Reference of the article: Magallanes, S., Llorente, F., Ruiz-López, M. J., la Puente, J. M. D., Ferraguti, M., Gutiérrez-López, R., … & Figuerola, J. (2024). Warm winters are associated to more intense West Nile virus circulation in southern Spain. Emerging Microbes & Infections.


What is known about the subject?

The West Nile virus (WNV) is a pathogen mainly spread among birds through the bite of infected mosquitoes of the Culex genus. Birds represent the natural reservoir of this pathogen. It can accidently reach other hosts, like mammals, which are generally considered dead-end hosts of WNV because they develop low viremias during infection that are not sufficient to infect mosquitoes feeding on their blood. An estimated 80% of WNV infections are asymptomatic and 19% may show mild symptoms like fever and myalgia. However, 1% of people who get bitten and become infected may develop more severe illnesses that can lead to meningoencephalitis and even death.

WNV is considered a re-emerging pathogen in Europe due to its increasing incidence. Endemic in Spain, WNV circulation is known in the southern area since 2003 as confirmed by seroconversion of resident birds and horses, and by the detection of the virus in mosquitoes and birds. The first human case was detected in 2004, and thereafter a few cases were sporadically detected in the region until 2020, when an unprecedented outbreak caused 77 clinical cases and eight fatalities. Cases of severe infection are being reported every year since then.


What does this study add to the existing literature?

Several studies suggest that climatic conditions can have an effect on the incidence of WNV in Europe. However, to date scarce long-term studies have analysed the relationship between climate and WNV incidence.

As a matter of fact, this is the first study in Europe to analyse a population of common coots (Fulica atra) over 16 years and explore how climatic variations can affect the presence of antibodies against WNV in this bird species. Animals exposed to WNV develop short-lived infections, usually of one or two weeks, and antibodies which end up making the virus disappear. Main findings of the study show that the higher the maximum temperature in the winter, the higher the proportion of birds that will have antibodies against WNV the following autumn. This means that a warm winter may favour an increased incidence of WNV the following year.


What are the implications of the findings?

 This study confirms that WNV has been endemic in Spain for over at least two decades, particularly in the South West region. Findings show how temperature is a determining factor in the incidence of this pathogen, especially winter temperature which could be associated with the phenology of mosquitoes and the survival of female mosquitoes during the winter period. Average winter temperatures, especially maximum temperatures, have risen by 1 ºC in the studied area over the past two decades. With this trend, we can expect an increase in the incidence of WNV in Spain in the coming years. However, the real impact on human health will depend on the ability to design and implement management policies that include efficient surveillance programmes, vector control and vector population awareness. Long-term studies on wildlife are an effective tool to identify environmental impact, including climatic conditions, on the dynamics of transmission and the incidence of this and other zoonotic pathogens.



Clara Bermúdez Tamayo, MPH PhD
Technical Director 
Andalusian Health and Environment Observatory
Andalusian School of Public Health.
Campus Universitario de Cartuja, Cuesta del Observatorio, 4, 18011 Granada