Term: West Nile Virus

Description: West Nile virus (WNV) is a mosquito-transmitted viral disease, known as a flavivirus, that is believe to be spread when a mosquito bites an infect bird and then bites a person or horses. Other animals, such as dogs and cats, can get the virus from a mosquito bite, but they are not very susceptible. The disease was first discovered in 1937 in Uganda in eastern Africa and brought to the U.S. the summer of 1999. Since then, WNV has been a seasonal epidemic in the U.S. that flares up in the summer and fall, with a peak in September. The majority of people who become infected will never realize they have the disease, and it can spread through blood transfusions, organ transplants, pregnancy or through breastfeeding. Risk factors for severe symptoms to WNV include: weakened immune system, very young or over 50, and pregnancy. Spending more time outdoors will increase risk of becoming infected.

There are various modes of transmission; however transmission by the mosquito is the primary mode. The West Nile virus is maintained through cycle between birds and mosquitoes. The genus culex of mosquito plays the largest role in transmission of the virus. In the U.S., corvids (including crows and jays) are mainly affected and often become ill and are dying at extreme rates. Humans and other animals (such as horses) have insufficient viremia (the presence of viruses in the blood) to infect native mosquito. Some rare modes of transmission are organ transplant, blood transfusion and breastfeeding. There was 1 case of West Nile virus infection with possible transmission via blood transfusion reported in Colorado. There were 2 patients with probable West Nile virus transmission via organ transplant and blood transfusion in Louisiana in 2008. And in 2006, 2 recipients of blood and organs from a single donor were infected with the virus. West Nile virus transmission through breastfeeding is rare. There were 10 reported cases of maternal or infant West Nile virus illness while breastfeeding since 2003. 6 cases were maternal illness, 5 tested infants had no illness or detectable antibodies after onset of maternal illness and 1 infant was not tested, had rash but was otherwise well 1 week after onset of maternal illness. Among 45 milk samples from women infected with West Nile virus during pregnancy, 2 had West Nile virus RNA and 14 had IgM antibodies to West Nile virus.

1) None. About 4 of 5 people (80 percent) infected with WNV will not show any symptoms.

2) Mild. Mild signs and symptoms of a West Nile virus generally go away on their own. Approximately 20 percent of people will begin to experience mild symptoms, typically arising within two weeks of becoming infected. A mild case of the disease, generally called West Nile fever, could last for a few days or up to a month and are typically flu-like:
  • Stomach aches
  • Sore throat
  • Diarrhea
  • Nausea and/or vomiting
  • High fever
  • Headache
  • Loss of appetite
  • Sore muscles
  • Rash
  • Swollen lymph nodes

3) Severe. More severe cases of WNV are extremely rare but can be life threatening. They may be called West Nile encephalitis or West Nile meningitis, depending on what part of the body is affected. Signs of a more severe case needing immediate medical attention, include:
  • Confusion
  • Loss of consciousness or coma
  • Muscle weakness
  • Stiff neck
  • Weakness of one arm or leg
  • Vision loss
  • Flaccid Paralysis

There are several tests used to detect WNV, including:
  • Complete blood count (CBC)
  • Head CT Scan
  • Head MRI Scan
  • Lumbar puncture and cerebrospinal fluid testing (CSF)

Diagnosis is suspected in patients with symptoms including fever, headache, myalgia, meningitis and/or flaccid paralysis. A history of living in endemic areas, especially during warm weather and exposure to mosquitoes where West Nile virus exists are also considered when making a diagnosis. Diagnosis is usually confirmed using serology, a medical science dealing with blood serum especially in regard to its reactions and properties. Nucleic acid testing or viral isolation can also be done for definite diagnosis using blood or cerebrospinal fluid. However, it’s not often used clinically due to short-lived nature and low level of viremia in humans.

There is no treatment to cure WNV. The virus is not caused by antibodies, so it cannot be treated with antibiotics. Doctors may provide medicine to help cope with symptoms, but patients typically recover on their own within a few weeks. In more severe cases, patients may need hospital care where they can receive supportive treatment including intravenous fluids, help with breathing and nursing care. Treatment is mainly focused on supportive care. Oral or IV fluids may be needed in patients with nausea and vomiting if weakness occurs. Patients with meningitis may need pain control for headache or encephalitis mechanical. Patients with severe encephalopathy (a disease that affects the function or structure of the brain) may need ventilation. There is limited evidence for medication use. Interferon alpha-2b (a man made protein that boosts your immune system) and IV immunoglobulin (a sterile solution of concentrated antibodies extracted from people) reported to be effective in some case series and case reports.

Currently no human vaccine exists, but there is one for horses. Several vaccines for humans are in development. The experimental DNA vaccine immunogenic (capable of inducing an immune response; antigenic) has been tested in 30 healthy adults and was successful.

The best way to prevent being infected with WNV is to avoid mosquito bites.
  • Limit time outdoors during peak mosquito season
  • Always apply bug spray containing DEET when outside
  • Protect skin with long sleeves and pants when outdoors
  • Drain pools of standing water, such as trash bins, gutters, or planters (mosquitoes breed in standing water)
  • Inquire about community spraying for mosquitoes
  • Use well-fitting screens on doors and windows

Web Resources:

Related Terminology:


In the map above, the pink are counties with positive cases of West Nile virus. This map is from data collected by the CDC and you may see all of the data and zoom into each state here: http://diseasemaps.usgs.gov/wnv_us_human.html

r7_westnileviruscycle.jpgexternal image WNVhouse03L.jpg
Above is West Nile virus etiology and prevention diagram.