05/10/2026
West Nile Virus - where does it originate and how has it spread?
The second part in a series of questions about West Nile virus (WNV) that I'm going to answer here one by one in a quest to find an evidence based answer to the overarching question: am I going to vaccinate my pony?
WNV is a virus. It affects horses 🏇🐎, birds 🐦🦉🦜🦅 AND 👨👩👦👦 people.
Mosquitoes 🦟 🦟, specifically those of the Culex variety, transmit the virus via blood. However, they can only transmit it from birds to other mammals (mainly humans and horses, although infections in other mammals have been described). In horses and people the amount of virus in the blood stays too low to be transmitted, the so called "dead end host".
This means that you are not an infection risk for your pony and vice versa.
The way WNV spreads depends on the movement of infected birds and the presence of mosquitoes. This in turn depends a lot on the season and the weather. Below a certain temperature there are no mosquitoes, but as temperatures rise there are reports of WNV infections as late as November.
Research indicates that WNV has adapted to overwinter in mosquito populations in Europe, suggesting the potential for year-round transmission in some temperate areas, a shift that poses significant challenges to current control strategies
So far we know of 9 different lineages of WNV. Seeing which ones cause infection where, helps to determine the pattern of spread.
Although there are exceptions, in general, lineage I (clade Ia) viruses can cause severe human neurologic disease whereas lineage I (clade Ib), and lineage II viruses generally cause a mild, self-limiting disease.
The first report of human infection was in 1937 in Uganda, which, you guessed it is west of the river Nile. Since then it had spread world wide. The first noted infections in Southern Europe were around 20 years ago and in the last 5 years this has spread to Northern Europe. Initially the virus was noted in birds and wild animals, but has since caused an outbreak in horses and humans in France and the Netherlands in the last year.
Further reading:
de Best PA, Braks M; One Health PACT consortium; National West Nile Virus Response team, working group; Timen A, Sikkema RS, Koopmans MP; One Health PACT Consortium. A multidisciplinary approach to the detection of and response to West Nile virus in the Netherlands between 2020 and 2023: best practices, challenges and opportunities. Euro Surveill. 2026 Mar;31(10):2500276. doi: 10.2807/1560-7917.ES.2026.31.10.2500276. PMID: 41822983; PMCID: PMC13077186.
Klitting R, Gondard M, Pezzi L, Migné CV, Bellone R, L'Ambert G, Helle T, Mignotte A, Gutiérrez-Climente R, Lacour G, Mocq J, Ayhan N, Lucchese F, Piorkowski G, Dumarest M, Amaral R, Bollore K, Hanin J, Courot O, Hirchaud E, Beven V, Blanchard Y, Karch A, Jakerian G, Durand GA, Grard G, Herbreteau N, Duvignaud A, Gutierrez S, Aubert L, Cannet A, Parisey M, Simonin Y, Malvy D, Paty MC, Fournet N, Franke F, Laperche S, Gallian P, Failloux AB, de Lamballerie X, Fontaine A, Gonzalez G. Genomic Epidemiology of West Nile Virus in Paris. JAMA Netw Open. 2026 Feb 2;9(2):e2559588. doi: 10.1001/jamanetworkopen.2025.59588. Erratum in: JAMA Netw Open. 2026 Mar 2;9(3):e265966. doi: 10.1001/jamanetworkopen.2026.5966. PMID: 41697700; PMCID: PMC12910402.
Schwarzer A, Schopf F, Groschup MH, Bock S, Heenemann K, Herms L, Himmelreich A, Kenklies S, Kilwinski J, Konrath A, Michel F, Müller K, Priemer G, Rahner R, Sauerwald C, Scuda N, Siempelkamp T, Skuballa J, Wonnemann H, Keller M, Vahlenkamp TW, Ziegler U, Sadeghi B. Circulation dynamics of West Nile virus in Germany, 2023 and 2024. Virol J. 2025 Dec 18;23(1):58. doi: 10.1186/s12985-025-03043-8. PMID: 41413585; PMCID: PMC12955091.
Fania A, Capozza P, Cazzolla Gatti R, Amoroso N, Vasinioti VI, Elia G, Bellotti R, Pratelli A, Monaco A. Standardized Incidence Ratio dataset of Human West Nile Virus in Italy (2012-2024). Sci Data. 2025 Nov 24;12(1):1861. doi: 10.1038/s41597-025-06129-8. PMID: 41285906; PMCID: PMC12644758.
If you have other questions that are not on the list, please feel free to add them in the comments.
-> what is it and who does it affect ?
-> how does it affect people?
-> what are the clinical signs in horses?
-> is it likely to come to the UK and if so, when?
-> is there a link with bluetongue virus in sheep?
-> is there a treatment?
-> how do I prevent my horse from catching it?
-> how effective are the vaccines and are there any side effect?
-> is there enough vaccine available in case of an outbreak?
Please share and help educate others (but don't copy and paste my words).
