammalian species, that increases the risks to humans.鈥�
