Highly pathogenic avian influenza in mammals

Avian influenza caused by highly pathogenic avian influenza (HPAI) viruses, notably H5N1, has been causing global concern due to the unprecedented number of outbreaks since October 2021. As well as having a devastating impact on poultry farming, the spread of HPAI viruses to new geographical areas, wild bird die-offs, transmission to sea and land mammals, and potential for spill-over to humans, has heightened concerns.

While HPAI viruses primarily affect poultry and wild birds, they can occasionally be transmitted to mammals, including humans in close and repeated contact with infected birds. However, the viruses would need to undergo a number of adaptive mutations to allow efficient transmission between mammals.

Recent reports of H5N1 in mammals, likely infected by scavenging on wild birds, include foxes in Japan and the Netherlands, skunks, raccoons and bears in the USA, otters and seals in the UK and a cat in France. An outbreak in farmed mink in Spain (October 2022) and mass mortality of sea lions in Peru (January-February 2023) are more concerning.

Commenting on the cases in mammals, the World Organisation for Animal Health (WOAH) said, “The current situation highlights the risk that H5N1 avian influenza may become better adapted to mammals, and spill-over to humans and other animals. In addition, some mammals, such as mink, may act as mixing vessels for different influenza viruses, leading to the emergence of new strains and subtypes that could be more harmful to animals and/or humans. Recently reported infections in farmed mink are a concern because infections of large numbers of mammals kept in close proximity of each other exacerbate this risk. Several studies are currently on-going to further explore the virulence and transmissibility (including between mammals) of these viruses.”

In collaboration with its network of experts, WOAH is closely monitoring the situation to assess the risks to animals and humans.

The World Health Organization (WHO) has also called for increased monitoring of infections among mammals. At a press conference covering global health issues on 8 February 2023, Director-General Tedros Adhanom Ghebreyesus commented, “H5N1 has spread widely in wild birds and poultry for 25 years but the recent spill-over to mammals needs to be monitored closely.”

“For the moment, WHO assesses the risk to humans as low. Since H5N1 first emerged in 1996 we have only seen rare and non-sustained transmission of H5N1 to and between humans, but we cannot assume that will remain the case and we must prepare for any change in the status quo.”

He said that WHO is working with national authorities and partners to monitor the situation closely and to study cases of H5N1 infection in humans when they occur. WHO recommends countries strengthen surveillance in settings where humans and farmed or wild animals interact.

For more information from WOAH, see: Avian influenza

For more information from WHO, see: Influenza (avian and other zoonotic)

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