West Nile Virus - Fact Sheet

What is West Nile Disease?

West Nile virusWest Nile Virus, also known as West Nile Disease or West Nile Encephalitis, is a mosquito-borne disease which affects the central nervous system. Although approximately 80% of those infected with West Nile Virus will show no symptoms at all, it is potentially a serious or fatal disease. Animals can also be affected, especially horses.

According to the CDC, the year 2005 saw a total of 2,744 cases of human West Nile Disease in the US, up from 2,359 in 2004. Both years showed a decrease from the year 2003, which had a total of 9,862 cases.

The incidence of West Nile in the gulf coast following hurricanes Katrina and Rita was far less than anticipated - most likely due to the mass evacuations and to the aggressive use of pesticides and other insect control measures following the storms.


West Nile Virus is mainly transmitted by infected mosquitos. The mosquitos become carriers of the disease by feeding on the blood of an infected bird. They then transmit the disease by biting humans or other birds and domestic animals. Less commonly, West Nile disease can be spread by transfusions or organ transplants from an infected donor, and pregnant or breast feeding mothers could pass the disease to their babies. It is not spread through casual contact.

Symptoms of West Nile Virus

Most people infected with West Nile Virus (about 80%) will develop no symptoms at all. In those who are affected, symptoms begin 3 to 14 days after exposure to the disease.

Mild symptoms, lasting from a few days to a few weeks, occur in about 20% of people infected with West Nile Virus. These can include:

  • fever
  • Head and body aches
  • rash on the upper body
  • swollen lymph glands
  • nausea and vomiting

More severe symptoms occur in about one person in 150. The ill and those over age 50 are at higher risk. Severe symptoms lasting several weeks might include:

  • high fever
  • severe headache
  • stiff neck
  • tremors or convulsions
  • disorientation and confusion
  • coma
  • vision loss
  • numbness, paralysis or muscle weakness

Persons severely affected can suffer permanent neurological damage.


Unfortunately, there is no effective treatment for WNV. Those with severe cases should be hospitalized to receive supportive care including intravenous fluids and respiratory assistance.

If you develop any of the severe symptoms, especially an intense headache accompanied fever and a stiff neck, you should see your doctor immediately.


The best way to prevent West Nile Virus is to avoid mosquito bites. Ways to do this include wearing protective clothing, avoiding outside activity betweent dusk and dawn, eliminating standing water on your property, and wearing a mosquito repellent containing Picaridin, Deet or Oil of Lemon Eucalyptus when outdoors. For details and more tips, see our page on mosquito bite prevention.

Although birds die of many causes, dead birds can be a sign that West Nile is active in your area. Promptly report dead birds to your local health department - they may want to collect and have them tested. Never touch a dead bird unless you are wearing gloves or other protective gear.

Is a vaccine on the horizon?

In April of 2013 Hawaii Biotech re-acquired several patents for West Nile vaccine technology from pharmaceutical company Merck and announced plans to continue in the development of a vaccine for West Nile virus.

Merck had originally obtained the patents from Hawii Biotech when it bought the research division and some assets of the company back in 2010 - an acquisition which enabled the Honolulu-based HBI to recover from bankruptcy. Now back on their feet financially, Hawaii Biotech is also working on some other important vaccines for mosquito-borne diseases, including vaccines for malaria, encephalitis and dengue fever.

A veterinary West Nile vaccine is already available for use in horses.

Further reading: