Characterization and Mosquito infection of the Tengah Isolate of Japanese encephalitis virus

Abstract

Japanese encephalitis (JE) is a brain infection caused by JE virus (JEV). It has an estimated worldwide incidence of 68,000 cases and 10,000 to 15,000 deaths annually. Despite large effective immunization campaigns, Japanese encephalitis remains a disease of global health concern, because the virus is spreading. There are five genotypes of JEV (genotype I – V), each associated with different geographical areas and associated epidemiology. The Muar strain of JEV, the fifth genotype, is believed to represent the oldest lineage from which genotype I - IV evolved. Muar was isolated in Singapore in 1952. At the same time as Muar was isolated; the Tengah strain of JEV was also isolated from a nearby location. However, Tengah and the characteristics of Tengah have largely remained unknown. Muar was considered the only known representative of genotype-V prior to 2009. Vector competence studies have examined genotype-I, II and III of JEV. However, genotype-IV and genotype-V have never been investigated in vector competence studies. Therefore, the infectivity of these viruses to mosquitoes is unknown. The competence of non-Asian mosquitoes to JEV has been demonstrated suggesting potential for emergence in some other regions. In Great Britain, JEV is considered a potential threat to animals and public health. However, the level of competence of British mosquitoes to any arbovirus is not known. The overall objective of this thesis was to characterise Tengah, investigate molecular and mosquito factors that might relate to the lack of circulation of genotype-V isolates and assess the potential of arbovirus (JEV) emergence in Great Britain. Molecular characterization of Tengah strain showed that it is another isolate of genotype-V, with 99% sequence similarity to Muar. Evolutionary analysis performed using the Bayesian Evolutionary analysis of Sampling Trees (BEAST) program estimated that JEV is evolving at a rate of 3.53 × 104 nucleotide substitution per site per year. Vector competence studies demonstrated Culex quinquefasciatus mosquitoes are able to transmit Muar with transmission rates of 23% at 21-days post infection. Comparison of transmission between Muar (genotype V) and Nakayama (genotype III) found no significant difference between the two genotypes. Ochlerotatus detritus, a British mosquito, was susceptible to JEV at both 23°C and 28°C as determined by the detection of virus in the saliva 7 days post infection. The overall transmission rate was 13% at 23°C and 25% at 28°C. There was no significant difference between the two temperatures. Infection rates for Ochlerotatus detritus and Culex quinquefaciatus were similar. This thesis has shown that on account of its similarity to the Muar isolate, Tengah represents a variant of Muar and is another genotype-V isolate. Evolutionary analysis showed that JEV originated from its ancestral virus in the year 1120 while the Time to Most Recent Common Ancestor for genotype V was in the year 1840. Muar has the ability to infect, replicate and be transmitted in Culex quinquefasciatus, suggesting that the limited distribution, isolation and circulation of genotype-V is probably not explained solely by mosquito factors. Ochlerotatus detritus is competent to transmit JEV and would therefore pose a threat should this virus occur in Great Britain