Due to the severity of the illness causing the horses to be unable to get up; they were euthanased.

Another nine cases have been treated in the Upper Hunter.

The illness is believed to be a virus carried by mosquitoes; however blood samples, including some from the Hunter, are still being tested at the NSW Government’s Elizabeth Macarthur Agricultural Institute and the Australian Animal Health Laboratory to determine the exact virus.

Signs of the illness include a lack of coordination, increased responsiveness to touch and sound, facial paralysis and weakness in the hindquarters.

Upper Hunter veterinarian Debbie Racklyeft said since the end of February she has seen about 10 cases, with the most recent being seen last week.

She said due to the unusual nature of the illness there were not large numbers of incidents; however she had seen cases that vary from one to 10 on a scale of seriousness.

Dr Racklyeft said the symptoms were quite variable and the horses that displayed more nervous system signs were taking longer to recover.

“The only treatment is to try to control the symptoms that come about because of the inflammation of the brain and spinal cord using anti inflammatory drugs and careful nursing care,” she said.

There have been about 100 cases spread across NSW with 10 per cent of these horses being destroyed.

Dr Racklyeft said although there was a fairly low chance of horses getting the disease, people should protect them against mosquitoes by applying long acting repellents, rugging their horses and keeping them inside if possible.

“I have seen horses from six months to 20-years-old affected, but I have not seen two horses on the one place,” she said.

“The illness doesn’t spread from horse to horse and can only affect a human if they are bitten by a mosquito themselves.

“Once the weather turns cold and the insects decrease I think the illness will disappear.”

NSW chief veterinary officer Ian Roth said extremely good progress had being made on the testing and they have found evidence to suggest that a majority of cases were due to infection with one or more Australian strains of virus in the Flavivirus group, that includes viruses like Murray Valley encephalitis virus and Kunjin virus.

“All the samples to date have been tested negative for Hendra virus and Japanese encephalitis.”