Summary
Highlights
The video introduces the different types of hepatitis viruses (A through E) and outlines key aspects to be covered: route of transmission, acute vs. chronic infection, and viral structure. Hepatitis A causes acute infections via the fecal-oral route, is non-enveloped, and is a positive-sense single-stranded RNA virus. Hepatitis B can cause both acute and chronic infections, is transmitted sexually, through blood, or perinatally, has an envelope, and is a partially double-stranded DNA virus. Hepatitis C also causes both acute and chronic infections, has similar transmission routes to B, is enveloped, and is a positive-sense single-stranded RNA virus. Hepatitis D primarily causes chronic infections, requires HBV for infection (co-infection or superinfection), is enveloped, and is a negative-sense single-stranded RNA virus. Hepatitis E mainly causes acute infections via the fecal-oral route, is non-enveloped, and is a positive-sense single-stranded RNA virus. All hepatitis viruses are RNA, except for HBV (DNA).
The video details how hepatitis viruses infect hepatocytes. RNA viruses (HAV, HEV, HCV, HDV) enter, uncoat, and release their single-stranded RNA. This RNA uses the host ribosome to synthesize viral proteins, including capsomeres, antigens, and RNA-dependent RNA polymerase. This polymerase then creates more viral RNA, leading to the assembly and release of new viruses. HBV, a DNA virus, enters, uncoats, and releases its partially double-stranded DNA into the cell's nucleus. Host DNA repair enzymes complete the double-stranded DNA. This DNA is then transcribed by host RNA polymerases into viral RNA, which can either become mRNA for protein synthesis or pre-genomic RNA. HBV, being a retrovirus, uses its reverse transcriptase to convert pre-genomic RNA back into partially double-stranded DNA, which is then packaged into new viruses. HBV DNA can also integrate into the host genome, leading to continuous replication.
Viral replication within hepatocytes leads to cell lysis and death. Additionally, the immune system plays a crucial role in causing damage. Infected hepatocytes display viral proteins on their MHC-1 complexes. Cytotoxic T cells (CD8+) recognize these viral proteins, become activated, and release perforins and granzymes. Perforins create holes in the cell membrane, allowing granzymes to enter and induce apoptosis (programmed cell death). This immune-mediated destruction contributes to inflammation and damage of liver cells.
Damage to liver cells triggers the release of cytokines (IL-1, IL-6, TNF-alpha), which enter circulation and cause systemic symptoms. These cytokines affect the central nervous system, leading to the production of prostaglandins (PGE2, PGF2) that alter the hypothalamic thermostat, resulting in fever and malaise. Liver damage also causes a buildup of hepatotoxins in the blood, which activate the chemoreceptor trigger zone in the brainstem, subsequently activating the emetic center. This leads to nausea and vomiting. Prolonged vomiting can cause dehydration, electrolyte abnormalities, and weight loss. Altered GI motility can also lead to diarrhea. These early, flu-like symptoms are characteristic of the prodromal phase of hepatitis.
As liver damage progresses, bilirubin metabolism is impaired. Unconjugated bilirubin, normally processed by the liver, accumulates in the blood, as does conjugated bilirubin released from damaged cells. This buildup, particularly of conjugated bilirubin, leads to icterus (yellowing of the sclera) and jaundice (yellowish discoloration of the skin, especially palms and soles). Increased conjugated bilirubin excretion by the kidneys results in dark urine. Simultaneously, decreased bile production due to liver malfunction leads to less bilirubin in the intestines, causing a reduction in urobilinogen and stercobilin. This manifests as clay-colored or acholic stools. Liver inflammation also causes hepatomegaly and right upper quadrant abdominal pain. Increased levels of liver enzymes (AST, ALT, alkaline phosphatase, GGT) and prolonged clotting times (increased PTT, PT/INR) are also observed due to liver cell damage and impaired clotting factor synthesis. This stage is called the icteric phase.
In specific hepatitis viruses (HBV and HCV), immune complexes formed from viral antigens and antibodies can deposit in various tissues, causing extra-hepatic manifestations. These include inflammatory arthritis (synovial joints), vasculitis (e.g., polyarteritis nodosa), myocarditis and pericarditis (heart), and glomerulonephritis (kidneys). These immune responses can also lead to thrombocytopenia, hemolytic anemia, and neutropenia, while viral infections typically cause lymphocytosis. The convalescent phase follows, where symptoms gradually resolve as the body clears the infection. However, chronic hepatitis (especially with HBV and HCV) can lead to ongoing cell death, increasing fibrosis and regeneration attempts. This can lead to cirrhosis and an elevated risk of hepatocellular carcinoma due to increased cellular replication and potential for dysplasia.