## Hepatitis D Virus (HDV) — Dependence on HBV **Key Point:** Hepatitis D virus is a defective RNA virus that requires the hepatitis B surface antigen (HBsAg) for its replication, assembly, and transmission. It cannot exist or replicate independently. **High-Yield:** HDV can only infect individuals who are HBsAg-positive. It occurs either as: - **Co-infection:** simultaneous infection with HBV and HDV - **Superinfection:** HDV infection in a chronic HBsAg carrier **Clinical Pearl:** Superinfection with HDV in chronic HBV carriers leads to more severe liver disease, accelerated cirrhosis, and higher risk of fulminant hepatic failure compared to HBV alone. **Mnemonic:** **D = Dependent** — HDV depends entirely on HBV for its lifecycle. ### Why HDV is Obligate Intracellular Parasite 1. Lacks genes for envelope protein synthesis 2. Uses HBsAg as its outer envelope 3. Cannot complete replication without HBV co-infection 4. Transmission requires HBsAg-positive status ### Comparison with Other Hepatitis Viruses | Virus | Genome | Dependence | Transmission | Chronic Infection | | --- | --- | --- | --- | --- | | **HAV** | RNA | Independent | Fecal-oral | No | | **HBV** | DNA | Independent | Parenteral, sexual | Yes (5-10%) | | **HCV** | RNA | Independent | Parenteral | Yes (80%) | | **HDV** | RNA | **Requires HBsAg** | Parenteral (co/superinfection) | Yes (if HBV+) | | **HEV** | RNA | Independent | Fecal-oral | No (except immunocompromised) | [cite:Robbins 10e Ch 18]
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