Changing paradigm in the epidemiology of Japanese encephalitis in a non-endemic region

Background & objectives: Japanese encephalitis (JE) is a mosquito-borne zoonotic disease. The JE virus (JEV) does not cause any disease among its natural hosts and transmission continues through mosquitoes belonging to Culex vishnui subgroup. This study was aimed to investigate the prevalence of JEV in mosquitoes and humans in the Thanjavur district, a non-endemic region for JE, in Tamil Nadu, by using standard available assays. Methods: A sero-surveillance study was conducted in Thanjavur district among the normal rural school children in the 5-12 yr age group, during the JE season (October) and post-JE season (February) from 2011 to 2013 for the detection of JEV infection. Vector abundance studies were carried out from 2011 to 2014. JE seropositivity and its association between the seasons were analysed statistically. Results: The occurrence of JE infection among children aged 5-12 yr was very high in the study area. The infection rates for JE in two consecutive seasons for 2011-12 and 2012-13 were 32.2 and 65.2%, respectively. The Cx. tritaeniorhynchus sp. dominated the catch, and was majorly responsible for the transmission. There was a significant difference in the human infection rate compared to the years 1991-92 and 1992-93; and a marked decrease in the cattle to pigs ratio (123 : 1) compared to the studies in 1991-93. Interpretation & conclusion: The study unearthed the prevailing situation of JE among children, who are at higher risk of developing the disease during the transmission season. The decrease in the cattle to pigs ratio might be the one of the reasons for increase in the JEV infection among the children population compared to 20 years before. This trend requires urgent attention as it could be prevented with effective surveillance systems and vaccines.