Research Article: Spatio-temporal epidemiology of Japanese encephalitis in Nepal, 2007-2015

Date Published: July 26, 2017

Publisher: Public Library of Science

Author(s): Dhan Kumar Pant, Tenzin Tenzin, Rakesh Chand, Barun Kumar Sharma, Padam Raj Bist, Naomi Forrester.


Japanese encephalitis (JE) is a major public health problem in Nepal. For the effective management and surveillance of JE, a clear understanding of its epidemiology is essential. Therefore, we conducted descriptive and spatial analyses to understand the spatio-temporal distribution of JE in human in Nepal. From 2007 to 2015, 1,823 JE cases were reported with a cumulative mean incidence of 0.735/100,000 population and a case fatality rate of 6.6%. The death rate in the up-to-24 years of age group was 74%. The JE cases were most commonly reported in the age group of 1–14 years. There is a strong seasonal pattern of JE occurrence in Nepal which peaked in August and declined by October each year, which corresponds to the monsoon season. The JE cases were reported in 63 of 75 districts (84%), expanding in the mountain and hill regions. There was a strong clustering of JE incidence in the south-western and south-eastern Terai region, which is endemic for JE. Therefore, the JE surveillance system should be improved to better understand the drivers of disease expansion in Nepal for instituting a control program.

Partial Text

Japanese encephalitis (JE) is a vector-borne viral disease caused by a flavivirus. The transmission cycle typically involves Culex tritaeniorhynchus mosquitoes and similar species, with aquatic birds and pigs acting as reservoir and amplifying hosts, respectively. Humans are the incidental host and play no role in perpetuating the virus [1]. The first epidemic of JE was reported in Japan during 1924 [1,2]. Thereafter, the epidemiological trend has been increasing in Asian countries, particularly in South East Asia [3]. In recent years, the epidemiological patterns and distribution of JE have changed. The disease is spreading in wider geographical areas due to climate change, land use patterns (especially cultivation of land and urbanization), changes in socioeconomic status (promotion of pig breeding as food source) and vector diversity [4–6]. Globally, approximately 3 billion people live in JE risk areas and nearly 68,000 clinical cases of JE, with 13,600 to 20,400 deaths, occur each year [7]. The case fatality rate of JE ranges from 10–20%, while 20–30% of the survivors have permanent neurological disorder [8]. Fundamentally, JE is considered as a disease of children, but all age groups are vulnerable in tropical and subtropical countries [3,8,9].

This study demonstrates that JE incidence is not distributed homogeneously in Nepal. Cases were clustered mostly in the southern Terai or low plain land. This might be due to high vector density, high intensity of cultivated paddy fields and favorable climatic factors for the JE vector. Terai is the grain basket of Nepal with high paddy cultivation and animal farming, including swine and birds. This ecological region has been reported as a JE prone area in the previous studies [11, 18, 19]. Therefore, preventive and control measures should be adopted before the outbreak period in this high risk zone. Although there has been a slight decline of JE cases in Nepal in the recent past, geographical expansion into hilly and mountain regions of the country has been documented between 2014 and 2015 (Fig 2). This might be associated with changes in socio-ecological determinants, including changing agriculture practices and land use patterns, climate change, expansion of pig farming and other anthropologic determinants. For example, Tibet, located in the Qinghai-Tibet Plateau of the western People’s Republic of China, has been recognized previously as free of JE because of high elevation. However, this region has recently demonstrated the emergence of JE virus in mosquitoes, humans and pigs, indicating global expansion of JEV into non-endemic areas [20]. Factors such as global warming, increased pig farming, and increased tourism and transportation may have contributed to the emergence of JE in Tibet. Among various driving predictors for JEV, climatic variation is regarded as a key factor. Change in both temperature and precipitation are capable of affecting JEV transmission [21–24]. Studies of JE epidemiology in Japan, Nepal, China and Taiwan demonstrated that transmission was associated with high temperature and low precipitation [25–28]. Nepal is one of the world’s vulnerable countries with respect to climate change. The increasing average annual temperatures in the hill and mountain regions of the country might have provided conducive environments for the survival and adaptability for JE vectors (C. tritaeniorrhynchus), thus favoring geographical expansion of JE in Nepal [29]. In addition, other climate-sensitive vector-borne diseases, including malaria, lymphatic filariasis, visceral leishmaniasis and dengue, have been reported in the hilly and mountain regions of Nepal, which were previously considered as non-endemic, indicating the adaptation of vectors in non-endemic regions [30, 31]. The study also detected anti-JE virus antibodies in swine populations sampled from four high-altitude mountain districts of Nepal indicating circulation of JE virus, where human infections were documented [32]. Similarly, a study conducted by Impoinvil et al. (2011) demonstrated JE clusters shifting to hilly and mountain regions of Nepal [26]. Therefore, a more comprehensive surveillance system should be designed and implemented based on a One Health approach for the early detection, preparedness and monitoring of the JE situation in Nepal.