West Nile fever

West Nile fever is an infection by the West Nile virus, which is typically spread by mosquitoes.^[1] In about 80% of infections people have few or no symptoms.^[3] About 20% of people develop a fever, headache, vomiting, or a rash.^[1] In less than 1% of people, encephalitis or meningitis occurs, with associated neck stiffness, confusion, or seizures.^[1] Recovery may take weeks to months.^[1] The risk of death among those in whom the nervous system is affected is about 10%.^[1]

West Nile virus (WNV) is usually spread by mosquitoes that become infected when they feed on infected birds, which often carry the disease.^[1] Rarely the virus is spread through blood transfusions, organ transplants, or from mother to baby during pregnancy, delivery, or breastfeeding,^[1] but it otherwise does not spread directly between people.^[4] Risks for severe disease include being very young, over 60 years old, having a weak immune system, and having other health problems.^[1][2] Diagnosis is typically based on symptoms and blood tests.^[1]

There is no human vaccine.^[1] The best way to reduce the risk of infection is to avoid mosquito bites.^[1] Mosquito populations may be reduced by eliminating standing pools of water, such as in old tires, buckets, gutters, and swimming pools.^[1] When mosquitoes cannot be avoided, mosquito repellent, window screens, and mosquito nets reduce the likelihood of being bitten.^[1][4] There is no specific treatment for the disease; pain medications may reduce symptoms.^[1]

The virus was discovered in Uganda in 1937, and was first detected in North America in 1999.^[1][4] WNV has occurred in Europe, Africa, Asia, Australia, and North America.^[1] In the United States thousands of cases are reported a year, with most occurring in August and September.^[5] It can occur in outbreaks of disease.^[4] Severe disease may also occur in horses, for which a vaccine is available.^[4] A surveillance system in birds is useful for early detection of a potential human outbreak.^[4]