Avian influenza, commonly referred to as 'bird flu', is a highly contagious viral disease that affects poultry and wild aquatic birds, and it can also be transmitted to mammals, including humans.¹ Avian influenza viruses are classified as either high or low pathogenic, based on their characteristics and their ability to cause disease and mortality in chickens. Low pathogenic avian influenza (LPAI) typically causes no noticeable or only mild clinical signs, while highly pathogenic avian influenza (HPAI) can lead to severe illness and even death.

HPAI is causing significant mortality rates amongst domestic and wild birds. Over the past two decades, various strains of the Avian influenza virus have been circulating in wild birds without causing any fatalities. However, the most recent H5N1 subtype has changed that, resulting in mass mortality among wild bird populations.² Additionally, there is a rising number of infections in both captive and wild mammals. The virus has been identified in and has caused fatalities in species such as minks, cats, raccoons, dolphins, foxes, sea lions, seals, and even polar bears.^1,3-5

Avian influenza causes significant suffering and high mortality to birds — poultry and wild species. Attempts to control its spread rely on the mass killings of both sick and healthy animals.⁶ Additionally, during outbreaks, free-range poultry are locked up in barns without outdoor access, sometimes for periods lasting several months.

When avian influenza is detected on a farm, all birds in the affected facility are killed to control the spread of the virus. Oftentimes, birds located in farms within a certain radius will also be killed.^1,7 This sadly means that not only infected birds are killed, but also healthy ones.¹ Between 2004 and 2024, HPAI caused the death and mass killing of more than 633 million poultry globally.⁸

In such emergency mass killings, animal welfare is not a priority. Methods of execution that cause unfair suffering are frequently used such as ventilation shutdown, house gassing, and house gassing with gas-filled foam, among others.^9,10 Approved methods for mass killing vary between countries.

When the risk of avian influenza outbreaks is high, farms with outdoor access must keep animals indoors to prevent contact with wild birds and further infections.⁷ Indoor confinement can last for months and negatively impacts animal welfare through, for example, overcrowding, stress and high ammonia levels. In the long run, it can make farmers favour barn-keeping over free-range or organic keeping due to the reduced profitability of the latter two. This would have additional impacts on the health and welfare of the animals kept in these facilities.

Concerns regarding avian influenza have escalated with the emergence of outbreaks in dairy farms. In March 2024, the subtype H5N1 was first reported in dairy cows in the USA. By April 2025, 998 cases in dairy herds in the USA had been confirmed.¹³ The clinical signs in cows include mild respiratory signs, reduced appetite and milk production.^14,15 In March 2025, the first case of an infected sheep was detected in the UK.¹⁶ The viral strain affecting dairy cattle originated in wild birds and was then transmitted to cattle, with infected cows then spreading the disease to other cows, cats, poultry and humans.^5,17,18

Avian influenza is killing wild birds and mammals at alarming rates. At least 485 bird species and 37 mammal species have been affected since 2021.¹⁹ Avian influenza has been reported in wild red foxes, skunks, common raccoons, mountain lions, and bears, amongst many other species.² Wild animals in captivity have also been infected, such as the reported cases in big cats in Vietnamese zoos.^{8, 20} This shows that avian influenza has became a conservation problem, threating biodiversity and ecosystems.  

Until now, the transmission of avian influenza viruses to humans has been rare, non-sustained and associated with exposure to infected birds or contaminated environments. According to data from the World Health Organisation (WHO), between 1 January 2003 to 22 April 2025, 973 cases of the H5N1 subtype were reported in humans, in 25 countries. Of these, 470 were fatal making it a 48.3% case fatality rate.¹⁷

While international organisations have assessed the current risk to humans as low, they have also warned that this may change rapidly as the viruses may further adapt to mammals and pose a risk to human health.²¹ In fact, the rising number of infections in mammals and the confirmed mammal-to-mammal transmissions, renew the concerns that the virus is quickly evolving, and as it continues adapting to mammals, it may become effectively transmissible between humans, which could trigger a new pandemic.⁴

Policy measures adopted to tackle avian influenza focus on biosecurity, mass killing and vaccination. However, they are not enough. There is an urgent need for measures that reduce the number of farmed animals and improve animal welfare.    

Vaccination cannot make farms immune to HPAI, just as biosecurity cannot bulletproof them against the virus. Outbreaks often occur in establishments without outdoor access, and poultry production systems with high biosecurity standards are also affected. Vaccination and biosecurity are important, but while they can help contain outbreaks, more must be done to effectively address the drivers of the disease: the poor keeping and treatment of poultry in factory farms.