Health authorities have detected mosquito-borne Japanese encephalitis in feral pigs in the Northern Territory, raising fears for residents in nearby areas.

More than 30 people across Australia have become infected with the virus in recent months and four people have died, including a person on the Tiwi Islands, north of Darwin.

Residents have been urged to guard themselves against mosquito bites after the virus was found in the West Daly region, near the West Australian border.

"Japanese encephalitis infection in humans is most commonly asymptomatic but, on rare occasions, it can result in severe disease and even death," NT Health entomologist Nina Kurucz said Friday.

"Symptoms can include fever, headache, neck stiffness, vomiting, confusion, seizures and paralysis."

The discovery of the virus in southern states has alarmed experts.

The virus is spread by mosquitoes and can infect animals and humans, but it can't be spread from person to person or by eating animal products.

Pigs with the infection don't transmit the infection to other animals but may infect mosquitoes if bitten while they still have the virus in their blood.

Other livestock such as horses, cattle, sheep and goats are dead end carriers and cannot transmit the disease back to mosquitoes.

The virus is often spread by migratory water birds and through the movement of infected mosquitoes, over long distances due to wind dispersal.

About 99 per cent of cases in people are asymptomatic but some may experience fever and headache.

Children aged under five and the elderly are more likely to develop severe infection.

People who believe they could be infected should seek urgent medical help.