Hepatitis and Viral Diseases Summary
Hepatitis Overview
Liver inflammation primarily caused by viruses, but can also be due to bacteria, alcohol, autoimmune diseases, or certain medications.
Can lead to acute illness with varying severity, and chronic infection in some cases, progressing to liver cell destruction (cirrhosis), liver failure, and hepatocellular carcinoma.
Associated with diverse hepatitis viruses, specifically Hepatitis A (HAV), B (HBV), C (HCV), D (HDV), and E (HEV).
Common symptoms include fatigue, nausea, vomiting, abdominal pain, dark urine, pale stools, and jaundice (yellow discoloration of skin and mucosa due to bilirubin accumulation).
Hepatitis Transmission
Hepatitis A (HAV): Transmitted via the fecal-oral route, typically through ingestion of contaminated water or food. It is highly contagious and often causes self-limiting acute illness with no chronic state. A vaccine is available for prevention.
Hepatitis B (HBV):
Primarily transmitted through percutaneous or mucosal contact with infected blood or body fluids.
Common routes include injection drug use (sharing needles), sexual contact, perinatal transmission (from mother to child during birth), needlestick injuries in healthcare settings, and sharing contaminated personal items (e.g., razors, toothbrushes).
Can be self-limiting in adults (with about 90-95% resolving) but often leads to chronic infection in infants infected perinatally (up to 90%). A highly effective vaccine and treatment options are available.
Hepatitis C (HCV):
Primarily transmitted through large or repeated percutaneous exposures to infected blood.
Main routes include injection drug use, blood transfusions (prior to 1992 when screening became available), organ transplants from infected donors, needlestick injuries, and maternal transmission (less common than HBV).
Sexual transmission is possible but less efficient than for HBV. There is no vaccine available for HCV.
Hepatitis D (HDV): Requires co-infection with Hepatitis B virus (HBV) to replicate and cause disease. Transmission routes are similar to HBV (blood, sexual contact). HDV co-infection can lead to more severe liver disease and accelerated progression to cirrhosis.
Hepatitis E (HEV): Similar to HAV, primarily transmitted via the fecal-oral route through contaminated water or undercooked meat. While generally self-limiting, it can cause severe disease and high mortality rates in pregnant women.
Hepatitis A Clinical Progression
Key stages:
Viremia: The virus is present in the bloodstream, often before symptoms appear.
ALT increase: Liver enzymes, particularly Alanine Aminotransferase (ALT), rise significantly, indicating liver cell damage.
Symptoms: Onset of fatigue, nausea, vomiting, abdominal pain, and fever.
Jaundice: Yellowing of the skin and eyes, typically appearing after other symptoms and indicating cholestasis.
Serological markers: Detection of IgM anti-HAV indicates acute infection, while IgG anti-HAV indicates past infection and immunity.
The duration of clinical progression, from exposure to resolution, spans about 6-8 months, with acute symptoms usually lasting several weeks. HAV infection typically resolves completely without leading to chronic liver disease.
Hepatitis B: Features
Clinical outcome depends heavily on the age of infection: Acute infection in adults is often self-limiting (90-95% resolve), while perinatal infection has a high likelihood (90\%) of leading to chronic infection.
Viral replication involves a complex interplay of antigens and antibodies:
HBsAg (Hepatitis B surface antigen): Indicates active infection (acute or chronic).
Anti-HBs (Antibody to HBsAg): Indicates recovery and immunity or successful vaccination.
HBeAg (HBe antigen): Indicates active viral replication and high infectivity.
Anti-HBe (Antibody to HBeAg): Indicates lower viral replication and reduced infectivity (seroconversion).
Anti-HBc (Antibody to Hepatitis B core antigen): Indicates past or current infection. IgM anti-HBc suggests acute infection, while IgG anti-HBc suggests chronic or past infection.
Incubation period is 4-12 weeks. Progression to chronic disease is common when infected at a young age, leading to a risk of cirrhosis and hepatocellular carcinoma later in life.
Hepatitis C Characteristics
An enveloped RNA virus (Flaviviridae family) known for its high mutation rate, leading to diverse viral variants (quasispecies) that challenge the immune system and complicate vaccine development.
Transmitted primarily via blood products, intravenous drug use, needlestick injuries, and congenital routes (mother-to-child during birth, though less efficiently than HBV). Sexual transmission is infrequent but possible.
Causes immune-mediated inflammation and damage in the liver, leading to a chronic infection in 75\%-85\% of cases. This chronic inflammation can progress to liver fibrosis, cirrhosis, and hepatocellular carcinoma over decades.
Diagnosis involves initial antibody tests (anti-HCV) to screen for exposure, followed by quantitative PCR to detect HCV RNA and confirm active infection, as well as determine the viral load and genotype.
Treatment with Direct-Acting Antivirals (DAAs) is now highly effective, achieving sustained virologic response (SVR) in over 95\% of patients, often with short treatment durations. However, these treatments can be expensive, and access remains a challenge in many regions.
No vaccine is available due to the virus's genetic variability, making prevention through harm reduction strategies (e.g., safe injection practices, blood screening) crucial.
Rabies Overview
An enveloped RNA virus belonging to the Rhabdoviridae family, characterized by its bullet-like shape.
Has a wide range of animal reservoirs, including wild animals (e.g., bats, raccoons, skunks, foxes, coyotes) and domestic animals (e.g., dogs, cats) in unvaccinated populations.
Causes acute encephalitis in warm-blooded animals and humans.
Accounts for over 50,000 annual deaths worldwide, predominantly in developing regions of Asia and Africa, where dog bites are the primary source of human infection.
There is no effective antiviral treatment once clinical symptoms appear. Post-exposure prophylaxis (PEP) is critical and consists of wound care, administration of rabies immune globulin (RIG) for immediate passive immunity, and a series of rabies vaccines for active immunity.
Rabies Pathogenesis
The virus typically enters the body through a bite or scratch from an infected animal, or less commonly via aerosolization in bat caves.
1. Viral Entry & Local Replication: The virus initially replicates in muscle cells near the site of inoculation for days to weeks.
2. Neural Invasion: It then binds to acetylcholine receptors on peripheral nerve endings and enters the peripheral nervous system (PNS).
3. Retrograde Axonal Transport: The virus travels unidirectionally up the axons to the central nervous system (CNS) at a rate of 12-100 mm/day.
4. CNS Replication & Spread: Once in the CNS (spinal cord and brain), it replicates extensively, causing widespread neuronal dysfunction but little structural damage.
5. Centrifugal Spread: From the CNS, the virus spreads centrifugally (away from the center) via cranial nerves to highly innervated tissues, including salivary glands, skin, and cornea. Viral shedding in saliva begins at this stage, enabling further transmission.
Symptoms progress from non-specific prodromal signs (fever, headache, malaise, pain/paresthesia at wound site) to neurological manifestations, which can be furious (hydrophobia, aggression, hyperactivity, anxiety, seizures) or paralytic (muscle weakness, paralysis, coma). Death usually occurs due to respiratory failure.
Diagnosis in humans is primarily through clinical signs and a history of exposure. Laboratory confirmation is often done post-mortem by detecting viral antigens in brain tissue using immunofluorescence. Ante-mortem diagnosis can involve检测 viral RNA in saliva by RT-PCR, or antibodies in serum/CSF, and viral antigen in skin biopsies from the nape of the neck.
Hantavirus Overview
An enveloped RNA virus belonging to the Bunyaviridae family.
Transmitted to humans through inhalation of aerosols from dried rodent excreta (urine, feces, or saliva) of infected rodents, such as deer mice (for Hantavirus Pulmonary Syndrome in North America) and various other mice and rats (for Hemorrhagic Fever with Renal Syndrome globally).
Causes two main severe clinical syndromes:
Hantavirus Pulmonary Syndrome (HPS): Primarily in the Americas, characterized by fever, myalgia, and rapid onset of respiratory distress, leading to pulmonary edema and acute respiratory failure.
Hemorrhagic Fever with Renal Syndrome (HFRS): Primarily in Asia and Europe, characterized by fever, headache, myalgia, abdominal pain, and renal dysfunction (kidney failure, hemorrhage).
Symptoms are often non-specific initially, including fever, chills, myalgia, and headache, before progressing to the specific pulmonary or renal manifestations.
There is no specific antiviral treatment or vaccine currently licensed for HPS-causing hantaviruses; treatment is supportive.
Prevention focuses on rodent control, reducing rodent habitats (e.g., sealing entry points in homes), safe cleanup practices of rodent droppings/nests (e.g., wetting down areas before cleaning, using gloves and masks), and avoiding direct contact with rodents or their excreta.