Nature of the Infectious Agent

HDV is approximately 36 nm in diameter with an RNA genome associated with delta antigen (HDAg) surrounded by an outer lipoprotein coat of HBV, HBsAg. The genome is a closed circular RNA molecule of between 1670 and 1685 nucleotides with some genetic heterogeneity, and resembles those of the satellite viroids and virusoids of plants. Unlike the plant viroids, HDV codes for delta antigen. This is encoded by an ORF in the antigenomic RNA but four other ORFs that are also present in the genome do not appear to be used.

There appear to be three genotypes. Genotype I is the commonest and isolates have been obtained from all parts of the world. Genotype II represent isolates in Taiwan, and this genotype appears to be associated with less severe disease. Genotype III from South America is associated with a fulminant form of hepatitis.

HDV replicates only in hepatocytes and most probably uses the same cell receptors as HBV. Once inside the cells, HDV can replicate in the nuclei in the absence of HBV.

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