Risk of exposure to West Nile fever virus in wild birds, horses, and people in western Castilla-La Mancha

    Wild birds from rural environments such as horse farms in western Castilla-La Mancha are more likely to be infected by Flavivirus such as West Nile fever virus, representing a risk for transmission of the virus to horses and people in these environments.

    The annoying mosquitoes are responsible for transmitting numerous diseases that can have serious consequences for the health of animals and people. Diseases caused by viruses of the genus Flavivirus, such as the West Nile fever (WNF), have experienced a considerable increase in Spain since 2010, causing numerous outbreaks in horse farms and human clinical cases. The year 2020 was a fateful year, in which 139 outbreaks of WNF were registered in horses and 77 cases in people in the southwest of Spain with 8 people dying due to the severe evolution of meningoencephalitis caused by the WNF virus.

    The cycle of the Flavivirus that causes West Nile fever is very complex, and involves birds, in charge of replicating, amplifying, and transmitting the virus to mosquitoes that can later bite horses and people; and mosquitoes, which are responsible for transmitting the virus between birds and to people and horses.

    Risk of exposure to West Nile fever virus in wild birds, horses, and people in western Castilla-La Mancha


    Schematic representation of the life cycle of the WNF virus, which is transmitted by mosquitoes of the Culex genus and continues to circulate among birds and mosquitoes, from which it can infect horses and people, in which it can cause serious disease.

    The existence of favorable environmental conditions for the replication of the West Nile fever virus in a territory, such as abundant populations of Culex mosquitoes and wild birds, can promote the circulation of the virus and the appearance of new cases of disease. In these scenarios it is very important to determine the risk of animals or people suffering from WNF, especially when they are generated in areas where the virus appears to be spreading. This is the case in western Castilla-La Mancha, where several cases have been documented in domestic and wild animals, but not in humans for now.

    Scientists from the Research Team in Ecology of Vector Diseases, from the Research Group in Health and Biotechnology (SaBio) of the Instituto de Investigación en Recursos Cinegéticos (IREC – CSIC, UCLM, JCCM), in collaboration with researchers from the Centro de Investigación en Sanidad Animal (CISA-CSIC), have studied the exposure of wild birds from rural environments (horse farms) to the West Nile fever virus in western Castilla-La Mancha, and have compared the results with those obtained from wild birds from less humanized environments around these farms.

    To do this, they designed a study in 9 horse farms distributed throughout the areas where virus circulation had been detected in the provinces of Ciudad Real and Toledo, considering a decreasing interaction gradient between birds, mosquitoes and horses/people. On each farm, they sampled wild birds and mosquitoes on the farms themselves, in the immediate neighborhood of the farms, and in an area further away from the farms where there were only wild animals, analyzing in each case the exposure to the WNF virus in these birds and potentially conditioning factors of exposure risk such as habitat, climatic conditions and abundance and diversity of birds and mammals.

    Risk of exposure to West Nile fever virus in wild birds, horses, and people in western Castilla-La Mancha


    Schematic representation of the different areas of interaction between wild animals, mosquitoes, and horses in the environment of the farms selected for the study.

    The results, recently published in the scientific journal Pathogens, showed that 9% of wild birds present in horse farms are exposed to Flavivirus, especially the WNF virus, compared to 5,6% and 5,8% of birds around farms. This indicates that horse farms are an optimal environment for West Nile virus transmission among birds, in which we also identified a higher abundance of Culex mosquitoes than in their environment, and therefore that the risk of transmission of the virus to horses and people is greater in these farms.

    The scientists also observed how the greater diversity of wild bird species in horse farms implies a greater risk of infection by these Flavivirus, suggesting that the implementation of measures that discourage birds from using farms, and even those focused on reduce the abundance of mosquitoes, could reduce the risk of WNF virus transmission to horses and people.

    The authors of the study recommend that managers of horse farms adopt preventive measures to reduce the use of farms by wild birds, but considering that they are ecologically relevant species, many of which are protected and must be respected. Studying the effectiveness of these bird-friendly measures in the future will be essential, especially if, as predicted, global warming has a positive impact on a greater abundance of mosquitoes and an increase in the circulation rate of this virus in the environment.

    This work provides highly relevant information on the risks and determinants of exposure to the West Nile fever virus, laying the foundations to act more effectively in preventing the risk of infection for animals and people with specific action measures. Such measures could result in better prevention of the disease in horses, for which there is a vaccine, and in people, for whom this vaccine is not available and for whom prevention is the only tool to fight the disease.

    The scientific publication of this research work is available at: