The wet spring and summer across large areas of Australia have resulted in an increased risk of insect-borne infections.
The Australian Horse Industry Council says horses are a dead-end host for mosquito-borne arbovirus infections, which means that a horse is not a source of new infection for people or other horses.
Reports of horses with nervous signs have originated from:
Some horses have been severely affected.
Most horses with clinical signs recover over several weeks with good husbandry and veterinary care. However, as of March 30, of the 100 cases in New South Wales, around 15 have died or have had to be euthanised for animal welfare reasons.
Laboratory testing of samples from the horses with soreness indicates that most infections are probably due to Alphavirus infections, including Ross River virus.
Testing of samples from horses with unusual nervous signs suggests that a majority of cases are due to infection with one or more viruses belonging to the Flavivirus group of viruses that includes viruses such as Murray Valley encephalitis virus and Kunjin virus.
Testing of samples to date has ruled out Japanese encephalitis virus.
The deadly Hendra virus infection has also been ruled out from all four states - Queensland, New South Wales, Victoria and South Australia.
Necropsies are taking place on infected dead horses to determine the cause of the infections.
Veterinarians and any assistants have been urged to take stringent precautions when performing necropsies on horses showing neurological clinical signs.
They have been warned that great care should be exercised when handling brain and spinal cord tissue and appropriate personal protective equipment should be ued as part of a risk management approach to personal safety.
The horse council said there had been no increased incidence of muscle/joint soreness or nervous signs in horses reported in Queensland.
It has urged horse owners to reduce the exposure of their animals to insect bites, in particular mosquitoes.
Owners can use registered repellent products, rug their horses and use fly masks.
Horses suffering from these arboviruses commonly display clinical signs including, but not limited to:
Owners who notice that their horses is displaying unusual signs should immediately contact their private veterinarian.