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for vertebrate host include vector and host behaviour and density, chemical signalsemitted by the host, and environmental variables (Edman and Speilman, 1988). Ifthe vertebrate host behavior is altered, virus infection may influence the rate oftransmission, since infected animals may be lethargic and exhibit reduced defenseagainst mosquito and tick bite. Interestingly, the reduced defense against mosquitoattack by immature nestling birds appear to be offset by their intrinsically lowerattractiveness to blood -seeking mosquitoes (Scott et aI., 1990).2.3 DENGUE VIRUSESDengue Virus (DEN) is a single-stranded positive-polarity enveloped RNA(of approximately l Ikb in length), a Flavivirus that causes DF, DHF/DSS (Diamondet al., 2000). They comprise four distinct serotypes (DEN-l to DEN-4), which noweo-circulate in many localities. The four serotypes constitute distinct anti geniccomplex. DENl and 3 forms a sub complex defined by monoclonal and polyclonalantibodies. Monoclonal antibodies have also shown unexpected relationships at thesub complex level [e.g. between DENl and 3, 2 and 4, and between DEN andviruses of the JE, and TBE complexes (Monath and Heinz 1996). Within the DENs,amino acid sequence positional homology of 63% to 68% was found, compared to44% to 51% between DEN, YF, and WNV. Genetic variants of DEN-2 showedmore than 90% similarity (Monath and Heinz, 1996). DEN genome is ofapproximately 10,700 bases in length, surrounded by a icosahedral nucleocapsid andcovered by a lipid envelope containing the envelope and the membrane proteins(Leitmeyer et al., 1999). The genome contains a single open reading frame, whichencodes a precursor polyprotein and is flanked by two nontranslated regions (5' and3'NTR). COe and posttranslational proteolytic cleavage of the precursor results in theformation of three structural proteins, capsid (C), membrane (M), and envelope (E),and seven nonstructural proteins arranged in the order 5' C-prm-E-NSI- NS2a­NS2b-NS3- NS4a- NS4b- andNS5 3' (Chambers et al., 1990; Rice, 1996). The viralgenome is the only mRNA found in the infected cells and is translated as a singlepolyprotein (350kDa) which is cleaved eo- and post - translationally by both hostcell and virus-encoded protease to generate individual proteins (Chambers et al.,1990), These authors observed that the glycosylated prM protein (22 to 24 kDa) isreleased by host cell signalase cleavage of the polyprotein. They further reported20