The world is in the throes of its biggest ever outbreak of bird flu and there is emerging evidence of mammals becoming infected.
ird flu has been recorded in various animals, including foxes and mink, but it is not clear whether the virus can be transmitted between mammals.
The latest outbreak began in October 2021 with the spread of the H5N1 subtype among seabird colonies as well as farmed poultry and wild flocks across the US and Europe. This has resulted in the deaths of up to 15 million domestic birds and more than 193 million birds being culled to curb the spread to other flocks.
The real concern is how transmissible bird flu is among humans
Since November 2022, all farmed birds in the UK have been subjected to a housing order, requiring them to be kept inside. This has led to the “free range” label disappearing from egg cartons.
Last November, outbreaks of bird flu were confirmed in two poultry flocks in Co Monaghan, resulting in 8,000 birds being culled. These numbers are low in comparison to the millions of birds that had to be culled in countries across Europe and the US. Part of this is due to the ability to apply a housing order in Ireland where owners are required to keep poultry indoors, which is not always possible in other countries with more intensive outdoor poultry farming.
Since the beginning of the epidemic to September 2022, more than 2,500 outbreaks in poultry, 227 in captive birds and almost 4,000 detections in wild birds were notified in 37 European countries. The persistence of highly pathogenic avian influenza (HPAI) in wild birds and poultry throughout the summer of 2022 was unusual in that there was no clear separation between the end of the first year of the epidemic and the beginning of the 2022 HPAI season in October 2022.
Avian flu has been found in foxes, which are believed to have eaten dead wild birds infected with the virus. Photo: Getty
Apart from the risk to both wild birds and the poultry industry, there is equal concern that the virus has been detected in mammals, including foxes in England, mink in Spain, grizzly bears in the US and seals in the Caspian Sea.
The World Health Organisation for Animal Health (WOAH) says there have been 119 recorded outbreaks of bird flu affecting mammals with about 200 individual cases recorded. The Animal and Plant Health Agency (APHA) in the UK, says there have been outbreaks in otters and foxes.
How mammals become infected is not clear. Research from the Wageningen Bio veterinary Research in the Netherlands, shows there is no spread of H5N1 between wild animals such as foxes, suggesting that animals become infected independently of each other by eating infected wild birds.
“What we don’t have evidence of is that it can then go from fox to fox, or otter to otter – so these are what we call dead-end infections,” said Professor Ian Brown, scientific services director at the APHA.
A future vaccine would need to be effective for all different strains of the virus, as it changes over time, to be feasible for widespread use.
The real concern is how transmissible bird flu is among humans.
Humans can catch bird flu, although it is rare. The WHO says that over 20 years, across multiple outbreaks there have been almost 870 cases of human infection with H5N1, of which 457 cases were fatal. Most of these occurred in Africa and Asia and were linked to the handling of infected live poultry. To date, there is no evidence of person-to-person spread of avian flu. The worry is that if the H5N1 virus mutates to transmit between mammals, it could soon jump between human hosts.
“There is concern about it having pandemic potential,” said molecular virologist Wendy Blay Puryear, of Tufts University, Boston. “Before Covid was on anybody’s radar, this was the one that we were all watching very closely.
‘The virus is currently considered a low risk to humans. But anything that has the ability to replicate and evolve rapidly, and anything that has that ability to infect a lot of different hosts is kind of on borrowed time,” she said.
“The chances of this happening are small, but the impact – if it does happen – is very big, because it means that we have a new influenza pandemic,” said Professor Thijs Kuiken, of the Erasmus University Medical Centre, Rotterdam.
Ian Barr is the deputy director of the WHO’s Collaborating Centre for Reference and Research on Influenza in Melbourne. He suggests the virus would probably require more than one or two changes to enable human-to-human transmission.
‘The more viruses that are out there, the more species that they infect, the longer they hang around for, then the more chance there is for something to mutate or go awry,” he said.
The presence of the virus in wild birds as well as farmed poultry has allowed it to ping-pong between populations and amplify its spread.
Professor of Biology at the University of Massachusetts, Nichola J Hill suggests this has created a new era “that’s a far more complex situation in terms of figuring out how to control it and predict where it will go next”.
While avian flu vaccines are available, they are not being used on a large scale on poultry farms because this hinders the ability to conduct surveillance testing, which helps detect the virus in unvaccinated flocks and limit the spread of the disease.
A future vaccine would need to be effective for all different strains of the virus, as it changes over time, to be feasible for widespread use.
What is clear, said Prof Hill, is the role our food system has played in getting us to this point.
“All of these amount to a situation which is ripe for the virus to get a foothold and then evolve towards virulence,” she said.
Dr Catherine Conlon is a public health doctor in Cork and former director of human health and nutrition with Safefood
