Avian Influenza

Key points

• Avian influenza (AI) is a contagious viral disease primarily affecting avian species. Avian influenza is divided into two pathotypes – high pathogenicity avian influenza (HPAI) and low pathogenicity avian influenza (LPAI).
• HPAI can be spread by domestic and wild birds. Domestic birds can become infected through exposure to virus from wild birds, and vice versa.
• The current outbreak, and the 2024 HPAI H7Nx outbreaks that occurred in Australia, are not the same as the HPAI subtype H5Nx clade 2.3.4.4b occurring overseas. At the time of writing, HPAI H5Nx 2.3.4.4b has not been detected in Australia. All HPAI outbreaks in Australia to date are suspected to have arisen from viral lineages already present in Australia.
• Overseas, HPAI H5Nx 2.3.4.4b has resulted in illness and deaths in birds.
• HPAI H5Nx 2.3.4.4b viruses have also been detected in numerous wild and captive mammalian species globally, including domestic livestock (dairy cattle, goats, alpaca, and a pig) in the United States (USA). These cases in the USA were the first known detections of HPAI H5Nx 2.3.4.4b in domestic livestock, with the virus having now been detected in dairy cattle across many states in USA.
• More details on the locations of historic and current outbreaks are available at WAHIS.
• Though the likelihood of HPAI H5Nx being introduced to Australia through migratory birds’ annual flight paths has increased with the globalisation of HPAI H5Nx, the likelihood is still considered to be low.
• Humans are susceptible to infection with AI viruses. Natural exposure to some AI pathotypes, including HPAI subtypes, has caused human disease in various forms, ranging from mild or inapparent infection to death. Most of the HPAI viruses circulating in birds do not readily infect humans. Where human infection does occur, it is usually via direct contact with the infected animal or its excretions.
• The likelihood of human infection with AI in Australia through normal contact with healthy birds is very low.
• There is no evidence that HPAI virus can be transmitted to humans through properly handled and prepared food.

What is the current Australian HPAI H7 situation?

• On 8 February 2025, Agriculture Victoria confirmed an outbreak of HPAI H7N8, with an initial detection of the virus in poultry at an egg farm in northern Victoria.
• Confirmation of this new outbreak occurred less than a week after Australia declared freedom from the three different H7 HPAI outbreaks in Victoria, New South Wales and the Australian Capital Territory in 2024, which were successfully eradicated.
• Laboratory testing has confirmed that this is a different strain to the strains in the 2024 Australian outbreaks.
• Further specific details about the current outbreak situation can be accessed via the Agriculture Victoria webpage and outbreak.gov.au

What is the current international HPAI H5 situation?

• HPAI H5Nx viruses have not been detected in animals in Australia.
• HPAI H5Nx 2.3.4.4b is spreading globally causing unprecedented outbreaks in domestic poultry and wild birds. Numerous species of terrestrial and aquatic mammals have also been infected.
• Avian influenza (AI) viruses are being detected in a growing number of bird orders. With this increase, HPAI may be becoming endemic in geographical regions such as Europe where HPAI has historically been seasonal. This is resulting in resurgent outbreaks and difficulty in eradication.
• HPAI H5Nx 2.3.4.4b viruses have been detected in domestic livestock, including dairy cattle, goats, alpaca and a pig in the United States (usda.gov). Wild migratory birds are believed to be the original source of the virus and cattle movements between herds are thought to have played a role in virus spread.
• The global situation with AI is dynamic, and continued vigilance is necessary.
  

About avian influenza

AI viruses are divided into two pathotypes — high pathogenicity AI (HPAI) and low pathogenicity AI (LPAI) — based either on the lethality of the virus in experimentally inoculated chickens or on the molecular characteristics of the virus. Wild birds are the natural host for LPAI and it does not result in significant disease.

Influenza A viruses have been isolated from most major bird orders —over 500 species, of which more than half are newly-affected since 2021. Experimentally, AI virus can infect almost all commercial, domestic and wild avian species. Humans are susceptible to infection with influenza A viruses.

AI viruses are widely distributed throughout the world, and outbreaks have occurred in Australia. Wild birds in Australia actively carry and shed LPAI virus without suffering noticeable clinical signs. All Australian outbreaks to date have arisen from endemic LPAI H7 viruses which evolved to acquire high pathogenicity.

The new HPAI subtype H5Nx (‘x’ due to the variety of neuraminidase subtypes detected, including H5N1 and H5N8) viruses have the ability to infect and cause disease in a wide range of avian species including poultry and wild birds. These new strains (particularly H5Nx 2.3.4.4b) differ significantly from previous HPAI H5 viruses in their increased outbreak frequency, geographic range and ability to spread via a wide range of avian species.

Signs of HPAI infection in birds

The clinical signs of HPAI virus infection are variable and influenced by the virulence of the virus, the species infected, the age of the infected individual, concurrent infection with other disease-causing agents, acquired immunity and environmental factors such as temperature. Pathogenicity in poultry can vary during an outbreak. Some birds may remain sub-clinically infected.

In severe forms, the disease appears suddenly and birds can die within 24 hours, sometimes without showing signs of the disease. In many cases, an increase in flock mortality is the first indicator of infection with HPAI. In chickens and turkeys, clinical signs include:

• lack of energy and reduced feed and water consumption
• severe respiratory signs with excessively watery eyes and sinusitis
• neurological signs such as tremors and paralysis
• cyanosis of the comb and wattles
• oedema of the head, resulting in swelling
• misshapen or soft-shelled eggs
• significant drop in egg production.

Ducks tend to predominantly display neurological signs, with decreased activity and lethargy as a common finding.

Of the cases in cattle in the United States, they showed clinical signs including decreased lactation, reduced appetite, lethargy, fever and dehydration.

Clinical signs alone are suggestive but not confirmatory. The possibility of HPAI must be investigated by laboratory testing. HPAI is nationally notifiable.

Could an HPAI H5 outbreak occur in Australia?

The current outbreak and the 2024 HPAI H7 outbreaks in Australia are not the same as the HPAI H5Nx 2.3.4.4b strain overseas. However future HPAI outbreaks (either H5 or H7) may occur in Australia.

Strains of HPAI virus could be introduced to, or arise in Australia through:

• migratory birds and other natural non-migratory movements of infected wildlife
• illegal importation of contaminated goods
• importation of contaminated poultry products, fomites, inanimate objects or people
• evolution of domestically-circulating Australian lineage LPAI viruses to acquire high pathogenicity.

Key resources to help manage an avian influenza response

Printable factsheet

Key resources to help better understand the disease

Key resources for biosecurity planning on farm