A spate of mysterious horse deaths across three separate Victorian properties has sparked an investigation.
Ten horses are confirmed to have died suddenly at properties in the Mornington Peninsula, southeast Melbourne and southwest Victoria since July 4.
All the horses are believed to have died with short-term, non-specific signs of illness.
Agriculture Victoria has launched an investigation into the illnesses and deaths but the probe remains in its early stages.
"It is not yet known what has caused these deaths and to date common links are not evident," Victoria's chief veterinary officer Graeme Cooke said.
Early testing for some known diseases in horses such as anthrax and hendra virus has been negative.
Investigators are also considering feed and environmental factors as potential causes.
Multiple horse deaths on a single property are uncommon.
Investigations typically look at infectious causes and ingestion of toxins but it is not always possible to pin down a cause.
"What has happened here is very, very rapid onset of symptoms and rapid death," Dr Cooke told ABC Melbourne.
"That does lead one to look at a wide variety of causes, which could be non-infectious."
There have been reports on social media channels of further affected properties, with Agriculture Victoria calling on any owners who have not reported deaths to contact them or their veterinarian.
"We know this is a difficult time for horse owners, that's why we are working with veterinarians and a range of experts to find answers," Dr Cooke said.
"Please do not share unconfirmed information online or on social media."
Dr Cooke advised horse owners to monitor their animals and feed, but said there was no reason at this stage to separate the horses.
It's not the only zoological mystery for the state.
More than 90 native waterbirds have been found stranded across Victoria in the past two months.
Werribee Open Range Zoo associate veterinarian Paul Eden said grebes - duck-like waterbirds with an evolutionary link to the flamingo - were turning up in strange places a long way from their usual freshwater habitat.
"People are finding these waterbirds inside their backyards, around industrial estates, and we've even had birds turning up at the casino," Dr Eden said.
Grebes usually migrate from inland freshwater locations to coastal areas during autumn and winter, but are unable to walk well on land and were mistaking roads and other shiny surfaces for waterways.
The birds usually congregate in social flocks in winter months and one of the theories is the wetter conditions caused by La Nina are leading the grebes to disperse to new wetland habitats and getting lost, Dr Eden said.
Veterinarians and wildlife experts from Melbourne and Werribee zoos and Healesville Sanctuary are rescuing the birds, and people who encounter them in distress should contact Wildlife Victoria.
