West Nile Virus

Type: Viral

Geography: Africa, Europe, the Middle East, North America, West Asia

Diagnosed Cases Per Year: 30 to 10,000

Fatality Rate: 4.5%

First Discovered: 1937 in the West Nile district of Uganda

Birds are the natural hosts of West Nile Virus. Mosquitos become infected when they feed on infected birds, and the virus is mainly transmitted to humans, horses, and other animals through the bites of infected mosquitos.

The incubation period is 3 to 14 days. Approximately 80% of people who are infected will not show symptoms, and therefore never receive an official diagnosis. Around 20% of people who become infected will develop West Nile fever with symptoms of fever, headache, tiredness, body aches, nausea, vomiting, skin rash, and swollen lymph glands. Severe disease symptoms include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. Approximately 1 in 150 people infected with the virus will develop a more severe form of the disease, with people over the age of 50 and those who are immunocompromised at the highest risk. It can cause a fatal neurological disease in humans.

The virus can cause severe disease and death in horses. Vaccines are available for use in horses but not yet available for people.

Photo: West Nile Virus (yellow) – Electron micrograph.