West Nile Virus

West Nile fever is caused by the West Nile virus (WNV), a Flavivirus transmitted by the bites of infected mosquitoes. The virus is mainly found in (migratory) birds and can be transmitted to humans by certain species of mosquitoes of the genus Culex. In addition to humans, mammals such as horses can also be infected, though these are accidental hosts because the transmission cycle is only between birds and mosquitoes.

What is West Nile virus? 

  • Although there are eight types or “lineages” of the West Nile virus, only two of them cause disease in humans. WNV lineage 1 is present in many parts of the world (America, Africa (including the Nile basin), West Asia, Middle East and Europe), while WNV lineage 2 is more limited to Africa and Europe.  
  • The virus is mainly found in birds (migratory). Humans and horses are so-called “dead-end” hosts for the virus because they no longer contribute to the transmission of the disease. This is because the virus does not grow in these hosts, leaving an insufficient quantity of virus in the blood to infect a mosquito when bitten.
  • Most human cases have been reported in North America and Canada. In Europe, the virus was originally introduced by migrating birds from Africa, and is currently endemic  in several regions of southern and central Europe. Since the start of the surveillance of West Nile fever in Europe, the highest number of cases was observed in 2018. In recent years, autochthonous human cases were reported for the first time in Germany (2019 and 2020) and the Netherlands (2020).
  • In exceptional cases, human-to-human transmission is possible through blood transfusion, organ transplant or during pregnancy, childbirth or breastfeeding.


  • After an incubation period of approximately 2 to 14 days, 1 in 5 infected people experience symptoms such as headaches, muscle pain, fever, a skin rash and swollen lymph nodes.
  • Most people recover completely, but fatigue can sometimes last for several weeks or even months.
  • In rare cases (1% of infections), infected people develop a serious illness, such as encephalitis (inflammation of the brain) or meningitis (inflammation of the meninges, i.e. the coverings of the spinal cord and the brain). The severe form is more common in the elderly and can be fatal (for 1 in 10 people affected).

Diagnosis, treatment and prevention

  • If the doctor suspects West Nile virus infection, the diagnosis can be made using a blood test or lumbar puncture. Depending on the stage of the disease, different tests can be used, such as a PCR test and serology.
  • There is no specific treatment for West Nile fever, symptoms are treated with painkillers and rehydration in the case of fever. Severe cases require hospitalisation.
  • There is no vaccine available for humans, but there is one for horses.
  • Prevention is done by avoiding mosquito bites.

Sciensano is responsible for the epidemiological surveillance of West Nile Fever in humans in Belgium.

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