Ninety-nine percent of the cases of SLEV are asymptomatic. If symptoms develop, it is usually five to fifteen days after the mosquito bite. When children and young adults are infected, they often only present with fever, headache, and mild meningitis. When older adults are infected, 90% develop encephalitis. Their symptoms are confusion and disorientation, unsteadiness and tremors, and dizziness. A few will become comatose and some may die.
There is no specific antiviral treatment and no vaccine.