West Nile Virus

West Nile virus (WNV), primarily transmitted by mosquitoes of the genus Culex (see photos), is the most important mosquito-borne pathogen in North America. It has infected thousands of humans and countless wildlife since its arrival in the USA in 1999. It can make people sick and even cause death.

West Nile fever (WNF) is the name of the milder form of illness caused by West Nile virus in humans. Over the past two decades, WNF has become the most common mosquito-borne illness in North America. Cases occur during mosquito season, which starts in the summer and continues through fall in most parts of the country. Warmer areas may have a longer mosquito season.  Transmission also occurs via blood transfusions, organ transplants, and possibly pregnancy.

Nearly 7 million cases of WNF are estimated to have occurred in the U.S. since 1999 (70-80% of cases are unreported). In 2022, 1,036 cases of West Nile virus-related disease were reported to the Centers for Disease Control and Prevention (CDC). Of these, 737 (71%) were classified as the more serious form of the disease, because it affects the central nervous system. Those who contract this more serious form of West Nile illness may suffer long-term disabilities or death.

Which Mosquito Is the Primary Vector?

WNV is transmitted primarily by Culex species mosquitoes, which become infected when they feed on infected birds. The mosquitoes then transmit the virus to humans through bites. These mosquitoes are active from two hours before dusk until two hours after sunrise.

Several species of mosquitoes that commonly bite people are able to spread JCV. The Aedes, Culiseta, and Anopheles species are the primary vectors. These mosquitoes emerge in the spring and are often found in wooded areas.

Where is WNV Found

West Nile virus has been detected in all of the “Lower 48” states of the U.S., the District of Columbia, and Puerto Rico.  WNV is the leading cause of mosquito-borne disease in the continental United States.

West Nile virus-related diseases occur in different parts of the U.S. from year to year. In 2022, the states with the highest number of human cases were Arizona, California, Colorado, Nebraska, Illinois, South Dakota, and New York. The largest identified single outbreak in U.S. history of West Nile virus occurred in Arizona in the summer and fall of 2021. Over 1,600 cases of West Nile virus disease were reported, with the outbreak centered in Maricopa County.

California leads the nation in total WNV cases – with more than 7,000 human cases and 300 deaths since 2003. The mosquitoes that spread WNV are found throughout the state. High-incidence areas have been identified in southern California (Los Angeles, Orange, Riverside, and San Bernardino) and California's Central Valley (Stanislaus, Kern, Fresno, Butte, Sacramento, and Tulare). Despite containing only 2% of California's area and 17% of the state's population, these high-burden areas accounted for 44% of California’s WNV cases.

Symptoms

Most people infected with West Nile virus do not experience any symptoms. Another 19% develop a mild infection called West Nile fever (WNF). Symptoms usually appear within 3 to 14 days of infection. Common signs and symptoms may include:

• Fever
• Headache
• Body aches
• Vomiting
• Diarrhea
• Fatigue
• Skin rash

Signs and Symptoms of More Severe Disease (West Nile Encephalitis or West Nile Meningitis) May Include

• High fever
• Severe headache
• Stiff neck
• Disorientation or confusion
• Coma
• Tremors or muscle jerking
• Seizures
• Partial paralysis or muscle weakness
• Vision loss
• Numbness

Who Is Most at Risk?

West Nile virus causes serious symptoms in about 1% of those who become infected, with increased illness and death occurring in individuals 50 years of age and older. Adults with the severe form of the illness may experience encephalitis (inflammation of the brain). Children with the severe form of the disease develop meningitis (inflammation of the membranes that surround the brain and spinal cord). The fatality rate in older adults is 14%. It is about 1% in children.

Treatment

No vaccine or specific medicines are available for West Nile virus infection.

Prevention

The best way to prevent WNV is to protect against mosquito bites. Mosquitoes capable of infecting humans with WNV are active from dusk till dawn so try to avoid being outside at those times. If you must be outside, use repellents containing DEET (or other EPA-registered active ingredients) according to label directions. The American Academy of Pediatrics and the Centers for Disease Control and Prevention note that products containing up to 30% DEET are appropriate for children over two months of age. For more information on avoiding mosquito bites, please visit our prevention page.