Study area and data collection
The study was conducted in the state of Uttar Pradesh (UP), where there are 18 administrative divisions divided into a total of 75 districts (https://www.up.gov.in, accessed 3 May 2023). The state of UP has 3 distinct seasons: winter from November to February, summer from March to May, and monsoon from June to October (https://www.nri.up.gov.in, accessed 3 May 2023). Five districts in eastern UP in Gorakhpur and Basti divisions (Gorakhpur, Maharajganj, Kushinagar, Sant Kabir Nagar and Siddarthnagar districts) are frequently affected by floods due to the topography of the region(Kumari & Joshi, 2012). During 2011‑2018, the four districts of Gorakhpur division (Gorakhpur, Maharajganj, Deoria and Kushinagar), and the three districts of Basti division (Basti, Sant Kabir Nagar and Siddarthnagar) contributed to 86% of the total number of reported acute encephalitis syndrome cases in UP (A. K. Singh et al., 2020).
Pig serum samples were collected from three divisions of eastern UP: Gorakhpur (Gorakhpur, Kushinagar, Deoria and Maharajganj districts), Basti (Basti, Sant Kabir Nagar, Siddarthnagar districts) and Azamgarh (Azamgarh, Ballia, and Mau districts; Figure 1). The samples from Gorakhpur and Basti divisions were collected from February 2013 to December 2022, whereas samples from Azamgarh division were collected from 2017 to 2022, following increased reporting of Japanese encephalitis (JE) cases in people in Azamgarh division. Samples were collected by veterinarians of the State Animal Husbandry Department and Indian Veterinary Research Institute and submitted to the Division of Veterinary Public Health, Indian Veterinary Research Institute, Bareilly, UP, for sero-diagnosis of JE. Variables recorded at the time of sample collection included pig age (categorised into 3 groups: 4-5 months, 6-12 months and >12 months), date of sample collection, and district from where the sample was collected.
Monthly total rainfall, mean relative humidity and mean minimum temperature measured at Gorakhpur Station (latitude 26.74, longitude 83.45) during the study period were obtained from ‘The Visual Crossing global weather database’ (https://www.visualcrossing.com/weather-data, accessed 5 May 2023).
Animal ethics approval was not required for this study, because collection of samples and analysis are conducted for pre-existing and ongoing surveillance programmes for animal and public health.