Toll from spate of mysterious horse deaths rises to 13

The first of the horses died on July 4, with the 13 dead spread across five separate properties in Victoria on the Mornington Peninsula, southeast Melbourne near Cranbourne, and near Ballarat and Colac in the state's west.

All of the horses died with short-term, non-specific signs of illness and there were no obvious common links between them.

Agriculture Victoria is in the early stages of investigating the deaths, with some of the most recently reported cases involving horses that died weeks ago.

Authorities urged horse owners to monitor their animals and immediately report any rapid onset of illness or sudden deaths.

"Agriculture Victoria is working with private veterinarians and industry experts, responding to calls and building a bigger picture of the situation," Agriculture Victoria said.

"Those who are concerned about sick horses should seek advice from their vet.

"Rapid reporting after death will assist Agriculture Victoria and veterinarians to further investigate the situation."

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.

Early testing for some known diseases in horses such as anthrax and hendra virus has been negative.

Investigators will test for a range of other possible infectious and non-infectious causes.

Some horse owners have reported cases to the emergency animal disease hotline, that are not believed to be linked to the mystery spate of deaths.

There was no need for owners to restrict horses' movements or separate them at this stage, Agriculture Victoria said.

Australian Associated Press