Health officials recently investigated 10 people hospitalised with acute encephalitis and said four of those were now confirmed to be the Japanese variety.
Seven of those people remain in hospital. One person has died.
Health department spokesman Chris Lease said with three of the four cases acquired locally, it was important for people to avoid being bitten by mosquitoes.
"People planning activities around the River Murray are warned to be especially vigilant, particularly between dusk and dawn when mosquitoes are most active," Dr Lease said.
Most people who are infected with flaviviruses such as Japanese encephalitis virus, Kokobera virus, West Nile virus, and Murray Valley encephalitis virus are asymptomatic or develop a mild illness.
However, a small number of infected people will develop encephalitis, which may be fatal or cause long-term neurological damage.
Symptoms of encephalitis include confusion, headaches, neck stiffness, tremors, drowsiness and seizures.
Dr Lease said while there are some vaccines available for flaviviruses, personal and household protective measures remained the first line of defence, including the use of an insect repellent and wearing long, loose-fitting clothing.
He said mosquito coils could also be useful outside along with insect sprays.
"It is important that people remain vigilant in protecting themselves against mosquito bites, particularly during periods of warmer weather when mosquito activity is high," Dr Lease said.
