Mosquito-borne viral encephalitis is a virus carried by mosquitoes. This virus can lead to encephalitis , which is an inflammation of the brain. In the United States, there are five main types of mosquito-borne viral encephalitis:

  • Eastern equine encephalitis
  • Western equine encephalitis
  • West Nile encephalitis
  • St. Louis encephalitis
  • LaCrosse encephalitis

Outside of the United States, the most common types of mosquito-borne viral encephalitis are Japanese Encephalitis and Venezuelan Equine Encephalitis. Mosquito-borne viral encephalitis is a potentially serious condition that requires care from your doctor.


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Mosquito-borne viral encephalitis is caused by a bite from a mosquito that carries the virus from animals to humans. When mosquitoes bite an infected bird, horse, or other animal, they can pass the infection on to humans. It usually takes between 4 to 15 days for a person to have any symptoms after they have been bitten by an infected mosquito. Very rarely, the infection can also be passed through organ transplants or blood transfusions.

Risk Factors

The following factors increase your chance of developing mosquito-borne viral encephalitis:

  • Living in an area where outbreaks of viral encephalitis have occurred
  • Spending a lot of time outdoors for work or play
  • Age: over 50 or under 15
  • Having a weak immune system for any reason, including:
    • HIV/AIDS
    • Taking immunosuppressant drugs


Most people who become infected with the viruses that can cause encephalitis do not develop any symptoms, and the infection runs its course without being dangerous. Many other people develop only mild symptoms, including:

  • Mild fever
  • Headache
  • Nausea
  • Body ache
  • Restlessness

A small number of people who become infected with one of these viruses actually develop encephalitis, which can cause death or brain damage. The more serious symptoms of encephalitis include:

  • Seizures
  • High fever
  • Coma
  • Weight loss
  • Weakness
  • Changes in mental state
  • Stiff neck
  • Tremors
  • Paralysis
  • Vision loss
  • Numbness

If you experience any of these symptoms, do not assume it is due to mosquito-borne viral encephalitis. These symptoms may be caused by other, less serious health conditions.


Your doctor will ask about your symptoms and medical history, and perform a physical exam.

The doctor will ask questions about:

  • Travel to areas that have had mosquito-borne viral encephalitis outbreaks
  • Recent mosquito bites
  • Exposure to dead animals

Tests may include:

  • Neurological exam—a series of tests to measure reflexes, memory, and other brain functions
  • Blood tests—to look for signs of infection in the blood
  • Spinal tap—removal of a small amount of cerebrospinal fluid to check for signs of infection
  • CT scan —a type of x-ray that uses a computer to make pictures of structures inside the head
  • MRI scan —a test that uses magnetic waves to make pictures of structures inside the head
  • Electroencephalogram (EEG) —a test that records the brain’s activity by measuring electrical currents through the brain


Talk with your doctor about the best treatment plan for you. There is no drug to treat mosquito-borne viral encephalitis. Instead, doctors usually prescribe supportive care, which means treating the symptoms while the immune system fights the disease. Supportive treatment options may include:

  • Intravenous fluids
  • A respirator to help with breathing
  • Anticonvulsants to treat seizures
  • Sedatives to treat restlessness
  • Pain relievers to treat headache and fever
  • Corticosteroids (anti-inflammatory drugs) to reduce brain swelling


The best way to reduce your chances of getting mosquito-borne viral encephalitis is to avoid being bitten by mosquitoes. Steps that can help include:

  • Limiting outside activities where mosquitoes are present
  • Wearing long sleeve shirts and long pants at dusk and dawn, when mosquitoes are most active
  • Using bug repellent that contains DEET
  • Emptying sources of standing water around the home, such as bird baths and gutters, where mosquitoes may breed
  • Other prevention tips include the following:
    • Do not handle dead birds or other animals that can carry the virus.
    • If you plan to take a long visit (greater than one month) to areas in Asia where outbreaks have occurred, your doctor may suggest that you take the vaccine for Japanese encephalitis.