Epidemiological Analysis On Japanese Encephalitis In Inner Mongolia

Japanese encephalitis (JE) is an acute infectious diseases caused by Japanese encephalitis virus (JEV) which is transmitted by mosquitoes. The mortality of JE is about 5-30% and about 30-50% of survivors have serious sequelae of the nervous system. All of these create a heavy burden to the society and family. JE cases were mainly reported in Asia and the Pacific, and our country is one of the major JE epidemic countries.Inner Mongolia Autonomous Region (Inner Mongolia) was an area where JE had a high prevalence. Outbreaks had occurred repeatedly in the history. In recent decades, JE cases had significantly reduced. To understand the prevalence of JEV and mosquito-borne virus in the local area, the epidemiological data of JE cases in Inner Mongolia from 1950 to 2007 were analyzed and at the same time mosquitoes were collected from 2007 to 2008 in eastern, central and western area of Inner Mongolia to carry out the investigation of etiology, The results were reported as follows.1. Analysis of epidemiological data of JE in Inner Mongolia from 1950 to 2007This study used the method of descriptive epidemiology to analyze the epidemiological data of JE cases from 1950 to 2007 in Inner Mongolia, and the result showed as follows:(1) Inner Mongolia has reported a total of 4850 cases of JE from 1950 to 2007. Outbreaks have occurred repeatedly in the history. The incidence and the mortality rate of JE in 1974 was the highest, whose numbers were 9.66/100 thousand and 1.51/100 thousand, respectively. (2) According to the incidence of JE every year in Inner Mongolia, the epidemic period can be divided into three phrases: platform period (1950-1968), the high-epidemic period (1969-1984) and the effective control period (1985-2007). From the year 1990 to 2007 the annual incidence of JE in Inner Mongolia was below the national average level during the same period.(3) From 1950 to 2007, 10 cities in Inner Mongolia had reported the cases of JE. Chifeng city and Tongliao city had reported more cases, accounted for about 84.8 percent of total number of cases. And the reported cases mainly concentrated in the years from 1969 to 1984. Wuhai and Alashan have never reported JE case;(4) The incidence of JE in Inner Mongolia has obvious seasonal characteristics. The incidence in July began to increase. The cases occurred mainly in August and September each year, and then a dramatic drop in October. Compared with incidence of the average level in China and Guizhou Province, the peak of the incidence of JE in Inner Mongolia reached a month later. (The average high incidence period in China is in July and August.).(5) The sex and age distribution characteristic of JE cases during the period from 1988 to 2007 is as follows: From 1988 to 2007 JE cases were reported in each age group, including children under the age of 15 accounted for a total of 28.61%, significantly lower than the average level 88.03% of China. Male and female ratio was 1:0.48 ;(6)From 1991 to 2007, the career order of JE cases in Inner Mongolia was farmers (47.71%), students (23.85%) and the scattered children (8.72%), while the order of the nation was the scattered children (51.7%), students (26.2%) and care young children (10.9%).2. The etiology investigation of Japanese encephalitis and the mosquito-borne arbovirus in Inner MongoliaIn order to understand the distribution of Japanese encephalitis virus and mosquito-borne arbovirus of Inner Mongolia, four areas from eastern, central and western Inner Mongolia were chosen to collected mosquitoes for two consecutive years to carry out the investigation of mosquito vector and mosquito-borne arbovirus (1) Survey for mosquito vector:Total 10,542 mosquito specimens were collected in July and August during the year 2007 and 2008. These samples were collected from 11 areas which belong to Tongliao, Bayannur, Hohhot and Wuhai in Inner Mongolia. The mosquito specimens were belong to 3 genera and total 7 species, including Culex modestus, Culex pipiens pallens, Aede vexans, Aede dorsalis, Aede caspius, Aede flavidorsalis and Anopheles sinensis. Culex tritaeniorhynchus-the major vector of JEV was not collected during the two-year collection period. The dominant mosquito species varied in different areas, Aede dorsalis in Tongliao, Aede caspius in Bayannur, Culex modestus and Aede dorsalis in Hohhot, Aede flavidorsalis in Wuhai.(2) Isolation and identification for virusAll mosquito specimens in accordance with each pool with 25 to 100 after grinding and centrifugation were inoculated into tissue culture cells (C6/36 and BHK-21 cells) for virus isolation. Totally six positive virus isolates which can cause cytopathic effect were separated. These positive viruses isolate from Culex modestus and Aede dorsalis in Huhot, Culex modestus in Tongliao; Aede flavidorsalis in Bayannur. According to serology (ELISA, IFA), molecular biology (SDS-PAGE, Real-time PCR and RT-PCR, etc.) and bio-informatics methods research, the results showed that two were Tahyna virus belongs to the Bunyaviridae, which is the second time that Tahyna virus was isolated since 2006 when it was firstly isolated in Xinjiang. One virus was Banna virus belonged to the reoviridae. The three other virus isolates were Culex pipiens pallens densovirus belongs to Brevidensovirus, Parvoviridae genus. Japanese encephalitis virus was not identified from the Positive isolates.3. Other workThis study also carried out mosquitoes and mosquito-borne arbovirus survey in Jinzhou and Dandong of Liaoning Province, in 2008. A total of 9296 mosquito from 3 genera and 5 species were collected, including the Culex tritaeniorhynchus, Culex pipiens pallens, Aede vexans, Aede dorsalis, and Anopheles sinensis. Culex tritaeniorhynchus was the dominant mosquito species in Dandong of Donggang and Anopheles sinensis was the dominant mosquito species in Beining of Jinzhou. Four virus isolates were obtained from Culex tritaeniorhynchus in Dandong and Anopheles sinensis in Jinzhou, among which one Japanese encephalitis virus, two Banna viruses, a hybrid strains of Banna virus and Liaoning virus were identified by the serological and molecular biology tests. The analysis of sequence evolution of the whole genome the new separation of Japanese encephalitis virus indicated that Japanese encephalitis virus isolated from mosquitoes in Dandong in 2008 belongs to gene type I JEV, similar with those isolated in 2002 and 2007 in the same area. Furthermore, the key amino acid sites for virus antigen and virulence in E gene of Japanese encephalitis virus isolated in 2008 remained unchanged compared with vaccine strain (SAM-14-2 strain).