West Nile virus (WNV) is a mosquito-borne virus that causes West Nile fever and has a wide geographical distribution. Rising temperatures are likely to increase transmission and extend the distribution of the WNV and the length of the transmission season, hence increasing infection risk in existing hot spots as well as in previously unaffected regions within Europe.

West Nile Fever notification rate (map) and reported cases (graph) in Europe
Source: ECDC, 2023, Surveillance Atlas of Infectious Diseases

Notes:

Map and graph show data for the EEA member and cooperating countries, excluding Denmark, Switzerland and Türkiye due to absence of data. The boundaries and names shown on this map do not imply official endorsement or acceptance by the European Union.
The disease is notifiable at the EU level, but the reporting period varies among the countries.
When countries report zero cases, the notification rate on the map is shown as '0'. When countries have not reported on the disease in a particular year, the rate is not visible in the map and is labelled as 'unreported' (last updated in April 2023).

Source & transmission

The WNV occurs in a remarkably large number of different (bird) species, which explains its wide geographical distribution (Blitvich, 2008). While birds act as the primary host of the virus, humans and other mammals can become sick when bitten by a mosquito infected with the WNV. Yet, mammals are unable to infect mosquitos themselves (Chancey et al., 2015). Constant infections between mosquitos and birds in mosquito-active seasons result in the maintenance of high viral amounts, which leads to consistently high risks for human infection. Throughout the winter season in Europe, the WNV can persist in mosquitoes (Rudolf et al., 2017).

The WNV is predominantly transmitted by Culex mosquitos, and to a lesser extent by Aedes mosquitos. Culex mosquitos are widely spread across Europe (ECDC, 2022a,b). There is nevertheless a higher likelihood of WNV transmission in Southern compared to Northern Europe, since higher temperatures accelerate the transmission potential of Culex mosquitos (Colpitts et al., 2012; Vogels et al., 2017). Mosquitos can also transmit the WNV to their eggs and larvae, hence maintaining virus circulation (Colpitts et al., 2012).

Apart from the infection route with mosquito vector, the WNV can also be transmitted through blood transfusions, organ transplants, or maternal transmission from mother to unborn child (Hayes et al., 2005).

Health effects

Only 20% of people infected with the WNV show symptoms. About a fifth of these patients develop a fever, which is often accompanied by other symptoms such as headaches, pains, vomiting, diarrhoea, or rashes. Most people developing a fever recover fully but may experience weakness and fatigue for an extended period.

A minority of infected people develop a severe illness, i.e., West Nile Neuroinvasive Disease (WNND). In the case of organ donation however, the risk to develop WNND is relatively high: 40% of the people receiving an organ infected with the WNV get WNND (Anesi et al., 2019). WNND can include meningitis (inflammation of the membranes surrounding the brain and the spinal cord), encephalitis (inflammation of the brain itself), or in rare cases poliomyelitis, which may lead to partial paralysis and damage to heart or lung muscles. Symptoms include high fever, headaches, neck stiffness, tremors, convulsions, loss of vision, numbness or even paralysis and coma. Patients with severe symptoms may not fully recover and sometimes the WNND has a fatal outcome.

Morbidity and mortality in Europe

In the EEA member and cooperating countries (excluding Denmark, Switzerland and Türkiye due to absence of data), in the period 2008-2021:

  • 5,399 cases
  • The EU/EEA notification rate was 0.1 cases per 100 000 population in 2019, compared to 0.3 for 2018
  • The case fatality among infections with known outcome was on average 12% in the period 2016-2019
  • More than 90% of cases with reported hospitalisation status were hospitalised between 2016 and 2019
  • An increasing number of infections identified as locally acquired, with >90% of cases locally acquired between 2016 and 2019.
  • No clear trend in the number of reported locally acquired infections could be discerned between 2010 and 2019. However, peaks occurred in 2010, 2012, 2013, 2016 and 2018.

(ECDC, 2014-2021)

Distribution across population

  • Infection rates increase with age and are the highest in the age group with the highest disease rate in Europe: >65 years old
  • Infection rates are higher among men than woman (ECDC, 2014-2021)
  • Groups at risk of severe disease course: elderly and people with low immunity
  • Groups at higher risk of infection: migrant workers and travellers

Climate sensitivity

Climatic suitability

The WNV can infect Culex mosquitos at temperatures as low as 18 °C. Yet, higher temperatures lead to shorter incubation periods (i.e., the period of virus development within the mosquito), quicker virus mutation and evolution and an amplified viral load (Leggewie et al., 2016). The Culex mosquito species thrive between approximately 11 and 35 °C, with faster development rates and longer seasons at higher temperatures (Mordecai et al., 2019; Rueda et al., 1990). Natural or artificial containers filled with water are needed for reproduction.

Seasonality

In Europe, most cases occur between July and October with a peak of infections mainly in August. The seasonality in infections coincides with the period when mosquito vectors are most active and the ambient temperature is high enough for virus multiplication in vectors across Europe (ECDC, 2014-2021).

Climate change impact

A warmer climate in Europe will generally lead to a shorter incubation period of the WNV and accelerate the virus evolution rate, hence increasing the viral load within host populations. Moreover, in higher temperatures, Culex mosquitos develop faster, extend their reproductive season, and feed more often. Hence, increasing temperatures are likely to lead to faster transmission and wider distribution of the WNV, longer transmission seasons and a higher risk for local acquisition of human WNV infections in both existing transmission areas and previously unaffected European regions (Leggewie et al., 2016).

Prevention & Treatment

Prevention

  • Personal protection: long-sleeved clothes, mosquito repellents, nets or screens, air conditioning and limiting outdoor activities at night-time
  • Mosquito control: environmental management, e.g., minimizing reproduction opportunities in open natural and artificial waters, and biological or chemical measures, e.g., insecticides and water treatment chemicals (e.g. see the activities of the mosquito control action group in Germany)
  • Active monitoring and surveillance of mosquitos, disease cases and environment to prevent transmission (e.g. see the case studies of the ‘Mückenatlas’ initiative, the EYWA project, or the West Nile virus surveillance in Greece)
  • Awareness raising about disease symptoms, disease transmission and mosquito bite risks
  • Screening of blood and organ donors
  • Currently, no WNV vaccines are licensed to be administered to humans (DeBiasi and Tyler, 2006)

Treatment

  • No specific and effective antiviral therapy
  • Symptom treatment with pain control or rehydration therapy
  • Close monitoring for patients with encephalitis or inflammation of the brain. Ventilator support or heart massages to avoid respiratory or heart failure (Chancey et al., 2015; DeBiasi and Tyler, 2006).

References

Anesi, J. A. et al., 2019, Arenaviruses and West Nile Virus in solid organ transplant recipients: Guidelines from the American Society of Transplantation Infectious Diseases Community of Practice, Clinical Transplantation 33(9), e13576. https://doi.org/10.1111/ctr.13576

Blitvich, B. J., 2008, Transmission dynamics and changing epidemiology of West Nile virus, Animal Health Research Reviews 9(1), 71–86. https://doi.org/10.1017/S1466252307001430

Chancey, C. et al., 2015, The Global Ecology and Epidemiology of West Nile Virus, BioMed Research International e376230, 1-10 http://dx.doi.org/10.1155/2015/376230

Colpitts, T. M. et al, 2012, West Nile Virus: Biology, Transmission, and Human Infection, Clinical Microbiology Reviews 25(4), 635–648. https://doi.org/10.1128/CMR.00045-12

DeBiasi, R. L. and Tyler, K. L., 2006, West Nile virus meningoencephalitis, Nature Clinical Practice Neurology 2(5), 264–275. https://doi.org/10.1038/ncpneuro0176

ECDC, 2014-2021, Annual epidemiological reports for 2012-2019 – West Nile virus infection. Available at https://www.ecdc.europa.eu/en/west-nile-fever/surveillance-and-disease-data/annual-epidemiological-report. Last accessed April 2023.

ECDC, 2022a, Culex modestus - current known distribution: March 2022, Online mosquito maps, ECDC, Stockholm. Available at https://www.ecdc.europa.eu/en/publications-data/culex-modestus-current-known-distribution-march-2022. Last accessed December 2022.

ECDC, 2022b, Culex pipiens group - current known distribution: March 2022, Online mosquito maps, ECDC, Stockholm. Available at https://www.ecdc.europa.eu/en/publications-data/culex-pipiens-group-current-known-distribution-march-2022. Last accessed December 2022.

ECDC, 2023, Surveillance Atlas of Infectious Diseases. Available at https://atlas.ecdc.europa.eu/public/index.aspx. Last accessed April 2023.

Hayes, E. B. et al., 2005, Epidemiology and Transmission Dynamics of West Nile Virus Disease, Emerging Infectious Diseases 11(8), 1167–1173. https://doi.org/10.3201/eid1108.050289a

Leggewie, M. et al., 2016, Culex pipiens and Culex torrentium populations from Central Europe are susceptible to West Nile virus infection, One Health 2, 88–94. https://doi.org/10.1016/j.onehlt.2016.04.001

Mordecai, E. A. et al., 2019, Thermal biology of mosquito‐borne disease, Ecology Letters 22(10), 1690–1708. https://doi.org/10.1111/ele.13335

Rudolf, I., et al., 2017, West Nile virus in overwintering mosquitoes, central Europe, Parasites & Vectors 10(452), 1-4. https://doi.org/10.1186/s13071-017-2399-7

Rueda, L. M. et al., 1990, Temperature-Dependent Development and Survival Rates of Culex quinquefasciatus and Aedes aegypti (Diptera: Culicidae), Journal of Medical Entomology 27(5), 892–898. https://doi.org/10.1093/jmedent/27.5.892

Vogels, C. B., et al., 2017, Vector competence of European mosquitoes for West Nile virus, Emerging Microbes & Infections 6(e96), 1-13. https://doi.org/10.1038/emi.2017.82

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