Avian influenza (AI) is an infectious viral disease caused by strains of influenza A virus. It mainly affects birds. Water birds such as ducks, swans and geese (Anseriformes) and gulls, terns and shorebirds (Charadiformes) are the natural reservoirs for avian influenza A viruses.
Avian influenza strains are classified as either low pathogenicity (LPAI) or high pathogenicity (HPAI), depending on how severely the strain affects poultry.
Avian influenza strains also have subtypes. The names of the subtypes reflect the surface glycoproteins (haemagglutinin [HA] and neuraminidase [NA]) present (for example, H5N1).
High pathogenicity avian influenza (HPAI) strains
HPAI strains cause severe outbreaks in poultry and outbreaks of HPAI H7 subtypes have occurred in Australia.
These include:
- May 2024: HPAI H7N3 and HPAI H7N9 in Victoria
- June 2024: HPAI H7N8 New South Wales and the Australian Capital Territory.
These outbreaks were the result of Australian lineage LPAI strains ‘spilling over’ from wild birds into poultry farms, where the virus mutated to become HPAI. All previous outbreaks of HPAI in poultry in Australia have been linked to similar spillover events.
See the Agriculture Victoria, NSW Department of Primary Industries and ACT Government webpages for updates.
In Australia, no strains of HPAI have ever been detected in wild birds. The new severe strain affecting wild birds and some mammals across the world – H5 avian influenza (or H5 bird flu) – has not been found in Australia to date.
Low pathogenicity avian influenza (LPAI) strains
LPAI strains are common in wild birds in Australia, and affected birds are usually not sick.
When wild birds with LPAI have direct contact with poultry, their water, food, or environment, they can infect poultry with LPAI. While the poultry may show no disease or mild disease signs, this gives the LPAI strain the opportunity to mutate into an HPAI strain, particularly when the H5 or H7 subtypes are present.
Key differences between LPAI and HPAI
The table below summarises the differences between LPAI and HPAI.
| LPAI | HPAI |
| H1–H16 subtypes | Only H5 or H7 subtypes |
| Common in wild birds (including in Australia) | Cause severe outbreaks in poultry |
| No disease or death in wild birds | Associated with disease and death in poultry and sometimes in wild birds Note: HPAI has never been detected in wild birds in Australia |
| Occasional mild disease in poultry |
Source: Table adapted from a Michelle Wille graphic https://www.michellewille.com/
