Jpn. J. Infect. Dis., 62, 331-336, 2009
Review
Japanese Encephalitis in Mainland China Wang Huanyu, Li Yixing1, Liang Xiaofeng1, and Liang Guodong* State Key Laboratory for Infectious Disease Control and Prevention, Department of Viral Encephalitis, Institute for Viral Disease Control and Prevention and 1National Immunization Program, Chinese Center for Disease Control and Prevention, Beijing, People’s Republic of China (Received February 16, 2009. Accepted July 31, 2009) CONTENTS: 1. Introduction 2. Incidence and mortality 3. Seasonal distribution of cases 4. Geographic distribution of cases 4-1. Highly endemic areas 4-2. Moderately endemic areas 4-3. Slightly endemic areas 4-4. Non-endemic areas
5. Age and gender distribution 6. Occupational distribution 7. Prevalence of JEV isolates 8. Control and prevention of JE 8-1. JE vaccine 8-2. Surveillance of JE 9. Conclusion
SUMMARY: Japanese encephalitis (JE) is a seasonal epidemic disease with a 50-year recorded history in China. Its characteristics can be summarized as follows: (i) it is a seasonal epidemic disease; approximately 90% of cases are recorded in July, August, and September each year. The peak of JE onset is 1 month earlier in South China than in the north of the country; (ii) the disease is highly sporadic. It is rare for more than two cases to appear simultaneously in one family; (iii) most affected children are under 15 years old; (iv) the disease is widely distributed in all areas of the nation except Qinghai Province, Xinjiang Uygur Autonomous, and Tibet. Due to widespread application of the JE vaccine, the number of JE cases has decreased significantly nationwide, from 174,932 cases of morbidity in 1971 to 5,097 cases in 2005. tending to expand. For instance, in 1995, an outbreak of JE occurred in Papua New Guinea and among the original inhabitants of the northern islands of Australia, and even appeared in the northern areas of Australian mainland (5,6). JE has consequently become one of the most prominent public health issues in the world. China is the main region of JE endemism. Our data reveal the occurrence and epidemic outbreaks of JE in all provinces except Xinjiang Uygur Autonomous, Tibet, and Qinghai. JE is one of four arbovirus diseases currently prevalent in China (7). In this paper, we summarize the epidemics of JE in mainland China in recent years and the prevention measures taken there.
1. Introduction Japanese encephalitis (JE) is an acute epidemic disease of the central nervous system (CNS) caused by infection with the Japanese encephalitis virus (JEV). JE mainly affects children and adolescents. According to the World Health Organization (WHO) statistics, approximately 35,000 cases of JE are reported each year, causing approximately 5,000 deaths---a mortality rate of 5 - 40%. Approximately 50% of JE patients present neurological and mental sequelae (1). JEV is transmitted by mosquitoes and the genus Culex, which is major vector. It is a perennial disease in tropical areas, but is clearly seasonal in temperate zones, with a peak incidence period between June and October each year. At present, JE is endemic in over 20 countries and areas, including the Pacific coastal areas of Far East Russia, Japan, China, North and South Korea, India, Vietnam, Laos, Myanmar, Thailand, Cambodia, the Philippines, Malaysia, Singapore, Bhutan, Indonesia, Nepal, Sri Lanka, Guam, Papua New Guinea, and Australia (2,3). The traditional endemic areas of JE are mainly distributed in the countries and regions of Asia, where epidemics are often reported. For example, an outbreak of JE in India in 2005 involved 5,737 reported cases and resulted in 1,344 deaths (4). At present, the areas of JE endemism are
2. Incidence and mortality JE case reporting is currently mandated by law in China. Since the establishment of a case reporting system in 1951, the incidence of JE has been recorded annually in China. Further, since 2004, each JE case has been electronically recorded and the data collected at the national level by the Chinese Center for Disease Control and Prevention (China CDC); the case reporting system has thus become both more sensitive and more efficient. Historically, there have been two major JE epidemics. The first, in 1966, had an annual incidence of >15/100,000 nationwide, whereas the second, in 1971, was associated with 174,932 cases of morbidity and an incidence of 20.92/100,000 (8-10). Since the 1980s, JE vaccination has been widely used in our country and, as a consequence, the number of morbidities has declined gradually year by year. Prior to the 1990s, the annual morbidities numbered between 20,000 and 40,000.
*Corresponding author: Mailing address: State Key Laboratory for Infectious Disease Prevention and Control, Department of Viral Encephalitis, Institute for Viral Disease Control and Prevention, Chinese Center for Disease Control and Prevention, 100 Ying Xin Street, Xuan Wu District, Beijing 100052, People’s Republic of China. Tel: +86 10 63510124, Fax: +86 10 63532053, E-mail:
[email protected] 331
Table 1. The incidence and death of JE in 1996 - 2005 Year
No. of JE cases
Incidence (1/100,000)
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005
10,308 10,060 12,490 8,556 11,779 9,795 8,769 7,860 5,422 5,097
0.8660 0.8343 0.9977 0.6889 0.9489 0.7707 0.6548 0.5829 0.4171 0.3898
Total
90,136
–
No. of death cases 379 370 510 348 375 246 229 366 200 214 3,237
Mortality (1/100,000)
Case fatality (%)
0.0318 0.0307 0.0407 0.0280 0.0302 0.0194 0.0171 0.0271 0.0154 0.0164
3.677 3.678 4.083 4.067 3.184 2.511 2.611 4.6565 3.6887 4.1985
–
–
Fig. 1. JE incidence in 1951 - 2006, China.
onwards, indicating a consistent annual pattern in the incidence and mortality of JE (13). The occurrences of JE cases in the north and south region of China have been shown to be slightly different. In the south, JE cases start to increase in July and decrease significantly in August, whereas in the north, JE cases begin to increase in August and clearly decrease in September (13-15).
However, the number of JE morbidities decreased from 11,779 to 5,097 between 2000 and 2005, the annual incidence declined from 0.9489/100,000 to 0.3898/100,000, the number of mortalities declined from 375 to 214, and the mortality rate also decreased from 0.0302/100,000 to 0.0164/100,000. Over this 6-year period, the annual mortality rate ranged from 2.51 to 4.66% (11,12). The trends in the incidence and mortality of JE in mainland China in recent years are illustrated in Figure 1 and Table 1.
4. Geographic distribution of cases A total of 31 provinces in mainland China have reported JE cases; the exceptions including Qinghai Province, Xinjiang Uygur Autonomous, and Tibet. The cases are scattered in various endemic localities with no obvious aggregation of the disease. Based on the average annual incidence of JE between 1996 and 2005, the regions of endemism in mainland China can be classified into the following four groups (Figure 3). 4-1. Highly endemic areas The average incidence in these areas is considered to be approximately >1/100,000. The areas include Sichuan Province, Guizhou Province, Chongqing City, Shaanxi Province, and Yunnan Province. Annually, the number of morbidities in these five areas accounts for 50% of the total cases nationwide, comprised as much as 74.1% of the total cases in 2002. The combined population of these areas, however, represents only 26% of the national population (10,11,13-16). 4-2. Moderately endemic areas The average incidence in these areas is considered to be between 0.5/100,000 and 1/100,000. The areas include the
3. Seasonal distribution of cases JE cases are reported from January to December each year nationwide and reveal a low incidence in the periods from November to May. However, the number of morbidities in June is typically double that occurring in May. The number of morbidities occurring in July and August each year are generally pooled. The number of morbidities in August accounts for 41.14% of the total annual morbidities. The relatively high morbidity level is maintained in September, declines in October, and then decreases significantly in November. The number of morbidities between June and October accounts for 97.42% of the total annual morbidity. A summary of 10 years’ data reveals that the monthly distribution trend of morbidity is consistent (Figure 2 and Table 2). Thus, the incidence of JE exhibits a clear seasonal distribution. In addition, the monthly distribution data shows that both the incidence and mortality rates increase significantly from June, peak in July and August, and then decline from October 332
Table 2. The incidence of JE cases by month in 1996 - 2005 1996
1997
1998
1999
Year 2000 2001
2002
2003
2004
2005
JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DEC
20 9 21 36 119 639 2,915 4,109 1,818 439 127 56
17 6 37 43 154 584 2,654 4,419 1,540 427 106 73
43 20 40 69 214 797 4,245 5,079 1,469 373 101 40
24 10 18 43 130 826 2,438 3,413 1,278 267 76 33
16 7 15 35 131 520 3,029 5,817 1,723 355 88 43
10 12 21 39 121 498 3,193 4,210 1,241 331 90 29
25 20 45 49 213 874 4,485 2,510 404 92 42 10
18 10 30 33 268 956 2,888 3,157 407 62 25 6
7 11 27 34 118 430 1,745 2,575 393 60 16 6
2 2 12 6 67 696 2,225 1,796 238 45 6 2
182 (0.20) 107 (0.12) 266 (0.30) 387 (0.43) 1,535 (1.70) 6,820 (7.57) 29,817 (33.08) 37,085 (41.14) 10,511 (11.66) 2,451 (2.72) 677 (0.75) 298 (0.33)
Total
10,308
10,060
12,490
8,556
11,779
9,795
8,769
7,860
5,422
5,097
90,136 (100.00)
Month
Total (%)
cases nationwide (10,11,17-19). The five provinces with an incidence >1/100,000 are located in southwest and middle China and lie adjacent to each other. The seven provinces with incidence between 0.5/ 100,000 and 1/100,000 are located on the eastern periphery of the abovementioned five provinces and are closely adjacent to them. The combined JE morbidities of these 12 provinces account for 80% of the total cases nationwide, whereas their combined population constitutes only 40% of the national population. In addition, the total number of JE morbidities in mainland China has decreased significantly in recent years compared to those reported in historical records, and the 5,097 cases reported in 2005 was the lowest annual figure to date. However, despite this downward trend, the ratio of the number of morbidities in the 12 high- and moderate-incidence provinces as a percentage of the national total cases is in-
Fig. 2. Seasonal distribution of JE case in China.
following seven provinces: Shanxi Province, Henan Province, Anhui Province, Hubei Province, Hunan Province, Jiangxi Province, and Guangxi Province. Between 2000 and 2002, the number of morbidities accounted for 20% of the
Fig. 3. Geographic distribution of JE in China.
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for 83.7% of total deaths, and among these, deaths in patients under 6 years of age account for 65.22% of JE fatalities. The number of males among JE patients has exceeded that of females, with a ratio of 1.3:1 in recent years (11).
creasing---from 82.7% in 1998 to 91.4% in 2002, and to 88.85% in 2005---demonstrating that the onset of JE is becoming more prevalent in these areas. 4-3. Slightly endemic areas The average incidence in the slightly endemic areas is considered to be between 0.1/100,000 and 0.5/100,000. The areas include the following seven provinces: Shanxi Province, Gansu Province, Jiangsu Province, Shandong Province, Fujian Province, Guangdong Province, and Zhejiang Province. In addition, there are a further 10 provinces and areas with an incidence
