Dairy Cows (c) Ben Andrew (rspb-images.com)
This blog is written by Emma Horton, Senior Policy Officer – Wildlife Disease and Susie Gold, Senior Conservation Scientist – Wildlife Disease. Here we cover the situation with avian flu in the United States, information on current risks and highlight how important it is that Governments take a “One Health” approach which recognises that human, environmental, domestic and wild animal health are inter-linked.
Since 2020, highly pathogenic avian flu (HPAI) has swept across the globe with devasting impacts on both wild birds and poultry. In addition, since the start of this global outbreak there have been sporadic cases of the virus reported in mammals, in some cases leading to mass mortality events. While most of these cases have been from exposure to infected birds, in some marine mammals, such as Elephant Seals, as well as in fur farms, there has been evidence of mammal-to-mammal transmission. As of last year, a further concerning development has been the emergence of avian flu strains in the United States (US) that are spreading through cattle.
What is the situation with avian flu in the United States?
The lineage of HPAI responsible for the current global outbreak first emerged in poultry in East Asia and has since spread around the world through both poultry and wild bird movements. Following introduction into the US, the virus has evolved into new strains through recombination with low-pathogenic viruses present in US bird populations.
On March 25th last year, there was an unexpected development when the US reported their first detection of one of these avian flu strains in dairy cattle. This was the first time avian flu had ever been reported in cattle, and the first time that spread had been reported in an herbivorous mammal. Since then, over 900 herds across 17 states have been reported as affected by HPAI. The virus is found at high concentrations in the milk from infected cows, and cow to cow spread has been linked to contaminated milking equipment. Unlike the high mortality seen in poultry from HPAI infection, infected cattle generally show mild symptoms, which can be missed, allowing spread from farm to farm through movement of undetected infected animals.
The spread in cattle comes on top of the significant HPAI outbreaks the US had already experienced over the last few years. In addition to the spread of the virus in wild birds and poultry, spillover has also been reported into both wild mammals and other domestic mammals, including in two pigs in a mixed-species farm. Cats have also been affected, with a considerable number of cases in domestic cats linked to farm outbreaks, as well as infections in pet cats, and big cats in zoos, attributed to consuming infected raw meat. Concerningly, the number of confirmed cases in US farmworkers exposed to infected animals (poultry and cattle) has also been rising.
What is the current risk to human health?
As you would expect, avian influenza viruses are adapted to spread in birds, which makes the current risk to the general public very low and human infections are rare. Even when people do catch HPAI, the virus is not well adapted to human cells so passing the virus onto other people is currently highly unlikely. No human-to-human transmission has been reported in the US to date, and sustained human-to-human transmission of H5N1 avian flu has never been observed globally. Spread of the virus between people would require the virus to evolve and adapt to human cells. This is why of all the reported cases of HPAI in humans worldwide, almost all have been associated with extremely close contact with infected birds (dead or alive) within populations of domestic poultry or more recently in the US, dairy cattle. Farmworkers who have extremely close contact with infected animals are at elevated risk, and good disease surveillance (in people and animals), management of outbreaks, biosecurity and Personal Protective Equipment (PPE) are essential to manage that risk.
There are two strains of avian flu currently circulating in North America that have caused recent human cases, neither of which has been detected elsewhere. One of these is the strain that was first detected in cattle (B3.13) and has since affected over 900 herds, as well as causing poultry outbreaks. The second strain (D1.1) has been circulating widely in wild birds and poultry, however, was recently also detected in cattle, showing multiple spillovers into cattle have occurred. Across both these strains, in the US over 60 detected HPAI infections have been reported in farmworkers. In addition, testing of farmworkers and vets who had contact with cattle has shown evidence for further cases that have gone undetected. There is concern that this increase in spillover into humans will provide the virus with more opportunities to adapt to human cells.
What would happen if an avian flu virus adapted to humans?
If spillover into humans of HPAI did lead to emergence of a strain that was able to spread effectively between people, it would be expected to essentially become a new ‘human’ influenza virus. Due to the differences in bird and human cells, it is likely that a human adapted strain would no longer be well-adapted to bird cells or spread easily in birds. So, at this point we would expect transmission to be mostly human to human – not from birds. And it is important to also highlight that there have been no detections outside of North America of the US strains which are causing concerns.
What about the human avian flu case recently reported in England?
The case reported in a person in England at the end of January is linked to a poultry farm outbreak and is not linked to the cases in the US which have been caused by North American strains of the virus. The UK Health Security Agency (UKHSA) has advised that the person had ‘close and prolonged contact with a large number of infected birds’ and that there is no evidence of onward human spread. This is also one of only seven cases in the UK since 2021, highlighting the rarity of such an event. All seven people have recovered fully.
Do UK wild birds pose a risk to people?
It’s important to emphasise that UKHSA advises that HPAI continues to pose a very low risk to the general public’s health. This is because the level of close contact which poultry workers have with infected animals, which causes their elevated risk, would not occur under normal circumstances between the public and wild birds. Close contact would include touching infected animals; contact with droppings or bedding; or inhalation of aerosols (e.g. feather dust). Additionally, molecular analyses indicate that HPAI viruses circulating in birds in Europe continue to be well-adapted to avian species.
It is however always sensible to take precautions around activities such as bird feeding to protect both yourself and wild birds, which includes following best practice hygiene advice which can be found here. Confirmed HPAI cases in birds that come to garden feeders are very low. However, there are other diseases which can impact wild birds and be transmitted at shared food and water sources, like Trichomonosis. Other pathogens can also be present at feeders, such as Salmonella Spp., which can affect people although the risk is considered low and sensible hygiene steps can mitigate this risk.
Government advice continues to be that the public should avoid touching any sick or dead birds, and should report any dead birds found to Defra (in Great Britain (GB)) or DAERA (in Northern Ireland (NI)) for disease monitoring purposes. Government guidance on removing and disposing of dead wild birds can be found here (for GB) and here (for NI). If you’re concerned about dead birds in public areas, please report them to your local authority. Dead birds on private land should be reported to the landowner or land manager.
Please also immediately report (to APHA in GB or DAERA in NI) any suspicion of notifiable diseases in domestic animals, or in wild mammals: unusual mortality (including two or more dead in one location); death near known/suspected disease outbreaks or neurological or respiratory symptoms.
The importance of a “One Health” Approach
Following the spread of avian flu in US cattle, key measures to reduce risk recommended by the World Organization for Animal Health (WOAH) include enhanced surveillance (in cattle, poultry and wild birds), preventative measures to reduce further spread within cattle, and the implementation of effective protection to reduce exposure risk to humans.
Avian flu continues to pose a risk to wildlife health and biodiversity conservation, domestic animals, human health, livelihoods, and economies globally. An integrated One Health approach, involving effective collaboration between human, domestic animal and wildlife health organisations is therefore essential.
Further reading
For more information on avian flu and public health please visit the UKHSA, NHS and World Health Organisation websites.