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University of Melbourne’s Dr Michelle Wille, Senior Research Fellow at the Centre for Pathogen Genomics and an honorary position at WHO Collaborating Centre for Reference and Research on Influenza at the Doherty Institute, explains.  

At the time of writing, various HPAI strains (H7N3, H7N9, H7N8) have been reported in eight poultry farms in Victoria, including Terang and Golden Plains Shire, in two in the Greater Sydney Basin (NSW) and in one in the Australian Capital Territory (ACT). The seven premises in the Golden Plains Shire are infected with the same strain (H7N3). While the property near Terang was epidemiologically linked, it was infected with a different strain (H7N9). A third strain (H7N8) first detected in NSW, was also found in the ACT. 

All of this has occurred amid growing concerns about the arrival of an overseas strain of HPAI H5N1, which is causing a global panzootic and requires a different risk and response strategy compared to our current HPAI H7 outbreaks. Oceania is the only part of the world remaining free from HPAI H5N1. 

HPAI vs LPAI, H7 vs N5 – what does it all mean?

There are many avian influenza viruses—some common, some rare, some lethal, some benign, some exclusively in seagulls, and some found in a huge diversity of birds. To address this, we classify these viruses by features of the virus (called subtypes) and disease severity (pathogenicity). 

The most common viruses are called “low pathogenicity avian influenza” (LPAI) because they do not cause disease in wild birds. They are found in wild birds globally, including Australia, and comprise 16 HA and nine NA subtypes. The different subtypes refer to the diversity of proteins on the surface of the virus called the HA and the NA, and can mix and match (e.g. H7N9, H7N8). Within the H5 subtype, we may also refer to genetic clades (such as clade 2.3.4.4b), and these genetic clades are somewhat similar to SARS-CoV-2 variants. 

In contrast, high pathogenicity avian influenza (HPAI) viruses which are lethal and make wild birds and poultry very sick. These HPAI viruses evolve when subtypes H5 and H7 infect poultry and undergo genetic changes, making the viruses more lethal. This is exactly what has occurred in Australia – l LPAI H7 viruses, which do not cause disease, jumped from wild birds to poultry, where they evolved into HPAI and caused outbreaks.  

This is not the first time an HPAI outbreak has happened in Australia. Unlike the HPAI H5N1 which is not present in Australia, Australia has faced a number of HPAI H7 outbreaks, the most recent being HPAI H7N7 outbreak in Victoria in 2020. To control the outbreak, movement and control zones have been set up, in both Victoria and NSW. Poultry must now be kept indoors in certain areas to prevent another virus jump event. Farms that have tested positive are being depopulated to stop virus spread and further evolution, and importantly, to prevent the HPAI virus entering wild bird populations.  

A similar process occurred in 1996 when HPAI H5N1 jumped from wild birds into poultry and then evolved in chickens. Unfortunately, this HPAI H5N1 strain was never controlled and has been spreading in poultry for decades. It developed mutations that allow for wild birds to carry it more effectively which resulted in massive global spread since 2021. HPAI H5N1 has caused the death of hundreds of millions of poultry, millions of wild birds, tens of thousands of mammals, as well as humans, making it a virus of serious concern. 

Why now?

The occurrence of the HPAI H7 outbreaks in Australia this year is likely not a coincidence. Previous research has shown a link between rainfall in the Murray Darling Basin and poultry outbreaks in the southeastern states. It’s not that rainfall directly has an impact on poultry, but rather that it affects what is happening in wild waterbirds. 

Following multiple La Niña years in a row with high rainfall, waterbird populations have surged resulting in a high number juvenile birds without antibodies against avian influenza viruses. As the region transitions into a period of less rainfall, the waterbirds begin to concentrate in permanent wetlands, leading to increased contact rates and higher LPAI prevalence among wild birds. With lots of LPAI in wild birds, the risk of spread to poultry increases.  

So, we hypothesise that, at present, there is likely to be a lot of LPAI H7, including many genotypes within H7, in the waterbirds of the Murray Darling Basin. As these birds have come into contact with poultry, there has been a virus jump from wild birds to free-range poultry on at least three occasions, resulting in three distinct viruses in poultry. 

To verify our hypothesis, we are out in the field catching ducks to understand what is happening in the wild bird population. In addition to resolving this hypothesis, this work is contributing samples and testing results to local authorities to understand the context of the spillovers and confirm that HPAI H7 from poultry has not jumped back into wild birds.  

What should I do?

If you see sick or dead wild birds or poultry, including backyard chooks, it is imperative that you call the Emergency Animal Disease Watch Hotline—1800 675 888—regardless of which state you are located in. 

If you have backyard poultry, please check the jurisdictional websites to see if you are in the restricted or controlled areas and keep your chickens inside if you are.  

The latest updates can be found on the Agriculture Victoria, ACT and NSW government websites.