Avian influenza (AI) is caused by members of the genus Influenzavirus A (Orthomyxoviridae family) and is further divided into two categories: low-pathogenicity and high-pathogenicity avian influenza (LPAI and HPAI, respectively). AI is a notifiable disease in NSW, however it should be noted that LPAI presents a significantly reduced disease risk and is known to circulate within wild bird populations within Australia (Wille et al., 2023). This does not negate the importance of AI surveillance in general, as LPAI has the potential to mutate into HPAI. HPAI is not currently present in Australia (Wille & Klaassen, 2023).
On the 13th February 2023, 20 dead wood ducks (later correctly identified as teal ducks Anas gracilis) were found at the decanting dam of the CSA Mine in Cobar, NSW. This was reported to the Emergency Animal Disease Hotline by the mine’s Environmental Advisor. Wildlife Health Australia describes a bird morality event as significant when five or more birds die within a defined area in a short period of time. The report was forwarded to Western Local Land Services (LLS) by the NSW Department of Primary Industries (DPI) duty officer. Two bird carcasses were provided to the Cobar LLS office and subsequently sent to the Australian Registry of Wildlife Health at Taronga Zoo (Registry) and Elizabeth Macarthur Agricultural Institute (EMAI) for post-mortem and notifiable disease exclusion.
On the 24th February, a site visit was conducted at the CSA Mine, with the Environmental Advisor interviewed. No further deaths were recorded since the initial report and a dam water sample was collected for analysis. Additional to this, the mine tested the water to an agricultural standard monthly. The Cobar Shire Council, Kidman Way Veterinary Surgery, Cobar Weekly and WIRES were also contacted to gauge their experience of similar bird deaths in town. These parties did not express any concerns at that time.
On the morning of the 27th February, 40-50 dead teal ducks were found scattered throughout Cobar. These were reported by locally based LLS staff, council rangers, the veterinary clinic as well as by members of the public who obtained District Veterinarian (DV) contact details from an unsanctioned social media post. Locations of bird deaths were recorded on an Arc GIS-based mapping platform created by the GIS team on DV request to record time and location data for epidemiological purposes.
14 fresh carcasses were collected by LLS staff under DV direction and submitted to the Registry and EMAI for similar post-mortem and notifiable disease (avian influenza, Newcastle disease) exclusion. Sick birds seen by the veterinary clinic were reportedly weak, lethargic and displayed neck drooping and difficulty walking. Water samples were also collected from the New ("Newey") and Old Reservoirs in Cobar. These were sent to the DPI Water Lab, Wollongbar for Standard Analysis for stock use.
Sporadic deaths, from sources of varying reliability, were reported over the next few days. Deaths ceased within a week.
One of the initial two carcases submitted was heavily autolysed, the other generally unremarkable on physical examination. These tested negative for avian influenza, Newcastle disease, infectious laryngo-tracheitis and west-Nile virus. A liquid chromatography/mass spectrometry (LC/MS) scan for approx. 600 pesticides was negative.
The carcases submitted from the second, more numerically significant, mortality event were in generally good body condition. They displayed an unusual, but consistent pattern of trauma characterised by skull contusions +/- fractures as well as haemorrhage surrounding the greater vessels and pectoral girdle-often associated with furcula or scapular fractures.
Water testing from the mine dam, and both old and new town reservoirs was unremarkable, with all samples suitable for livestock use.
Significantly, influenza A (H5 & H7) was detected by real time group PCR in birds from the second submission. The samples were negative for other notifiable diseases.
In both submissions, the NSW Environment Protection Agency (EPA) was notified and samples were forwarded for toxicological analysis (results pending as of April 2023).
Cobar is a western NSW town of approx. 3,600 (ABS, 2021) centred around an industry of copper and base metal mining. The CSA mine is located 5km north of Cobar and is a significant copper mine in NSW, employing 500 people and processing 40,000 tonnes of copper annually. Because of this, toxicological causes were considered and the EPA notified.
Due to the concurrent challenges of staff availability and vast travel distances within the region, a prolonged time (10 days) occurred prior to a DV field visit for the initial investigation. Despite this, the aforementioned carcases were collected and submitted within two days by local LLS staff.
The average temperature range at Cobar in the three days leading up both mortality events was 21.3°C-30.6°C (BOM, 2023). This did not present a significant avian thermal health risk (DEFRA, 2005).
The two reservoirs tested are the significant water bodies in the area, with the "Newey" being a major water bird habitat.
Interestingly, on 3rd February 2022 a dead wood duck tested positive to LPAI in Cobar, however this was not found in subsequent sampling of further animals.
It should be noted that there was a high degree of community interest in the duck deaths in Cobar from the 27th February event. That the pattern of deaths was relatively widespread through the town, as well as social and general media coverage, contributed to this.
Media engagement and advice based on DPI guidelines was put out with approval from the LLS Communications team to allay public fears and answer commonly asked questions about the incident.
The first set of bird deaths has an open diagnosis due to the non-specific nature of the findings.
The second set of results proved more diagnostically rewarding. The combination of skull contusions/fractures and haemorrhagic pattern seen in gross pathology findings of the second set of bird submissions is suggestive of acute, blunt force trauma, possibly associated with falling at speed from a considerable height. This aligns with anecdotal accounts from townspeople of ducks "falling from the sky" on the 27th February.
The presence of avian influenza and the absence of another clear pathogen or cause would position this as the presumptive diagnosis for this case.
Karrie Rose, Australian Registry of Wildlife Health
DPI Laboratory Services, Elizabeth Macarthur Agricultural Institute
Peter Grey, NSW Department of Primary Industries
Lauren Leicester, Kidman Way Veterinary Surgery