H5N1 bird flu is primarily an animal-health and biosecurity issue, affecting birds and, in some cases, other mammals. While human infections can occur in rare circumstances, the current risk to the general public in Australia remains low.

Over the weekend, H5N1 bird flu was confirmed for the first time in a vagrant migratory seabird on the Western Australia coast. This H5N1 strain has caused mass mortality in poultry, wild birds and sea mammals in all parts of the world except for Australia/Oceania. Water birds such as ducks, swans, gulls, terns and shorebirds are the natural reservoirs for avian influenza viruses.

Avian influenza (AI or bird flu) is an infectious viral disease caused by strains of the influenza A virus. Strains of avian influenza have subtypes, which reflect the presence of surface glycoproteins (haemagglutinin and neuraminidase) e.g. H5N1, H7N8.

Avian influenza strains are classified as either low pathogenicity (LPAI) or high pathogenicity (HPAI), depending on how severely the strain affects poultry. HPAI strains cause severe outbreaks and mortality in poultry. In Australia there were a series of outbreaks of HPAI involving H7 subtypes, including:

• 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. Importantly, Australia was successful in rapidly controlling these recent outbreaks in commercial poultry farms.

HPAI H5N1 bird flu is a highly contagious strain that can cause serious disease and mass mortalities in birds including poultry, wildlife species, and mammals. This strain:

• spreads more easily than other bird flu strains
  
• can kill large numbers of wild birds
  
• has no effective treatment
  
• leads to poor outcomes for infected birds and mammals e.g. cows, farmed mink, and cats.
  

At this stage, H5N1 has not affected other Australian wildlife or poultry farms. Biosecurity experts are working to ensure the risk of spread is minimised.

Risk to the public

The global public health risk from H5N1 has been assessed as low for the public, according to a joint assessment by the World Health Organization, Food and Agriculture Organization, and World Organisation for Animal Health.

Human infections are relatively rare, with cases usually linked to the handling of sick or dead poultry or wild birds or through contact with their droppings. Those most at risk are working in poultry farms or in processing facilities. There is no evidence that eggs, properly cooked poultry, or pasteurised dairy products transmit the H5N1 virus to consumers.

Human-to-human transmission has not been established, which is the key reason the current public risk remains low. With the possibility that the virus could evolve to spread efficiently between humans in the future being closely monitored by health authorities.

H5N1 is a major animal-health problem. The current risk to the general public remains low.