An outbreak in Australia of the equine influenza virus has spread to police horses in the state of New South Wales.
Several hundred horses are thought to be infected with the virus
Eight horses showed symptoms of the virus and all 36 animals have now been placed in quarantine, police said.
The move means that Mounted Police officers will be unable to help provide security for next week's summit of Asia-Pacific leaders in Sydney.
Deputy Police Commissioner Andrew Scipione said that security at the summit would not be affected.
Tests had confirmed the virus in a number of horses at the state Mounted Police headquarters in Redfern, a statement from New South Wales police said.
"Three horses were initially showing symptoms of EI (equine influenza) - now eight are believed to be suffering from the virus," it said.
The horses will remain quarantined until 30 days after the last horse is free of the virus.
This is the first time equine influenza has appeared in Australia. The outbreak began last week when horses at two sites in Sydney were found to be infected.
More than 400 horses across New South Wales and Queensland are now showing symptoms of the highly contagious virus, which does not affect humans.
Race meetings have been put on hold and the movement of horses banned, meaning that police horses could not be brought to the Sydney summit from another state.
Australian police are mounting a huge security operation for the Asia Pacific Economic Co-operation (Apec) summit next week. The leaders of 21 nations, including Russia, China and the US, will be attending.
Protesters are expected to turn out for the summit but Mr Scipione said that Mounted Police would no longer be part of the crowd-control effort.
"It won't stop us being able to protect dignitaries, the public or police at any planned protest," he said.
"The police presence will be backed up by the new water cannon that has been purchased by the state government for use in crowd control," he said.