## Hepatitis D Virus (HDV) — Defective Virus **Key Point:** Hepatitis D virus is a defective RNA virus that requires hepatitis B virus (HBsAg) for its replication, assembly, and transmission. It cannot replicate independently. ### Structural Features - **Genome:** Single-stranded, circular RNA (~1.7 kb) - **Envelope:** Derived from HBsAg (hepatitis B surface antigen) - **Core protein:** Delta antigen (HDAg) - **Replication requirement:** Absolutely dependent on HBV for packaging and secretion ### Clinical Significance - **Co-infection:** Simultaneous infection with HBV and HDV (acute hepatitis) - **Superinfection:** HDV infection in chronic HBV carriers (severe acute hepatitis) - **Transmission:** Parenteral route; same as HBV - **Prevention:** HBV vaccination prevents HDV infection (no separate HDV vaccine needed) **High-Yield:** HDV is the only defective human virus in clinical practice. It is often called a "satellite" virus because it depends entirely on HBV for its existence. **Clinical Pearl:** Superinfection with HDV in chronic HBV carriers leads to more severe liver disease and higher risk of fulminant hepatic failure compared to HBV alone.
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