Chapter 26: Hepatitis B & Hepatitis D Viruses

HBV stands unique as the only DNA-based virus in its class, belonging to the Hepadnaviridae family, and it utilizes a sophisticated replication mechanism that involves an RNA intermediate and a viral reverse transcriptase enzyme. The structure of the infectious virion, known as the Dane particle, includes essential components such as the surface antigen (HBsAg), core antigen (HBcAg), and a regulatory X protein linked to oncogenesis. Transmission is primarily driven by exposure to infected blood, sexual activity, or vertical transmission from mother to infant, with the latter posing a massive risk for chronic infection. Interestingly, the destruction of liver cells—hepatocytes—is largely an indirect consequence of the virus, triggered by the host’s cell-mediated immune response where cytotoxic T lymphocytes attack viral proteins expressed on cell surfaces. The clinical spectrum ranges from asymptomatic clearance to acute icteric hepatitis characterized by jaundice and dark urine, or even life-threatening fulminant liver failure. Chronic persistence remains a major global concern, as it serves as a reservoir for transmission and often culminates in permanent liver scarring (cirrhosis) or primary liver cancer (hepatocellular carcinoma). The chapter further explores the unique nature of HDV, a defective RNA agent that requires the presence of HBV to provide its outer envelope. When these two viruses interact, either through simultaneous coinfection or a secondary superinfection of a chronic carrier, the clinical severity intensifies, leading to more rapid progression of liver disease. Modern medicine manages these infections through diagnostic serology—tracking the rise and fall of specific antigens and antibodies to determine the stage of infection—and pharmaceutical interventions like pegylated interferon-alpha and nucleoside analogs. Ultimately, the widespread use of recombinant vaccines and immunoglobulin therapies remains the cornerstone of preventing these infections and reducing the global burden of chronic liver disease.