Biosecurity South Australia says it has now tested, or is in the process of testing, about 100 affected horses.
It said on Wednesday it was continuing to receive reports of horses showing unusual neurological signs in South Australia.
Since the middle of February 71 neurological cases in horses have been reported by private veterinarians to Biosecurity SA. Five have been euthanased while the remainder are recovering from their illness.
Results indicate infection with a range of insect-borne viruses, mainly Ross River and, probably, Kunjin.
A couple of horses have tested positive to Murray Valley Encephalitis. Some have been exposed to more than one virus.
Biosecurity SA is aware of three horses who have died or have been euthanised who have tested positive to a flavivirus, most of which are also insect-borne. They include Kunjin Virus and Murray Valley Encephalitis virus.
The agency says testing horses is a costly and time-consuming process and serology takes investigators only so far.
"We have established that there is an increase in general arbovirus infections but to establish the cause of the more severe neurological cases we need to isolate and type viruses," a spokesman said.
The time during which the live virus circulates in the blood for these diseases is short, so blood is not a good source for virus isolation.
Consequently, Biosecurity SA is focusing on severe neurological cases where it might be possible to obtain brain tissue for virus culture, should the animal die or be euthanised.
The mortality rate in all affected states appears to be about 10-15 per cent of cases affected.
Biosecurity SA expects the incidence of this syndrome to decline as the weather becomes cooler and mosquito activity declines.
Horse owners will need to be vigilant for a resurgence in spring, it said.
Signs shown by horses affected by the diseases include ataxia (wobbly in the legs and difficulty in walking), muscle tremors and sensitivity to touch.
Similar cases are still being reported in Victoria and New South Wales.
Since the middle of February a total of 55 neurological cases in horses have been reported by private veterinarians to Biosecurity SA.
These reported neurological cases are spread across the state from the Riverland, down the length of the Murray and in areas north and south of Adelaide, from Port Pirie to Victor Harbor.
All reported horses are being tested for a range of mosquito born viruses including Ross River Virus and the flaviviruses.
Laboratory results so far have indicated that a mosquito born flavivirus may be responsible for the clinical signs in some horses. The results also indicate that recent exposure to Ross River Virus has also occurred in some horses.
Further results for cases are pending, the agency says, but due to the increase in demand for laboratory testing these results may take a few weeks to be received.
It stresses that there is no suggestion that the deadly Hendra virus is involved in this syndrome.
There is no known risk to humans from direct contact with horses infected with mosquito transmitted viruses.
However humans, as with horses, can be infected with some diseases from the bite of an infected mosquito.
Owners can attempt to protect their horses from mosquitoes by using repellents, covering them up, keeping them indoors at times of high mosquito activity (dawn and dusk) and minimising possible mosquito breeding areas.
Horse owners should contact their local veterinarian if their horse is unwell or behaving strangely.