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Flaviviruses\ The arthropod-borne viruses (arboviruses) represent ecologic groupings of viruses with complex transmission cycles involving arthropods. The arboviruses are transmitted by bloodsucking arthropods from one vertebrate host to another. The vector acquires a lifelong infection through the ingestion of blood from a viremic vertebrate. The viruses multiply in the tissues of the arthropod without evidence of disease or damage. Some arboviruses are maintained in nature by transovarian transmission in arthropods. The major arbovirus diseases worldwide are yellow fever, dengue, Japanese B encephalitis, St. Louis encephalitis, western equine encephalitis, eastern equine encephalitis, Russian spring-summer encephalitis, West Nile fever, and sandfly fever. Classification & Properties of Flaviviruses

Arboviruses are in the Flavivirus genus in the Flaviviridae family. Initially, the flaviviruses were included in the togavirus family as "group B arboviruses" but were moved to a separate family because of differences in viral genome organization. The Flaviviridae family consists of about 70 viruses 4060 nm in diameter that have a single-stranded, positive-sense RNA genome. The viral envelope contains two glycoproteins. Some flaviviruses are transmitted between vertebrates by mosquitoes and ticks, whereas others are transmitted among rodents or bats without any known insect vectors. All flaviviruses are antigenically related. Flaviviruses are inactivated similarly to alphaviruses, and many also exhibit hemagglutinating ability.Replication of Flaviviruses The flavivirus RNA genome is positive-sense. A large precursor protein is produced from genome-length mRNAs during viral replication; it is cleaved by viral and host proteases to yield all the viral proteins, both structural and nonstructural. Flaviviruses replicate in the cytoplasm, and particle assembly occurs in intracellular vesicles. Proliferation of intracellular membranes is a characteristic of flavivirus-infected cells.

Antigenic Properties of Flaviviruses All flaviviruses share antigenic sites. At least eight antigenic complexes have been identified based on Nt tests. The envelope (E) protein is the viral hemagglutinin and contains the group-, serocomplex-, and type-specific determinants. Sequence comparisons of the E glycoprotein gene show that viruses within a serocomplex share over 70% amino acid sequences, whereas amino acid homology across serocomplexes is < 50%.

Reference:Jawetz, Melnick,& Adelbergs Medical Microbiology, 24th Edition1