**6. Conclusion**

*Ades aegypti* was introduced into the coastal cities of South East Asia from East Africa around nineteenth century via the shipping industry. With the eruption of World War II it

The 3' UTR region is thought to play a pivotal role in the DENV biology; it contains several conserved regions as well as 3' long Stable Hair Pin structure which is conserved among all the members of the family *Flaviviridae.* It has been proposed that this structure interacts with viral and host nucleic acid and protein factors to form a complex to regulate transcription and replication (Zhou, Y. et al.,2006. Therefore it appears to play a significant role in the efficiency of RNA- translation, and virus ability to cause infection, hence the role of 3-UTR in determining the severity of dengue disease seems plausible. The literature reviewed under this study did show considerable intra-serotype diversity at 3-UTR region with

A comparative analysis of 3' UTR conducted for DENV isolates from Bangkok, Thailand compared Thai sequences with 61 globally sampled isolates of DENV taken from patients with varying disease severity. Although some genetic variations were found both within and among the serotypes notably at 3' Long Hairpin Stable structure, however these mutations did not show consistent association with the clinical outcome of the DENV infection (Zhou, Y. et al.,2006). Study focusing on terminal 3' 5'UTR sequences of four DEN-2 from Thailand 1998 outbreak strains, showed complete homology for sequences at 5' UTR (highly conserved region) when compared with the prototype virus New Guinea C strain

Fig. 2. The Geographical distribution of mutations in DEN-2 and DEN-3 viruses detected at

*Ades aegypti* was introduced into the coastal cities of South East Asia from East Africa around nineteenth century via the shipping industry. With the eruption of World War II it

**5.3 Untranslated Region (UTR) mutations** 

greatest variability seen in DEN-4 followed by DEN-1.

different genomic loci of isolates from South East Asia

**6. Conclusion** 

deeply entrenched in many cities. The distribution of DHF outbreaks in SEA correlates with emergence of mosquito *A. egypti* in South East Asian countries due to uncontrolled urbanization leading to displacement of indigenous *A. albpictus* from the region.

Phylogenetic analysis suggests that there are foci of virus extinction and selection in South East Asian region, one such region is Thailand where the indigenous DEN-3 virus circulating up to 1992 has disappeared and replaced by two new lineages perhaps from a common ancestor. These studies point towards potential of regular extinctions of strains of dengue virus particularly DEN-3 virus and replacement by new variants in the region. Natural selection and / or genetic bottle neck are plausible causes for this variation. Since the extinction of pre 1992 strains and appearance of new epidemic strain in Thailand occurred during inter-epidemic period we therefore hypothesize that the genetic bottleneck is perhaps major cause of regional replacement. This is further supported by studies from India reporting shifting and dominance of the dengue virus serotype-3 (subtype III) replacing the earlier circulating serotype-2 (subtype IV) with emergence of increased incidence of DHF and DSS in subsequent outbreaks. Strains from the 2005 outbreak in Karachi (Pakistan) were found to be similar to those from Indian strains of dengue serotype 3, and were responsible for deadly outbreak in 2005-06.

Despite the growing genomic data base in the gene bank there are fundamental gaps in our understanding of epidemiological and evolutionary dynamics and its relation with disease severity. There are two possibilities that explain the association between clade replacement and increased viral virulence. The first is the possibility of these viruses to be better fit and therefore produce high viremia in infected humans, consequently with better transmission of virus by the vector. The other hypothesis to explain the possible virulence of emerging clades in the region is its improved ability to avoid neutralization by serotypes cross reactive antibodies (Kochel et al., 2005). Thus there is relative abundance of different serotypes and viral linage is continually changing in South East Asia. In face changing threshold of host immunity, periodic epidemics of DHF and DSS is due to local extinction and emergence of new clades. Over the period 1989 and 2000, a new genotype of DENV-1 and new clades of DENV-3 genotype III viruses have replaced older genotype and clades in this region and emergence of new clades coincided with severe epidemics.

Thus South East Asia displays greatest degree of genetic diversity, suggesting that it is the hub for the evolution of new epidemic strain. However, selection of specific clade and association of specific sequence variation with disease severity at various genomic levels reported in the literature reviewed in this study lacks strength of association i.e. reporting Relative Risk (RR)/ Odds Ratio (OR) limits our interpretation regarding causality or pin pointing specific clade with virus virulence, and therefore further studies are recommended.
