When the virus enters the bloodstream, it may cause inflammation of brain tissue and surrounding membranes. White blood cells invade the brain tissue as they try to fight off the infection. The brain tissue swells (cerebral edema), which may destroy nerve cells, cause bleeding in the brain (intracerebral hemorrhage), and brain damage.

Symptoms include:

• Clumsiness, unsteady gait
• Confusion, disorientation
• Drowsiness
• Fever
• Headache
• Irritability or poor temper control
• Light-sensitivity
• Stiff neck and back (occasionally)
• Vomiting

Emergency symptoms:

• Loss of consciousness, poor responsiveness, stupor, coma
• Muscle weakness or paralysis
• Seizures
• Sudden change in mental functions:
  • Diminished interest in daily activities
  • "Flat" mood, lack of mood, or mood inappropriate for the situation
  • Impaired judgment
  • Inflexibility, extreme self-centeredness, indecisiveness, or withdrawal from social interaction
  • Memory loss (amnesia), impaired short-term or long-term memory

An examination may show:

• Abnormal reflexes
• Increased intracranial pressure
• Mental confusion
• Mouth ulcers
• Muscle weakness
• Neck stiffness
• Signs in other organs, such as the liver and lungs
• Skin rash
• Speech problems

Tests may include:

• Brain MRI
• CT scan of the head
• Culture of cerebrospinal fluid (CSF), blood, or urine (however, this test is rarely useful)
• Electroencephalogram (EEG)
• Lumbar puncture and CSF examination
• Tests that detect antibodies to a virus (serology tests)
• Test that detects tiny amounts of virus DNA (polymerase chain reaction -- PCR)