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This article waslast modified on 23 March 2023.
What is it?

West Nile Virus (WNV) was first discovered as a cause of an infectious disease in Uganda, Africa in 1937. It then spread to the Middle East, Eastern Europe, West Asia and, more recently, the United States. It is a very uncommon condition in the UK, with only 2 cases recorded since 2002, both of which being associated with travel to Canada. 

WNV belongs to a group of disease-causing viruses known as flaviviruses. These lipid (fat)-enveloped viruses can be spread by insects, usually mosquitoes, to animals, including humans. Most human infections are mild, although in some cases serious illness such as encephalitis can result from infection. It is not contagious person-to-person.

The most common route of transmission is through a mosquito bite. When a mosquito bites an infected bird, such as a crow (crows are highly susceptible to infection), it can then transmit the virus to another animal it bites. Transmission of the virus commonly peaks during early spring when adult mosquitoes emerge and continues until autumn. It is estimated that 1 in 200 mosquitoes in the US harbours the virus.

There also has been some recent concern about transmission of WNV through donated blood or organs, as seems to have occurred in the case of four organ transplant recipients.   


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About West Nile Virus
  • Symptoms

    About one in five (20%) of those who are infected with WNV will experience symptoms. These usually are described as flu-like, with fever, gastrointestinal problems, headache, and body ache as well as potentially skin rash and swollen lymph glands. These symptoms typically last for only a few days and have no long-term health impact.

    Less than 1% (about 1 in 150) of those infected will develop more serious disease, in which there is brain involvement. This can lead to life-threatening encephalitis (inflammation of the brain) and/or meningitis (inflammation of the lining of the brain and spinal cord). Symptoms include high fever, extreme muscle weakness, neck stiffness, stupor, disorientation, coma, tremors, and convulsions. The fatality rate ranges from 3-15% and is highest among the elderly. Individuals with compromised immune systems also seem to be at increased risk of severe disease.

    The clinical presentation of paralysis in some individuals is reminiscent of that seen in the past with the polio virus. However, WNV is not in the same family of viruses. There is no explanation for this as of yet.


  • Tests

    Testing of samples is carried out by arrangement, via the local microbiology laboratory, at the Rare and Imported Pathogens Laboratory, UKHSA.

    The virus or its RNA can be detected in serum, cerebrospinal fluid (CSF), or other tissues using a method called polymerase chain reaction (PCR). However, this method is not currently recommended because of its low sensitivity. A better option is to use an antibody-capture assay, which can detect IgM antibody, the body's immune response to the virus, in spinal fluid and serum - usually within one week of the onset of illness. Further testing of serum samples is recommended to confirm a diagnosis of WNV using virus-neutralisation tests.

    Diagnosis usually is made through review of the patient's symptoms and exposure and confirmed by testing the patient's blood and CSF at the public health laboratory.


  • Prevention and Treatment

    No vaccine or drug treatment is available at this time, although research is ongoing and several experimental vaccines and antiviral therapies are showing promise. Vaccines for similar flaviviruses have been developed to protect humans, so it may not be a difficult task to produce one for WNV in the next few years. 

    Treatment currently is focused on symptom management. In more severe cases, admission to a hospital may be required. A more effective solution is preventing infection by protecting against mosquito bites by using insect repellent, wearing long-sleeved clothing and trousers, staying indoors at dawn and dusk, and eliminating standing water sources that attract mosquitoes.