Surveillance Open Access
Like 0



West Nile virus (WNV) is a flavivirus with an enzootic cycle between birds and mosquitoes; humans and horses are incidental dead-end hosts. In 2020, the largest outbreak of West Nile virus infection in the Iberian Peninsula occurred, with 141 clusters in horses and 77 human cases.


We analysed which drivers influence spillover from the cycle to humans and equines and identified areas at risk for WNV transmission.


Based on data on WNV cases in horses and humans in 2020 in Portugal and Spain, we developed logistic regression models using environmental and anthropic variables to highlight risk areas. Models were adapted to a high-resolution risk map.


Cases of WNV in horses could be used as indicators of viral activity and thus predict cases in humans. The risk map of horses was able to define high-risk areas for previous cases in humans and equines in Portugal and Spain, as well as predict human and horse cases in the transmission seasons of 2021 and 2022. We found that the spatial patterns of the favourable areas for outbreaks correspond to the main hydrographic basins of the Iberian Peninsula, jointly affecting Portugal and Spain.


A risk map highlighting the risk areas for potential future cases could be cost-effective as a means of promoting preventive measures to decrease incidence of WNV infection in Europe, based on a One Health surveillance approach.


Article metrics loading...

Loading full text...

Full text loading...



  1. Chancey C, Grinev A, Volkova E, Rios M. The global ecology and epidemiology of West Nile virus. BioMed Res Int. 2015;2015:376230.  https://doi.org/10.1155/2015/376230  PMID: 25866777 
  2. Kramer LD, Li J, Shi PY. West Nile virus. Lancet Neurol. 2007;6(2):171-81.  https://doi.org/10.1016/S1474-4422(07)70030-3  PMID: 17239804 
  3. Kaptoul D, Viladrich PF, Domingo C, Niubó J, Martínez-Yélamos S, De Ory F, et al. West Nile virus in Spain: report of the first diagnosed case (in Spain) in a human with aseptic meningitis. Scand J Infect Dis. 2007;39(1):70-1.  https://doi.org/10.1080/00365540600740553  PMID: 17366016 
  4. Connell J, Mckeown P, Garvey P, Cotter S, Conway A, O’Flanagan D, et al. Two linked cases of West Nile virus (WNV) acquired by Irish tourists in the Algarve, Portugal. Euro Surveill. 2004;8(32):pii=2517.  https://doi.org/10.2807/esw.08.32.02517-en 
  5. Ministerio de Sanidad. Informe de Situación y Evaluación del Riesgo de la Fiebre por Virus del Nilo Occidental en España. [Information report and evaluation of the risk of West Nile fever in Spain]. Madrid: Ministerio de Sanidad; 2017. Spanish. Available from: https://www.sanidad.gob.es/profesionales/saludPublica/ccayes/analisisituacion/doc/Evaluacion_de_riesgo-VNO-2017.pdf
  6. García-Bocanegra I, Jaén-Téllez JA, Napp S, Arenas-Montes A, Fernández-Morente M, Fernández-Molera V, et al. West Nile fever outbreak in horses and humans, Spain, 2010. Emerg Infect Dis. 2011;17(12):2397-9.  https://doi.org/10.3201/eid1712.110651  PMID: 22172565 
  7. Alves MJ, Poças JMD, Luz T, Zé-Zé L, Amaro F, Osório H. Infecção por vírus West Nile (Flavivírus) em Portugal. Consideraçoes acerca de um caso clínico de síndrome febril. [West Nile virus (Flavivirus). infection in Portugal. Considerations about a clinical case with febrile syndrome and rash]. Rpdi. 2012; 8(1):46-51. Portuguese.
  8. García San Miguel Rodríguez-Alarcón L, Fernández-Martínez B, Sierra Moros MJ, Vázquez A, Julián Pachés P, García Villacieros E, et al. Unprecedented increase of West Nile virus neuroinvasive disease, Spain, summer 2020. Euro Surveill. 2021;26(19):2002010.  https://doi.org/10.2807/1560-7917.ES.2021.26.19.2002010  PMID: 33988123 
  9. Lourenço J, Barros SC, Zé-Zé L, Damineli DSC, Giovanetti M, Osório HC, et al. West Nile virus transmission potential in Portugal. Commun Biol. 2022;5(1):6.  https://doi.org/10.1038/s42003-021-02969-3  PMID: 35013546 
  10. Casimiro-Soriguer CS, Perez-Florido J, Fernandez-Rueda JL, Pedrosa-Corral I, Guillot-Sulay V, Lorusso N, et al. Phylogenetic analysis of the 2020 West Nile virus (WNV) outbreak in Andalusia (Spain). Viruses. 2021;13(5):836.  https://doi.org/10.3390/v13050836  PMID: 34063166 
  11. Leblond A, Hendrikx P, Sabatier P. West Nile virus outbreak detection using syndromic monitoring in horses. Vector Borne Zoonotic Dis. 2007;7(3):403-10.  https://doi.org/10.1089/vbz.2006.0593  PMID: 17767410 
  12. Ward MP, Schuermann JA, Highfield LD, Murray KO. Characteristics of an outbreak of West Nile virus encephalomyelitis in a previously uninfected population of horses. Vet Microbiol. 2006;118(3-4):255-9.  https://doi.org/10.1016/j.vetmic.2006.07.016  PMID: 16971067 
  13. García-Bocanegra I, Belkhiria J, Napp S, Cano-Terriza D, Jiménez-Ruiz S, Martínez-López B. Epidemiology and spatio-temporal analysis of West Nile virus in horses in Spain between 2010 and 2016. Transbound Emerg Dis. 2018;65(2):567-77.  https://doi.org/10.1111/tbed.12742  PMID: 29034611 
  14. van Galen G, Calozet L, Leblond A, Tritz P, Dal Pozzo F, Porter SR, et al. Can horses be clinically screened for West Nile Fever? Vet Rec. 2013;172(4):101.  https://doi.org/10.1136/vr.101267  PMID: 23292842 
  15. Saegerman C, Alba-Casals A, García-Bocanegra I, Dal Pozzo F, van Galen G. Clinical sentinel surveillance of equine West Nile fever, Spain. Transbound Emerg Dis. 2016;63(2):184-93.  https://doi.org/10.1111/tbed.12243  PMID: 24899369 
  16. Barker CM, Reisen WK, Kramer VL. California state Mosquito-Borne Virus Surveillance and Response Plan: a retrospective evaluation using conditional simulations. Am J Trop Med Hyg. 2003;68(5):508-18.  https://doi.org/10.4269/ajtmh.2003.68.508  PMID: 12812335 
  17. Reed KD, Meece JK, Henkel JS, Shukla SK. Birds, migration and emerging zoonoses: west nile virus, lyme disease, influenza A and enteropathogens. Clin Med Res. 2003;1(1):5-12.  https://doi.org/10.3121/cmr.1.1.5  PMID: 15931279 
  18. Napp S, Montalvo T, Piñol-Baena C, Gómez-Martín MB, Nicolás-Francisco O, Soler M, et al. Usefulness of Eurasian magpies (Pica pica) for West Nile virus surveillance in non-endemic and endemic situations. Viruses. 2019;11(8):716.  https://doi.org/10.3390/v11080716  PMID: 31387316 
  19. European Commission (EC). Commission implementing decision 2018/945 of 22 June 2018 on the communicable diseases and related special health issues to be covered by epidemiological surveillance as well as relevant case definitions. Brussels: EC; 2018. Available from: https://eur-lex.europa.eu/legal-content/EN/TXT/PDF/?uri=CELEX:32018D0945
  20. European Centre for Disease Prevention and Control (ECDC). West Nile virus transmission season in Europe, 2020. Stockholm: ECDC; 2021. Available from: https://www.ecdc.europa.eu/en/news-events/epidemiological-update-west-nile-virus-transmission-season-europe-2020
  21. Eurostat. NUTS- Nomenclature of Territorial Units for Statistics. Luxemburg: Eurostat; 2021. Available from: https://ec.europa.eu/eurostat/web/nuts/background
  22. Ministerio de Sanidad. Meningoencefalitis por el virus del Nilo Occidental en España. Evaluación rápida de Riesgo. [West Nile virus meningoencephalitis. Rapid risk assessment]. Madrid: Ministerio de Sanidad; 2020. Spanish. Available from: https://www.sanidad.gob.es/profesionales/saludPublica/ccayes/alertasActual/docs/20200925_ERR_Nilo_Occidental.pdf
  23. Ministerio de Sanidad. Meningoencefalitis por virus del Nilo occidental en España (2a actualización). Evaluación rápida de riesgo. [West Nile virus meningoencephalitis in Spain (2nd update). Rapid risk assessment]. Madrid: Ministerio de Sanidad; 2020. Spanish. Available from: https://www.sanidad.gob.es/profesionales/saludPublica/ccayes/alertasActual/docs/20201203_ERR_Nilo_Occidental.pdf
  24. European Commission (EC). Animal Disease Information System. Brussels: EC. [Accessed: 28 Aug 2023]. Available from: https://food.ec.europa.eu/animals/animal-diseases/animal-disease-information-system-adis_en
  25. Benjamini Y, Hochberg Y. Controlling the false discovery rate: a practical and powerful approach to multiple testing. J R Stat Soc. 1995;57(1):289-300.
  26. Rao CR. Large sample tests of statistical hypotheses concerning several parameters with applications to problems of estimation. Math Proc Camb Philos Soc. 1948;44(1):50-7.  https://doi.org/10.1017/S0305004100023987 
  27. Real R, Barbosa AM, Vargas JM. Obtaining environmental favourability functions from logistic regression. Environ Ecol Stat. 2006;13(2):237-45.  https://doi.org/10.1007/s10651-005-0003-3 
  28. Fielding AH, Bell JF. A review of methods for the assessment of prediction errors in conservation presence/absence models. Environ Conserv. 1997;24(1):38-49.  https://doi.org/10.1017/S0376892997000088 
  29. Márcia Barbosa A, Real R, Muñoz A-R, Brown JA. New measures for assessing model equilibrium and prediction mismatch in species distribution models. Divers Distrib. 2013;19(10):1333-8.  https://doi.org/10.1111/ddi.12100 
  30. Lobo JM, Jiménez-Valverde A, Real R. AUC: A misleading measure of the performance of predictive distribution models. Glob Ecol Biogeogr. 2008;17(2):145-51.  https://doi.org/10.1111/j.1466-8238.2007.00358.x 
  31. Bombi P, D’Amen M. Scaling down distribution maps from atlas data: A test of different approaches with virtual species. J Biogeogr. 2012;39(4):640-51.  https://doi.org/10.1111/j.1365-2699.2011.02627.x 
  32. Bakonyi T, Haussig JM. West Nile virus keeps on moving up in Europe. Euro Surveill. 2020;25(46):2001938. https://doi.org/10.2807/1560-7917.ES.2020.25.46.2001938  PMID: 33213684 
  33. García-Carrasco J-M, Muñoz A-R, Olivero J, Segura M, Real R. Predicting the spatio-temporal spread of West Nile virus in Europe. PLoS Negl Trop Dis. 2021;15(1):e0009022.  https://doi.org/10.1371/journal.pntd.0009022  PMID: 33411739 
  34. Camp JV, Nowotny N. The knowns and unknowns of West Nile virus in Europe: what did we learn from the 2018 outbreak? Expert Rev Anti Infect Ther. 2020;18(2):145-54.  https://doi.org/10.1080/14787210.2020.1713751  PMID: 31914833 
  35. Bravo-Barriga D, Aguilera-Sepúlveda P, Guerrero-Carvajal F, Llorente F, Reina D, Pérez-Martín JE, et al. West Nile and Usutu virus infections in wild birds admitted to rehabilitation centres in Extremadura, western Spain, 2017-2019. Vet Microbiol. 2021;255(March):109020.  https://doi.org/10.1016/j.vetmic.2021.109020  PMID: 33677369 
  36. García-Bocanegra I, Franco JJ, León CI, Barbero-Moyano J, García-Miña MV, Fernández-Molera V, et al. High exposure of West Nile virus in equid and wild bird populations in Spain following the epidemic outbreak in 2020. Transbound Emerg Dis. 2022;69(6):3624-36.  https://doi.org/10.1111/tbed.14733  PMID: 36222172 
  37. Ulbert S. West Nile virus vaccines - current situation and future directions. Hum Vaccin Immunother. 2019;15(10):2337-42.  https://doi.org/10.1080/21645515.2019.1621149  PMID: 31116691 
  38. Marini G, Poletti P, Giacobini M, Pugliese A, Merler S, Rosà R. The role of climatic and density dependent factors in shaping mosquito population dynamics: The case of Culex pipiens in northwestern Italy. PLoS One. 2016;11(4):e0154018.  https://doi.org/10.1371/journal.pone.0154018  PMID: 27105065 
  39. Vogels CBF, Hartemink N, Koenraadt CJM. Modelling West Nile virus transmission risk in Europe: effect of temperature and mosquito biotypes on the basic reproduction number. Sci Rep. 2017;7(1):5022.  https://doi.org/10.1038/s41598-017-05185-4  PMID: 28694450 
  40. Brugueras S, Fernández-Martínez B, Martínez-de la Puente J, Figuerola J, Porro TM, Rius C, et al. Environmental drivers, climate change and emergent diseases transmitted by mosquitoes and their vectors in southern Europe: A systematic review. Environ Res. 2020;191:110038.  https://doi.org/10.1016/j.envres.2020.110038  PMID: 32810503 
  41. Reisen WK, Fang Y, Martinez VM. Effects of temperature on the transmission of west nile virus by Culex tarsalis (Diptera: Culicidae). J Med Entomol. 2006;43(2):309-17.  https://doi.org/10.1093/jmedent/43.2.309  PMID: 16619616 
  42. Hartley DM, Barker CM, Le Menach A, Niu T, Gaff HD, Reisen WK. Effects of temperature on emergence and seasonality of West Nile virus in California. Am J Trop Med Hyg. 2012;86(5):884-94.  https://doi.org/10.4269/ajtmh.2012.11-0342  PMID: 22556092 
  43. Paz S, Malkinson D, Green MS, Tsioni G, Papa A, Danis K, et al. Permissive summer temperatures of the 2010 European West Nile fever upsurge. PLoS One. 2013;8(2):e56398.  https://doi.org/10.1371/journal.pone.0056398  PMID: 23431374 
  44. Caminade C, Medlock JM, Ducheyne E, McIntyre KM, Leach S, Baylis M, et al. Suitability of European climate for the Asian tiger mosquito Aedes albopictus: recent trends and future scenarios. J R Soc Interface. 2012;9(75):2708-17.  https://doi.org/10.1098/rsif.2012.0138  PMID: 22535696 
  45. Miramontes R Jr, Lafferty WE, Lind BK, Oberle MW. Is agricultural activity linked to the incidence of human West Nile virus? Am J Prev Med. 2006;30(2):160-3.  https://doi.org/10.1016/j.amepre.2005.10.008  PMID: 16459215 
  46. Crowder DW, Dykstra EA, Brauner JM, Duffy A, Reed C, Martin E, et al. West nile virus prevalence across landscapes is mediated by local effects of agriculture on vector and host communities. PLoS One. 2013;8(1):e55006.  https://doi.org/10.1371/journal.pone.0055006  PMID: 23383032 
  47. Hardy JL, Houk EJ, Kramer LD, Reeves WC. Intrinsic factors affecting vector competence of mosquitoes for arboviruses. Annu Rev Entomol. 1983;28(1):229-62.  https://doi.org/10.1146/annurev.en.28.010183.001305  PMID: 6131642 
  48. Liang G, Gao X, Gould EA. Factors responsible for the emergence of arboviruses; strategies, challenges and limitations for their control. Emerg Microbes Infect. 2015;4(3):e18.  https://doi.org/10.1038/emi.2015.18  PMID: 26038768 
  49. World Health Organization (WHO). West Nile virus. Geneva: WHO; 2017. Available from: https://www.who.int/news-room/fact-sheets/detail/west-nile-virus
  50. Young JJ, Coulombier D, Domanović D, Zeller H, Gossner CM, European Union West Nile fever working group. One Health approach for West Nile virus surveillance in the European Union: relevance of equine data for blood safety. Euro Surveill. 2019;24(16):1800349.  https://doi.org/10.2807/1560-7917.ES.2019.24.16.1800349  PMID: 31014416 
  51. Ajuntament de Reus. Detectat un cas positiu de febre del Nil Occidental a Reus. [A positive case of West Nile fever has been detected in Reus]. Reus: Ajuntament de Reus; 2022. Spanish. Available from: https://www.reus.cat/noticia/detectat-un-cas-positiu-de-febre-del-nil-occidental-reus
  52. Ziegler U, Lühken R, Keller M, Cadar D, van der Grinten E, Michel F, et al. West Nile virus epizootic in Germany, 2018. Antiviral Res. 2019;162(162):39-43.  https://doi.org/10.1016/j.antiviral.2018.12.005  PMID: 30550796 
  53. Ministerio de Agricultura Pesca y Alimentación. Programa de vigilancia fiebre del Nilo Occidental 2022. [West Nile Fever Surveillance Program 2022]. Madrid: Ministerio de Agricultura Pesca y Alimentación; 2021. Spanish. Available from: https://www.mapa.gob.es/es/ganaderia/temas/sanidad-animal-higiene-ganadera/sanidad-animal/enfermedades/fiebre-nilo-occidental/F_O_Nilo.aspx
  54. Healy JM, Reisen WK, Kramer VL, Fischer M, Lindsey NP, Nasci RS, et al. Comparison of the efficiency and cost of West Nile virus surveillance methods in California. Vector Borne Zoonotic Dis. 2015;15(2):147-55.  https://doi.org/10.1089/vbz.2014.1689  PMID: 25700046 
  55. Chevalier V, Lecollinet S, Durand B. West Nile virus in Europe: a comparison of surveillance system designs in a changing epidemiological context. Vector Borne Zoonotic Dis. 2011;11(8):1085-91.  https://doi.org/10.1089/vbz.2010.0234  PMID: 21548765 
  56. García-Carrasco J-M, Muñoz A-R, Real R. Anticipating the locations in Europe of high-risk areas for West Nile virus outbreaks in 2021. Zoonoses Public Health. 2021;68(8):982-6.  https://doi.org/10.1111/zph.12877  PMID: 34242480 
  57. Riccò M, Peruzzi S, Balzarini F. Epidemiology of West Nile virus infections in humans, Italy, 2012-2020: a summary of available evidences. Trop Med Infect Dis. 2021;6(2):61.  https://doi.org/10.3390/tropicalmed6020061  PMID: 33923347 
  58. Papa A, Danis K, Baka A, Bakas A, Dougas G, Lytras T, et al. Ongoing outbreak of West Nile virus infections in humans in Greece, July-August 2010. Euro Surveill. 2010;15(34):1-9644.  https://doi.org/10.2807/ese.15.34.19644-en  PMID: 20807489 
  59. Papa A, Tsioka K, Gewehr S, Kalaitzopouou S, Pervanidou D, Vakali A, et al. West Nile fever upsurge in a Greek regional unit, 2020. Acta Trop. 2021;221:106010.  https://doi.org/10.1016/j.actatropica.2021.106010  PMID: 34129841 

Data & Media loading...

Supplementary data

Submit comment
Comment moderation successfully completed
This is a required field
Please enter a valid email address
Approval was a Success
Invalid data
An Error Occurred
Approval was partially successful, following selected items could not be processed due to error