West Nile Virus What is West Nile virus (WNV)? West Nile Virus is a mosquito-borne disease commonly found in Africa, West Asia, and the Middle East. The first recorded outbreak in North America happened in New York City in 1999. The virus can infect humans, birds, mosquitoes, horses and some other mammals. In the temperate zone of the world (i.e., between latitudes 23.5° and 66.5° north and south), WNV cases occur primarily in the late summer or early fall. In the southern climates, WNV can be transmitted year round. What are the symptoms of WNV infection? WNV affects the central nervous system. Most people (80%) do not develop symptoms. Some people (20%) develop fever, headache, body aches, nausea, vomiting, and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms typically last a few days. About one in 150 people infected with WNV will develop severe illness. Symptoms include high fever, headache, neck stiffness, stupor, disorientation, coma, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent. People with weaker immune systems and people with chronic diseases, are at greater risk for serious health effects. The overall risk of serious health effects increases with age. How does it spread? The virus is transmitted by the bite of infected Culex mosquitoes. They become infected when they feed on infected birds and then spread WNV to humans and other animals when they bite. In a very small number of cases, WNV also has spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby. What can I do to prevent WNV infection? There is so far no vaccine for this disease. The easiest and best way to avoid WNV infection is to prevent mosquito bites. How is WNV infection treated? There is no specific treatment for WNV infection. In severe cases, supportive treatment is indicated.