Ecology and geographical expansion of Japanese encephalitis virus

Japanese encephalitis virus (JEV) (Flavivirus: Flaviviridae) is a leading cause of encephalitis in eastern and southern Asia. The virus is maintained in a zoonotic cycle between ardeid wading birds and/or pigs and Culex mosquitoes. The primary mosquito vector of JEV is Culex tritaeniorhynchus, although species such as Cx. gelidus, Cx. fuscocephala, and Cx. annulirostris are important secondary or regional vectors. Control of JEV is achieved through human and/or swine vaccination, changes in animal husbandry, mosquito control, or a combination of these strategies. This review outlines the ecology of JEV and examines the recent expansion of its geographical range, before assessing its ability to emerge in new regions, using the hypothetical establishment in the United States as a case study.

Past, present, and future of Japanese encephalitis

Japanese encephalitis (JE), a vector-borne viral disease, is endemic to large parts of Asia and the Pacific. An estimated 3 billion people are at risk, and JE has recently spread to new territories. Vaccination programs, increased living standards, and mechanization of agriculture are key factors in the decline in the incidence of this disease in Japan and South Korea. However, transmission of JE is likely to increase in Bangladesh, Cambodia, Indonesia, Laos, Myanmar, North Korea, and Pakistan because of population growth, intensified rice farming, pig rearing, and the lack of vaccination programs and surveillance. On a global scale, however, the incidence of JE may decline as a result of large-scale vaccination programs implemented in China and India.

[Prospects for treating mosquito-borne flavivirus encephalitides]

The Japanese encephalitis virus and West Nile virus belong to the family Flaviviridae, genus Flavivirus; they have a similar transmission cycle, with birds serving as the natural vertebrate host and mosquitoes, primarily, Culex species, both serving as the enzootic vectors and infecting humans. These arboviruses have caused more human cases of severe neuroinvasive disease worldwide during the past decade than other mosquito-borne flaviviruses. The current data and concepts on the clinical manifestations, pathogenesis, diagnosis, and treatment of infections caused by West Nile virus and Japanese encephalitis virus, are considered. The need for developing innovative approaches to etiotropic therapy for West Nile fever and Japanese encephalitis is emphasized; these approaches should be based on the comprehensive insight and extensive studies of the pathogenesis of flaviviral infections.

Influence of the distribution of host species on adult abundance of Japanese encephalitis vectors Culex vishnui subgroup and Culex gelidus in a rice-cultivating village in northern Vietnam

A field study was conducted in a village in northern Vietnam to investigate how host distribution influences Japanese encephalitis (JE) vector abundance. Indoor and outdoor collections were conducted from 50 compounds. We collected three JE vector species--Culex tritaeniorhynchus and Culex vishnui that comprised the Culex vishnui group, and Culex gelidus. Spatial autocorrelation was not observed in the mosquito assemblies at any scale larger than the house compounds. Multivariate analyses revealed that the Cx. gelidus density correlated positively with both the host proximity to the breeding sites and cattle density; however, the Cx. vishnui subgroup density correlated positively only with cattle density. These results showed that the number of cattle in a compound influenced the JE vector abundance in that compound, and the abundance of Cx. gelidus, not of the Cx. vishnui subgroup, was affected by the host proximity to the breeding sites in the village.

Molecular epidemiological analyses of Japanese encephalitis virus isolates from swine in Japan from 2002 to 2004

To characterize Japanese encephalitis virus (JEV) strains recently prevalent in Japan, JEV surveillance was performed in pigs from 2002 to 2004. Eleven new JEV isolates were obtained and compared with previous isolates from Japan and other Asian countries. All of the isolates were classified into genotype 1 by nucleotide sequence analysis of the E gene. Two new isolates with different levels of neurovirulence and neuroinvasiveness, but with only one nucleotide difference in the E gene, Sw/Mie/34/2004 and Sw/Mie/40/2004, were isolated at the same farm on the same day. Sw/Mie/40/2004 displayed higher neurovirulence and neuroinvasiveness in mice than the other four new isolates. Another new isolate, Sw/Hiroshima/25/2002, was neutralized by antiserum to Beijing-1 at a level similar to the homologous Beijing-1 strain, whilst seven other new isolates were neutralized at 10-fold-lower titres. However, there were no amino acid differences in the E protein among these eight isolates. The present study indicated that the 11 new JEV isolates were genetically similar, but biologically and serologically heterogeneous.

Weather variables and Japanese encephalitis in the metropolitan area of Jinan city, China

OBJECTIVE

To identify weather-related risk factors and their roles in Japanese encephalitis transmission and to provide policy implications for local health authorities and communities.

METHODS

Data on notified cases of Japanese encephalitis and weather variables over the period 1959-1979 were collected from Jinan city, a temperate city in China. Due to seasonality of the disease, the data analysis was restricted to five months from June to October each year. Spearman correlation analysis and time-series adjusted Poisson regression analysis were performed to quantify the relationship between weather and the number of cases. The Hockey Stick model was used to detect potential threshold temperatures.

RESULTS

Monthly mean maximum and minimum temperatures, monthly total rainfall and monthly mean relative humidity were positively correlated to monthly notification of Japanese encephalitis, while monthly mean air pressure was inversely correlated. Lag times varied from one to two months. All these weather variables were significant in the adjusted Poisson regression model. Thresholds of 25.2 degrees C for maximum temperature and 21.0 degrees C for minimum temperature were also detected.

CONCLUSIONS

Weather variables could have affected the transmission of Japanese encephalitis in this urban area of China. Public health interventions should be developed at this stage to reduce future risks related to climate change.

Ecological study on mosquito vectors of Japanese encephalitis virus in Bellary district, Karnataka

BACKGROUND & OBJECTIVE

Japanese encephalitis virus (JEV) has been a serious public health problem in Bellary district since 1981. Though JE is a vector borne disease, no published record was available regarding prevalence of mosquito species in this region. A two years ecological study was therefore conducted to determine species composition, seasonal prevalence and relative abundance of culicid taxa, particularly JE vectors endemic to this area.

METHODS

Three villages with recent JE out breaks, were selected for regular mosquito collections. During daytime, adult mosquitoes resting inside human and domestic animal habitations were collected using aspirators and flashlights at weekly intervals. Similarly, adult mosquitoes were collected from cattle sheds and pig enclosures at dusk hours.

RESULTS

A total of 120113 mosquitoes collected at dusk belonged to 5 genera and 24 species, including 13 species those have already yielded JE virus in India with Cx. tritaeniorhynchus contributing to 70.3 per cent. General mosquito population showed bimodal pattern of peak occurrence during February and October in both the peaks, Cx. tritaeniorhynchus contributed maximum JE. Substantial densities of An. subpictus and An. peditaeniatus during JE season suggest the supportive role of these species. Other vector species viz., Cx. quinquefasciatus, Cx. pseudovishnui, Cx. vishnui, Cx. gelidus, Cx. fuscocephala, Cx. bitaeniorhynchus, Ma. annulifera, Ma. uniformis and An. barbirostris perhaps have a limited role in the transmission of JE virus due to their extremely poor density.

INTERPRETATION & CONCLUSION

Cx. tritaenorhynchus appears to have a major role in the transmission of JE virus in Bellary district. Risk of transmission of JEV to humans can be reduced by house spraying with residual insecticides and intermittent paddy irrigation.

Short report: the first isolation of Japanese encephalitis virus from mosquitoes collected from mainland Australia

In response to an incursion of Japanese encephalitis virus (JEV) on Cape York Peninsula, Australia, in 2005, 23,144 Culex mosquitoes were processed for virus detection. A single isolate of JEV was obtained from a pool of Culex sitiens subgroup mosquitoes. This is the first reported mosquito isolate of JEV from the Australian mainland.

Host range, amplification and arboviral disease emergence

Etiologic agents of arboviral diseases are primarily zoonotic pathogens that are maintained in nature in cycles involving arthropod transmission among a variety of susceptible reservoir hosts. In the simplest form of human exposure, spillover occurs from the enzootic cycle when humans enter zoonotic foci and/or enzootic amplification increases circulation near humans. Examples include Eastern (EEEV) and Western equine encephalitis viruses (WEEV), as well as West Nile (WNV), St. Louis encephalitis (SLEV) and Yellow fever viruses. Spillover can involve direct transmission to humans by primary enzootic vectors (e.g. WNV, SLEV and WEEV) and/or bridge vectors with more catholic feeding preferences that include humans (e.g. EEEV). Some viruses, such as Rift Valley fever, Japanese encephalitis and Venezuelan equine encephalitis viruses (VEEV) undergo secondary amplification involving replication in livestock animals, resulting in greater levels of spillover to humans in rural settings. In the case of VEEV, secondary amplification involves equines and requires adaptive mutations in enzootic strains that allow for efficient viremia production. Two of the most important human arboviral pathogens, Yellow fever and dengue viruses (DENV), have gone one step further and adopted humans as their amplification hosts, allowing for urban disease. The ancestral forms of DENV, sylvatic viruses transmitted among nonhuman primate reservoir hosts by arboreal mosquitoes, adapted to efficiently infect the urban mosquito vectors Aedes aegypti and Ae. albopictus during the past few thousand years as civilizations arose. Comparative studies of the sylvatic and urban forms of DENV may elucidate the evolution of arboviral virulence and the prospects for DENV eradication should effective vaccines be implemented.

Does 1-octen-3-ol enhance trap collections of Japanese encephalitis virus mosquito vectors in northern Australia?

The responses of Japanese encephalitis virus (JEV) mosquito vectors to 1-octen-3-ol (octenol) and CO2 were evaluated using Centers for Disease Control (CDC) light traps at 3 sites in northern Australia. There was no significant difference between the number of Culex sitiens subgroup mosquitoes or Cx. gelidus collected in CDC light traps baited with either CO2 alone or CO2 + octenol on Badu Island. At both mainland locations, using octenol in combination with CO2 significantly increased collections of Cx. sitiens subgroup mosquitoes. Collections of nontarget species, such as Ochlerotatus spp., Anopheles spp., and Verrallina spp. were also significantly increased with the addition of octenol. At all 3 locations, reducing collections of nontarget mosquitoes by not using octenol increased the proportion of Culex spp. collected, thus potentially reducing the time and resources required to sort and process collections for JEV detection. Our results also indicate that trials into the efficacy of using octenol as an attractant should be carried out in each area prior to the implementation of a mosquito-based arbovirus surveillance system.

Laboratory transmission of Japanese encephalitis and West Nile viruses by molestus form of Culex pipiens (Diptera: Culicidae) collected in Uzbekistan in 2004

We evaluated the molestus form of Culex pipiens pipiens (L.) (hereafter referred to as "molestus") captured near Tashkent, Uzbekistan, for their ability to transmit Japanese encephalitis (family Flaviviridae, genus Flavivirus, JEV) and West Nile (family Flaviviridae, genus Flavivirus, WNV) viruses under laboratory conditions. These molestus were highly competent laboratory vectors of WNV, with infection and dissemination rates of 96 and 81%, respectively. Approximately 75% of female molestus that fed after development of a disseminated infection transmitted virus by bite. Therefore, approximately 60% of those molestus taking a second bloodmeal between 16 and 25 d after an infectious bloodmeal would be expected to transmit WNV by bite. In contrast, these molestus were less efficient vectors of JEV, with infection and dissemination rates of 51 and 25%, respectively. In addition, only 33% of individuals with a disseminated infection transmitted JEV by bite, indicating a significant salivary gland barrier. Therefore, only approximately 8% of orally exposed individuals would be expected to transmit JEV by bite if they took a second bloodmeal 16-25 d later. These data indicate that the molestus form of Cx. p. pipiens should be considered a potentially important vector of WNV in Uzbekistan and may become involved in the transmission of JEV, should this virus be introduced into Uzbekistan.

Natural survivorship of immature stages of Culex vishnui (Diptera: Culicidae) complex, vectors of Japanese encephalitis virus, in rice fields in southern India

The development and survival of immatures of Culex vishnui (Diptera: Culicidae) complex, vectors of Japanese encephalitis virus, were studied in transplanted rice, Oryza savita L., fields during three crop growing seasons. The total duration of development from the first instar to adult emergence varied from 6 to 8 d. Survival rate estimates ranged from 0.003 to 0.524, but they generally were <0.1. Unusually high survival rates (0.192-0.524) were observed in summer and long-term monsoon crop seasons during 1993. A multiple regression method using backward elimination was used to analyze the factors responsible for these variations. The model identified nine parameters related with survival rates that explained 99% of the variance. Area of water surface and hardness were negatively related, whereas green algae, notonectid adults, anisopterans, dytiscids, salinity, water depth, and dissolved oxygen were positively associated with immature survival.

Long-term study of Japanese encephalitis virus infection in Anopheles subpictus in Cuddalore district, Tamil Nadu, South India

OBJECTIVE

To investigate the role of Anopheles subpictus Grassi as a vector of Japanese encephalitis virus (JEV) transmission in Cuddalore, an area of Tamil Nadu endemic for the disease.

METHOD

We collected 98 pools (4,900 specimens) of wild adult male An. subpictus mosquitoes outdoors during dusk hours and screened them for JEV antigen by antigen-capture Enzyme Linked Immunosorbent Assay. Additionally, over a period of 1 year, we tested 166 pools (8,300 specimens) of wild adult female An. subpictus mosquitoes collected indoors for JEV.

RESULTS

Four pools of male An. subpictus tested positive. This indicates possible natural transovarial transmission of the virus through An. subpictus. Nineteen female pools were positive with a minimum infection rate of 2.3. From January through March the maximum infection rate was highest: 5.0 compared with 1.7 between April and September and 2.1 from October to December, although the difference was not statistically significant. From the 19 positive female pools, four isolates were confirmed as JEV by insect bioassay.

CONCLUSION

The role of An. subpictus as a secondary vector in JEV transmission in Cuddalore, Tamil Nadu lends support to the hypothesis of periodic epidemics in the region.

Differential tolerance of two morphological variants of Culex tritaeniorhynchus (Diptera: Culicidae), a Japanese encephalitis vector, to pyrethroid insecticides in Mysore, India

Five synthetic pyrethroids, deltamethrin, cypermethrin, permethrin, alpha-cypermethrin, and lambdacyhalothrin, were tested on the larvae of 2 morphological variants of Cx. tritaeniorhynchus collected from Mysore City (ground pools) and outside Mysore (paddy fields), in Karnataka State, India. The morphological characters, ie, length and width of siphon, siphonal index, comb scale numbers, pecten teeth numbers, length of anal gills and anal gill index of larvae of two Cx. tritaeniorhynchus populations were found to be significantly different (p<0.05). To elicit further detail of these two variants, pyrethroid bioassays were undertaken. In general, the toxicity ranking of these pyrethroids tested on the Cx. tritaeniorhynchus variants from Mysore City was deltamethrin > lambdacyhalothrin > cypermethrin > permethrin > alpha-cypermethrin. However, for the rural variety it was deltamethrin > lambdacyhalothrin > alpha-cypermethrin > permethrin > cypermethrin. Of the 5 pyrethroids tested, deltamethrin was the most effective on both variants. Alpha-cypermethrin and lambdacyhalothrin were respectively 2.17 and 2.09 times more effective on the city variety (p<0.05), based on LC50 values. The results suggest that, in addition to morphological differences, the 2 tested varieties of Cx. tritaeniorhynchus also differ in susceptibility status.

Illustrated keys to the mosquitoes of Thailand. IV. Anopheles

Illustrated keys for the identification of the larvae and adult female Anopheles mosquitoes of Thailand are presented along with distribution maps, tabulated bionomics information, and a checklist. A total of 73 species are treated, including 71 previously and newly described species (An. cracens = dirus B, An. scanloni = dirus C, An. baimaii = dirus D, An. latens = leucosphyrus A, and An. epiroticus = sundaicus A). Also, two undescribed species are included, i.e., An. minimus C and a new species near An. gigas. Thirty-four chromosomal forms of 14 species are discussed, with suggestions provided for resolving their taxonomic status.

[Clinical and epidemiological aspects of Japanese encephalitis]

Japanese encephalitis has been well controlled in Japan. On the other hand, the Japanese encephalitis virus (JEV) transmission cycle still exists, as does the risk of exposure to JEV infection. Although Culex tritaeniorhynchus has been considered the major mosquito vector in Japan, recent surveys suggested Culex pipiens complex and Aedes albopictus as potential alternatives. Similarly, swine have reduced the role in amplification of JEV in paradomestic environments. Another surveys suggested attenuation of recent JEV strains and a shift in the major clinical manifestation caused by JEV infection from encephalitis to meningitis. This paper describes current situation of Japanese encephalitis in Japan from clinical and epidemiological aspects.

Monitoring of Japanese encephalitis virus infection in mosquitoes (Diptera: Culicidae) at Guandu Nature Park, Taipei, 2002-2004

Japanese encephalitis virus (JEV) infection in mosquitoes was monitored at Guandu Nature Park in Taipei City from September 2002 to December 2004. In total, 30,386 female mosquitoes consisting of six genera and 14 species were processed for virus in 1,229 pools by using Flavivirus NS5 gene sequences detected by reverse transcription-polymerase chain reaction (PCR) and nested PCR assay. Overall, 101 pools were positive, including 95, 1, 4, and 1 for Culex tritaeniorhynchus Giles, Culex sitiens Wiedemann, Culex rubithoracis (Leicester), and Aedes vexans noctunmus (Theobald), respectively.

Effect of irrigated rice agriculture on Japanese encephalitis, including challenges and opportunities for integrated vector management

Japanese encephalitis (JE) is a disease caused by an arbovirus that is spread by marsh birds, amplified by pigs, and mainly transmitted by the bite of infected Culex tritaeniorhynchus mosquitoes. The estimated annual incidence and mortality rates are 30,000--50,000 and 10,000, respectively, and the estimated global burden of JE in 2002 was 709,000 disability-adjusted life years lost. Here, we discuss the contextual determinants of JE, and systematically examine studies assessing the relationship between irrigated rice agriculture and clinical parameters of JE. Estimates of the sizes of the rural population and population in irrigated areas are presented, and trends of the rural population, the rice-irrigated area, and the rice production are analyzed from 1963 to 2003. We find that approximately 1.9 billion people currently live in rural JE-prone areas of the world. Among them 220 million people live in proximity to rice-irrigation schemes. In 2003, the total rice harvested area of all JE-endemic countries (excluding the Russian Federation and Australia) was 1,345,000 km(2). This is an increase of 22% over the past 40 years. Meanwhile, the total rice production in these countries has risen from 226 millions of tonnes to 529 millions of tonnes (+134%). Finally, we evaluate the effect of different vector control interventions in rice fields, including environmental measures (i.e. alternate wet and dry irrigation (AWDI)), and biological control approaches (i.e. bacteria, nematodes, invertebrate predators, larvivorous fish, fungi and other natural products). We conclude that in JE-endemic rural settings, where vaccination rates are often low, an integrated vector management approach with AWDI and the use of larvivorous fish as its main components can reduce vector populations, and hence has the potential to reduce the transmission level and the burden of JE.