## Hepatitis D Virus (HDV) — Structural and Replication Features ### Virion Structure **Key Point:** Hepatitis D virus is a defective RNA virus that requires hepatitis B surface antigen (HBsAg) for virion assembly and transmission. ### Genomic and Replication Characteristics - **Genome:** Negative-sense, single-stranded circular RNA (~1.7 kb) — the smallest human RNA virus - **Envelope:** Lipid bilayer envelope derived from host hepatocyte membrane - **Replication site:** Cytoplasm (unlike HBV, which replicates in nucleus) - **Dependency:** Obligate requirement for HBsAg for: - Virion assembly - Budding from hepatocyte - Transmission to other cells ### Comparison with Other Hepatitis Viruses | Feature | HAV | HBV | HCV | HDV | HEV | | --- | --- | --- | --- | --- | --- | | **Genome** | (+) ssRNA | dsDNA | (+) ssRNA | (−) ssRNA | (+) ssRNA | | **Envelope** | No | Yes | Yes | Yes | No | | **Replication Site** | Cytoplasm | Nucleus | Cytoplasm | Cytoplasm | Cytoplasm | | **Requires HBsAg** | No | — | No | **Yes** | No | | **Defective Virus** | No | No | No | **Yes** | No | **High-Yield:** HDV is the **only defective human virus** — it cannot complete its life cycle without HBsAg from HBV. This is why HDV infection occurs only as coinfection (with acute HBV) or superinfection (in chronic HBV carriers). ### Clinical Significance - HDV coinfection with acute HBV → more severe acute hepatitis - HDV superinfection in chronic HBV → rapid progression to cirrhosis - Prevention: HBV vaccination prevents HDV (since HDV cannot occur without HBV) [cite:Robbins 10e Ch 18]
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