According to the official, the virus has infected 84 residents so far after its outbreak across the province, Xinhua reported.
Symptoms of Japanese encephalitis, commonly known as "brain inflammation", include high fever, nausea and in severe cases, seizures, paralysis and coma.
Infants and the elderly are most vulnerable.
The Son La health department has urged parents to take their children under five years old to hospital if they spot any such symptoms.
It has also urged parents to follow a vaccination schedule, which is now seen as the most efficient way to deter their children from contracting the disease.