Serosurvey Reveals Exposure to West Nile Virus in Asymptomatic Horse Populations in Central Spain Prior to Recent Disease Foci.

The research shows that the West Nile virus, an infectious disease affecting horses, birds, and humans, was present in asymptomatic horse populations in Central Spain before the first reported outbreak of the disease in that area.

Objective of the Research

• The main purpose of this study was to carry out a retrospective serosurvey to identify evidence of West Nile virus (WNV) infections in asymptomatic horses in central Spain from 2011 to 2013, that is, before the first outbreaks of the disease were reported in the area.

Methodology

• Scientists gathered serum samples from 369 horses between September 2011 and November 2013 in Central Spain.
• These samples were then tested using a method called ELISA (enzyme-linked immunosorbent assay), specifically a type of ELISA called blocking and IgM.
• Positive results from the ELISA were then confirmed by a virus neutralization test. This test also checks for the specificity of the virus by using parallel titration with another flavivirus (Usutu virus).

Results and Findings

• Out of the 369 horse serum samples, 10 tested positive using the competitive ELISA. Out of the 10, five were confirmed as positive for WNV infection using virus-neutralization technique. This gives a seropositivity rate of 1.35%.
• One of the samples that tested positive for WNV was IgM-positive. Immunoglobulin M (IgM) is a type of antibody that is produced by the immune system in response to a specific antigen, in this case, WNV. Hence, the presence of IgM indicates a recent infection.
• Chronologically, the first positive samples, including the one that was IgM-positive were collected in 2012 in Madrid province.

Conclusion

• The study concluded that WNV circulated in asymptomatic equine populations of central Spain at least since 2012, quite before the first disease outbreak reported in the area.
• This finding is significant as it indicates that the virus had been spreading silently among hors populations before being detected in the first outbreak.