West Nile Virus

West Nile virus is a mosquito-borne infection that may develop into a life-threatening illness. The majority of individuals who become infected experience few signs, with mild symptoms - such as fever, headaches, fatigue and rash - displaying in about 1-in-5 individuals. However, some cases develop into serious illness, including encephalitis (inflammation of the brain) in humans and horses. It also causes mortality in certain domestic and wild birds.


The virus is transmitted when mosquitoes become infected after feeding on infected birds and then spread to humans and other animals. It cannot be transmitted from person to person. Exposure to mosquitoes where West Nile virus exists will increase chances for the infection to develop, so the best precautionary advice is to avoid these situations and to protect yourself using mosquito repellent.


West Nile virus is commonly found in humans, birds and other vertebrates in Africa, Europe and Western Asia. The first documented reports of the virus in the Western Hemisphere occurred in New York City (in 1999). Human, bird and mammal cases were subsequently reported through several Middle Atlantic states and New England. It has continued to spread throughout the United States, with the first cases reported in the St. Louis Metropolitan Area in October 2001.

Symptoms & Treatment

Most infections are mild, and symptoms include fever, headache and body aches, occasionally with skin rash and swollen lymph glands. More severe infection may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis and, rarely, death. West Nile virus is only fatal in 3-15% of all cases, and persons older than 60 years of age or those with certain medical conditions have the highest risk for the development of severe infections. Mild cases usually resolve in a few days, but individuals experiencing signs or symptoms of serious infection should seek medical care immediately.