**2. Historical background of dengue virus in South East Asian**

Geographically South East Asia comprises of land south of China to east of India extending as far as to the north of Australia. Although geographically the region is well defined, the list of countries included in this region varies due to political reasons. For the purpose of this review W.H.O based definition has been used. In addition status of dengue virus in further south of the region; including countries like Pakistan and Bangladesh have also been included to encompass the broader spectrum of the region.

### **2.1 Dengue vector evolution**

Evidences suggest that vectors *Ades aegypti* and *Ades albopictus* originated from darker sylvan forms found in African tropical forests. It is believed to have reached New World from West Africa via slave ships during the 17th century (Gubler, D.J.1998). *Adesaegypti* 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 became deeply entrenched in many cities (Gubler, D.J.1998). The mitochondrial genetic diversity studies have revealed circulation of two distinct clusters of *Ades aegypti* in South East Asia one with strains from French Polynesia, Guinea and Brazil while the other cluster is of strains that migrated from Europa Island in Mozambique and Amazonia (Mousson et al 2005). In contrast; *A. albopictus* is known to be native to South East Asia. It has spread within past few decades to various countries primarily due to introduction of trade of used tyres worldwide. Using ecological niche modeling Benedict and co-workers have predicted the risk of global invasion by *Ades albopitus* secondary to cargo trade and increasing air travel. Although temperate and humid climates are prerequisites for the optimum survival of both the vectors but *A. albopictus* is known to better acclimatize to the cold and dry weather due to its ability of efficient egg diapause during the extreme conditions, thus favoring its survival in the regions with exotic temperature ranges (Benedict, M.Q, *et al* 2007)*.* 

#### **2.2 Factors leading to disease spread in SEA**

The factors responsible for the insurmountable expansions of dengue in the region are complex and thought to be intricately linked with vector-host-virus triad, socioeconomic stresses and climatic variations. There are excellent reviews that discuss the impact of these factors in details (Aiken, S.R. 1978, Kendall, C. et al 1991, Halstead, S.B. 1966). Only salient factors in context of SEA will be discussed here. The distribution of DHF outbreaks in SEA correlates with emergence of mosquito A. aegypti in South East Asian countries perhaps due to displacement of indigenous A. albpictus in the region. This is considered to be associated with uncontrolled urbanization leading to shanty towns with inadequate pipe water supply and poor sanitation.

*A. albopictus* is semi domestic species that breeds on natural and man-made breeding sights; it feeds on variety of animals, birds and man. The A. aegypti on the other hand is more

of virus E- protein that confers the viral infectivity and host immune response of the virus (E.Descloux,2009) remains the focus of such studies. Sequence variation at different loci such as CprM, E/NS1, preM/E, C/prM/M and untranslated regions etc. have been investigated for its association with disease severity. This part of chapter will throw some light on our

Geographically South East Asia comprises of land south of China to east of India extending as far as to the north of Australia. Although geographically the region is well defined, the list of countries included in this region varies due to political reasons. For the purpose of this review W.H.O based definition has been used. In addition status of dengue virus in further south of the region; including countries like Pakistan and Bangladesh have also been

Evidences suggest that vectors *Ades aegypti* and *Ades albopictus* originated from darker sylvan forms found in African tropical forests. It is believed to have reached New World from West Africa via slave ships during the 17th century (Gubler, D.J.1998). *Adesaegypti* 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 became deeply entrenched in many cities (Gubler, D.J.1998). The mitochondrial genetic diversity studies have revealed circulation of two distinct clusters of *Ades aegypti* in South East Asia one with strains from French Polynesia, Guinea and Brazil while the other cluster is of strains that migrated from Europa Island in Mozambique and Amazonia (Mousson et al 2005). In contrast; *A. albopictus* is known to be native to South East Asia. It has spread within past few decades to various countries primarily due to introduction of trade of used tyres worldwide. Using ecological niche modeling Benedict and co-workers have predicted the risk of global invasion by *Ades albopitus* secondary to cargo trade and increasing air travel. Although temperate and humid climates are prerequisites for the optimum survival of both the vectors but *A. albopictus* is known to better acclimatize to the cold and dry weather due to its ability of efficient egg diapause during the extreme conditions, thus favoring its survival in

The factors responsible for the insurmountable expansions of dengue in the region are complex and thought to be intricately linked with vector-host-virus triad, socioeconomic stresses and climatic variations. There are excellent reviews that discuss the impact of these factors in details (Aiken, S.R. 1978, Kendall, C. et al 1991, Halstead, S.B. 1966). Only salient factors in context of SEA will be discussed here. The distribution of DHF outbreaks in SEA correlates with emergence of mosquito A. aegypti in South East Asian countries perhaps due to displacement of indigenous A. albpictus in the region. This is considered to be associated with uncontrolled urbanization leading to shanty towns with inadequate pipe

*A. albopictus* is semi domestic species that breeds on natural and man-made breeding sights; it feeds on variety of animals, birds and man. The A. aegypti on the other hand is more

current understanding of disease severity and it relation with genetic variation.

**2. Historical background of dengue virus in South East Asian** 

the regions with exotic temperature ranges (Benedict, M.Q, *et al* 2007)*.* 

**2.2 Factors leading to disease spread in SEA** 

water supply and poor sanitation.

included to encompass the broader spectrum of the region.

**2.1 Dengue vector evolution** 

acclimatized to urban set-up, once established the density of this mosquito is directly proportional to density of human population and artificial breeding sites (Merril S.A *et al* 2005), it feeds almost exclusively on humans. Moreover A. aegypti is considered to be more competent vector for dengue virus. Genetic traits that determines successful midgut infection by DEN virus have been mapped on several loci on A. aegypti chromosomes (Benedict, M.Q, *et al* 2007) indicating that vector competence is genetically determined.

The extent to which these mosquitoes compete with each other in the environment is not clear, nonetheless the balance of two species in the region is important, and the socioeconomic factors in SEA appear to be displacing *A.albopictus* in favour of *A.aegypti* leaving the population more susceptible. The poor socioeconomic conditions are major contributing factor to sustained vector activity with severe form of disease in the South East Asia. The breeding habitats of A.aegypti have been strongly associated with squatter settlements, inadequate piped water supply and sewage facilities (Halstead, S.B. 1966). In addition, there are impacts of higher environmental temperature in the region. High temperature is inversely related to the mosquito gonotropic cycle and viral extrinsic incubation period; this increases the egg laying episodes resulting in more blood meals and increased risk for viral transmission. In addition shorten extrinsic viral incubation period culminate to increase virus load at time of inoculation (Focks D.A. et al 1993). These effects have been proven for dengue vectors in simulation studies conducted by (Cox J et al 2001) and it has been projected that increase in global temperature would increase the length of transmission season in temperate regions.

#### **2.3 Dengue fever and dengue hemorrhagic fever**

The word dengue is believed to have originated from Swahili language "*ki denga pepo"*, which describes sudden cramp like seizure. The clinical symptoms suggestive of dengue virus infection can be traced back to Chinese Chin Dynasty (265-420 AD) where disease was considered as water poison and was known to be associated with water and insects (anonymous 2006).

Emergence of the disease in the new world can be traced back to the transmigration of the vector in the 17th century. There are reports that suggest possible epidemics of dengue like illness in three major continents (Asia, Africa and North America) as early as 1779 and 1780, within Asia Batavia (now known as Jakarta) was affected by this outbreak (Halstead,S.B. 1966). By early nineteenth century Dengue fever was known to be endemic in the rural areas of South East Asia probably due to the indigenous vector *A.albopictus*. It manifested as self limiting disease to which native population developed immunity at early age. With the advent of *A. aegypti* at Asian ports, the disease spread to the main inland cities and towns. It is assumed that unlike rural population, the urban populations of South East Asia remained susceptible to dengue virus and were then infected by newly imported vector. Dengue epidemics progressively became less frequent as urban population became immune to the disease, until 1953 when a new form of dengue fever was reported from Thailand and Manila, where children suffered from fever followed by bleeding diathesis; the disease was then called as *Philippine Fever* (Aiken, S.R. 1978). By 1960's the hemorrhagic form of disease had spread to Malaysia, Vietnam, Sri Lanka, Singapore and Indonesia (Halstead, S.B. 1966). The disease epidemiology extended and outbreaks of dengue hemorrhagic fever (DHF) were reported from India 1988) French Polynesia (1990), Pakistan (1992) and Bangladesh (2000).Until recently, DHF was considered to be disease of childhood, especially in South

Genetic Diversity of Dengue Virus and

and epidemic strains in different parts of the region.

Associated Clinical Severity During Periodic Epidemics in South East Asia 95

et al 2009). There after genetic mutation in the envelope protein and receptor binding domains resulted in its emergence as infectious pathogen in human population. The divergent forms of these sylvatic strains are often found to be circulating in human habitat, suggesting that enzootic cycles with some spill over in the surrounding human population. This has been shown in Malaysian populations settled near forest and marshy habitats (Wang, E. et al., 2000). The phylogenetic studies conducted based on envelope gene sequences of basal portion of sylvatic linage, DENV 1,-2,-4 of Malaysian descent suggest that endemic /epidemic strains of these viruses diverged from sylvatic ancestors more than 1000 years ago (Wang, E. et al., 2000). Thereafter, only micro evolutionary change within dengue serotypes have taken place, these changes have nevertheless resulted in substantial genetic diversity with emergence of endemic

Fig. 1. The effects of climatic and social change on vector evolution and disease severity

East Asia where mean age of cases under fifteen, and the modal age of five or slightly higher was reported from countries such as Thailand, Philippines and Malaysia, however, recent reports are now documenting increasing number of DHF and DSS in adult population as well (Khan E et al 2007). The precise cause of DHF/DSS remains elusive despite enormous research in this area. Evidences suggest interplay of multiple factors such as host genetic make-up with unique immune response and viral virulence may play a role in determining the severity of the disease.
