A community-based focal serosurvey for West Nile virus infection following a surge in cases in 2024 in Kerala, India: a cross-sectional analysis

Background: From January to May of 2024, 27 cases of neuroinvasive disease due to West Nile Virus (WNV) were confirmed from Kerala. The number of cases were more than those seen over past three years combined. Cases were not clustered geographically and were primarily reported from four districts, viz, Kozhikode, Thrissur, Malappuram and Ernakulum. To understand the circulation of WNV, a focal serosurvey was conducted in the regions from where cases were reported. Methods: A cross-sectional study was done in four districts of Kerala. Patients, family members and immediate neighbours were recruited at each case location. From 27 clusters across 26 villages/localities, 751 blood samples were collected. Due to cross-reactivity of WNV antibodies and that of Japanese Encephalitis (JE) virus, a micro-neutralization assay was done against both these viruses for all the samples. Results: The seropositivity of WNV infection was found to be 29.96 % (26.68-33.24). Males had higher seropositivity than females, though the difference was not statistically significant. The overall seropositivity was higher compared to previously published studies. There was no significant difference in seropositivity across age-groups and sex. Seropositivity of JE infection was 1.86 % (0.90-2.83). Discussion: Though a smaller proportion of infected get neurological involvement, WNV seropositivity among considerable number of people in a wide geography of the state is a public health concern. Conclusion: To deal with this concern, more studies on bird-human interaction and larger serosurveys might be needed. Close monitoring and intervention planning are warranted to control possible future WNV spread.