Scottish wild bird high pathogenicity: avian influenza response plan

2. Pathogen and routes of incursion for avian influenza

2.1. Disease

Avian influenza refers to the disease caused by infection with avian influenza Type A viruses. These viruses include a broad diversity of subtypes that naturally spread among wild birds worldwide and can infect domestic poultry and other captive birds.

There are four types of influenza viruses: types A, B, C and D with some strains of Influenza type A viruses of most significance to public health. Influenza type A viruses are classified into subtypes according to the combinations of different virus surface proteins hemagglutinin (HA) and neuraminidase (NA). There have been 16 different HA proteins and 9 NA proteins detected in viruses from wild birds. The H5 and H7 are considered to be the most important from an animal health perspective, as they are the only subtypes to have been identified as causing high pathogenicity infection in birds. Today’s most concerning clades are the 2.3.4.4b that dominates globally, but also clade 2.3.2.1a and 2.3.2.1c H5Nx viruses. A small number of people have been infected around the world with clade 2.3.4.4b HPAI H5Nx viruses and so precautionary steps are taken to mitigate this risk as much as possible.

All positive detections of HPAI in poultry and both wild and captive mammals are sequenced by the UK’s National Reference Laboratory (NRL) (Weybridge) (see roles and responsibilities for more details) and where derived from mammalian species, sequences are assessed for indications of mammalian adaptation and antiviral sensitivity. Whilst virus detections in mammals have occasionally contained early mammalian adaptations, the zoonotic risk of all clade 2.3.4.4b H5Nx viruses detected in GB to date remains very low.

The outbreaks of HPAI between 2020 and 2025 were caused by H5Nx (clade 2.3.4.4b) viruses, with H5N1 dominating since initial emergence in 2021. The continued detection of H5Nx infection in wild birds since 2020 demonstrates that the virus is still circulating in wild bird populations. Alongside H5N1, H5N5 has also been detected sporadically in wild birds with both poultry and mammalian cases detected in England during 2024/25.

Avian influenza is a notifiable animal disease in poultry and other captive birds. Failure to report is an offence. If you suspect any type of avian influenza in poultry or other captive birds, you must report it immediately by contacting:

• If in Scotland your local Animal and Plant Health Agency (APHA) Field Services Office.
• If in England call the Defra Rural Services Helpline on 03000 200 301.
• If in Wales, contact 0300 303 8268.

Avian influenza is not a notifiable disease in wild birds. However, members of the public should use the online reporting system or call the GB telephone (03459 33 55 77) if they find dead wild birds. See contact information at the end of this document for further details.

2.2. Routes of incursion

Avian influenza can be spread in several ways:

• Movement of infected birds, from bird-to-bird, by contact with contaminated body fluids and faeces.
• Movement of contaminated objects and surfaces.
• Ingestion of infectious material.

An avian influenza outbreak can occur at any point in the year. However, notifiable avian influenza is not endemic in poultry in the UK, rather the UK typically faces a seasonal increase in the risk of a notifiable avian influenza incursion associated with the winter migration patterns of wild birds to the UK. HPAI incursions in wild birds in the UK follow migratory patterns of Anatidae (ducks, geese and swans), which means infections are typically reported between autumn and spring. However, the recent and unprecedented spread of infections in Charadriiformes (shore birds) and Suliformes (orders of seabird species) have elongated the infection timeframe extending into late summer.

In late autumn or early winter, wild migratory wildfowl have the potential to carry the HPAI infection to the UK. In contrast, the risk of an avian influenza incursion during the summer typically decreases because environmental conditions (warm, dry, high UV exposure) can reduce virus survival in the environment.

The UK is at the centre of a number of bird migration flyways (see Figure 1). This represents challenges when trying to model potential transmission routes. The main flyways for migrating birds to and from the UK are:

• Greenland/Iceland/ North-East Atlantic,
• North East Arctic archipelagos
• Feno/Scandinavia
• Continental Europe
• South Atlantic

The FluTrailMap research project, funded by the Biotechnology and Biological Sciences Research Council (BBSRC) and the department for Environment, Food and Rural Affairs (Defra), supports environmental sampling for avian influenza virus as part of the project. The FluTrailMap project is a consortia of government and non-government organisations that, in Scotland, is supported through activities delivered by scientists at the Roslin Institute and the University of Edinburgh where environmental testing is being undertaken (see Research and Monitoring section).

The main clinical signs of HPAI in birds are listed in Annex B, and can be found online at: How to control avian influenza - Avian influenza (bird flu): how to spot and report the disease.