Most of those killed were paradise shelduck, a native taonga species which traditionally used oxidation ponds at the treatment plant as a moulting site. Photo: Dave Young 2008, Wikimedia Commons
An outbreak of avian botulism in Otago that led to the deaths of about 1000 waterfowl at the Waikouaiti wastewater treatment plant has been followed by another suspected outbreak in Timaru.
Otago Fish & Game said that the incident in Otago was the first known major outbreak of the bacterial disease in the region.
The majority of the dead birds collected so far were paradise shelduck, a native taonga species which traditionally used oxidation ponds at the treatment plant as a moulting site, it said.
Dead bird species also included native grey teal, mallard, Canada goose, black swan and black-backed gull.
Otago Fish & Game chief executive Ian Hadland said avian botulism outbreaks had occurred regularly in the North Island, but were rare in the South Island.
"There is an increasing risk of further outbreaks this far south given climate change predictions and ongoing water quality issues," he said. "Fish & Game has been liaising with Dunedin City Council (DCC) and other stakeholders concerning this outbreak, and we will continue to work with the DCC and other councils where appropriate."
He said the primary task was to constrain the outbreak and minimise the harm to wider bird populations, and Fish & Game staff were monitoring wetlands and estuaries nearby to see if it had spread further.
"We are pleased to see the DCC is taking a proactive role in removing dead birds to avoid further spread of the outbreak. We understand that while avian botulism is often viewed as a 'naturally' occurring problem, measures can be taken to prevent or mitigate outbreaks.
"The organisation will be asking district councils across the region to look at their management plans for wastewater treatment plants to ensure everything is being done to minimise the risk and severity of harm to wildlife caused by avian botulism."
Otago Fish & Game said many wastewater treatment plants provided the perfect microclimate for outbreaks to occur, and the toxin from avian botulism could spread if birds ate maggots from infected carcasses.
Avian botulism had killed an estimated 25,000 birds in the Auckland/Waikato region some years, it said.
Suspected Timaru outbreak
Wildlife rescuers in Timaru are scrambling to respond to a suspected similar outbreak of avian botulism.
South Canterbury Bird Rescue retrieved more than thirty dead black back gulls and ducks and more than a dozen sick birds from Washdyke Lagoon on Monday, and was seeking volunteers to help capture sick birds for treatment.
Alan Parker of Avian Rescue Otago said the symptoms identified in the birds - such as ascending paralysis - were consistent with botulism.
Because the affected birds were not endangered, there was little official assistance, he said.
"There's not a lot of organisations that can help, it sort of falls on the rescues that are mostly all voluntary."
Anyone who saw three or more sick or dead wild birds in a group was advised to report it immediately to Biosecurity New Zealand's Exotic Pest and Disease Hotline on 0800 80 99 66.
Risk to humans 'very low'
Dunedin City Council (DCC) said the outbreak was unrelated to avian influenza but was being treated as a significant animal welfare issue.
"This issue was first identified on Tuesday, 21 January 2025. Deceased birds were sent to the Ministry of Primary Industries (MPI) for testing that identified the contraction of the Avian Botulism type-C strain by the birds," a spokesperson said.
"The medical officer of health has confirmed that the risk to human health is very low, and botulism in wild birds has not been known to cause botulism in humans."
DCC said bird health was regularly monitored at treatment plants in Dunedin with avian botulism a known risk at water bodies, such as treatment ponds, across New Zealand.
"Avian botulism is caused when waterfowl eat the toxin produced by the Clostridium botulism bacteria. The bacteria occurs naturally in soils around ponds and wetlands and is found in oxidation ponds. It is harmless until certain environmental factors occur, such as low oxygen and warm temperatures," the spokesperson said.
This was the first recorded event at a Dunedin plant, they said.
"DCC has activated its management plan for collecting and disposing of dead birds and will continue for the next few weeks. This should help stop the spread and break the chain of infection."
DCC said as a precautionary measure, people visiting the Waikouaiti estuary were advised to take the following precautions:
- Don't let your pets eat dead wild birds or fish.
- Don't touch or handle sick or dead wild birds or fish.
- Don't eat undercooked or improperly prepared waterfowl or fish. Heating the food properly will kill the toxin.
- Don't harvest birds that appear to be sick or dying.
- Maintain good hand hygiene. Wash your hands either with soap and running water or use alcohol hand rubs often and thoroughly - especially before and after contact with animals and their environment.
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