Comprehensive Notes on Hepadnaviruses, Hepatitis D, and Parvoviruses

Hepadnaviruses

Hepatitis B Virus (HBV) Infection: Key Points

• HBV causes acute hepatitis and chronic infection, leading to chronic liver disease and hepatocellular carcinoma (HCC).
• HBV is a partially double-stranded DNA virus that uses a reverse transcriptase-like enzyme to replicate its DNA from an RNA intermediate.
• Transmission occurs through blood (e.g., IV drug use), sexual intercourse, and from mother to child during childbirth.
• Geographical variation in exposure is considerable, with the highest rates in the Far East, sub-Saharan Africa, Oceania, and South America.
• An effective vaccine is available, administered in three doses over 6 months; universal vaccination of young children will reduce chronic infection rates and long-term sequelae, including HCC.
• Treatment of chronic HBV infection includes interferon-α, pegylated interferon, or long-term suppressive therapy with nucleos(t)ide analogues.
• Pegylated interferon (peginterferon) is a chemically modified form of standard interferon with polyethylene glycol (PEG) attached; the PEG increases the duration the interferon stays in the body (blood).

Key Features of Hepatitis B Virus

• Genotypes: 8 genotypes (A–H) based on sequence variation.
• Carriers: 200x more likely to develop primary hepatocellular carcinoma (HCC) compared to non-carriers.
• Maternal-Neonatal Infections: 70-90% result in chronic infection.
• Transmission Routes: Sexual, parenteral, or perinatal.
• Survival: Survives outside the body for at least 7 days and can still cause infection.
• Susceptibility: Chimpanzees are the only non-human primates susceptible to HBV infection, but they are no longer used for research due to ethical concerns.
• Enveloped virion contains partial double-stranded circular DNA genome.
• Replication occurs through an RNA intermediate; the virus encodes and carries a reverse transcriptase.
• The virus encodes several antigenically and clinically predictive important proteins.
• Strict tissue tropism to the liver.
• HBV replicative intermediates and infection markers have been identified in extrahepatic lymphoid tissues, including the spleen, lymphoid nodes, and lymphoid cells.
• Virus-infected cells produce and release large amounts of HBsAg particles lacking DNA.
• Viral DNA can integrate into the host chromosome.

Nomenclature for Hepatitis B Virus Components

• HBV: Hepatitis B virus, a hepadnavirus (enveloped, partially double-stranded DNA virus); Dane particle = infectious HBV.
• HBsAg: Antigen found on the surface of HBV; also found on spheres and filaments in the patient’s blood; positive during acute disease; continued presence indicates carrier state.
• HBsAb: Antibody to HBsAg; provides immunity to hepatitis B.
• HBcAg: Antigen associated with the core of HBV.
• HBcAb: Antibody to HBcAg; positive during the window phase; IgM HBcAb is an indicator of recent disease.
• HBeAg: A second, different antigenic determinant on the HBV core; important indicator of transmissibility.
• HBeAb: Antibody to e antigen; indicates low transmissibility.
• Window Period: Period between the end of detection of HBsAg and the beginning of detection of HBsAb.

Replication Cycle of HBV

• Attachment to hepatocyte via human sodium taurocholate co-transporting polypeptide (huNTCP) receptor.
• Entry and uncoating.
• Transport to the nucleus; conversion to covalently closed circular DNA (cccDNA).
• Transcription of mRNAs by host RNA polymerase II.
• Translation of viral proteins.
• Reverse transcription of pgRNA into DNA within the capsid.
• Assembly and release of new virions.
• Recycling of capsid to the nucleus for persistent infection.
• Incomplete (+) DNA strand results in partially dsDNA virion genome.

Pathogenesis and Immune Response

• HBV Pathogenesis
  • Direct cytopathic effect is minimal.
  • Immune-mediated liver damage is the main cause of pathology.
  • Cytotoxic T lymphocytes (CTLs) target infected hepatocytes.
  • The severity of the disease correlates with the immune response strength.
  • Chronic infection is associated with a weak or tolerogenic immune response.
• Host Immunity to HBV
  • Innate immunity: Type I IFNs, NK cells.
  • Adaptive immunity:
    • CD8+ T cells kill infected cells.
    • CD4+ T cells aid B cell response.
    • B cells produce neutralizing antibodies against HBsAg.
  • Resolution of acute infection requires strong CD8+ and Th1 response.

Immunopathogenesis and Cancer

• Immunopathogenesis of Liver Injury
  • Infected hepatocytes display HBV peptides via MHC I.
  • CTL-mediated apoptosis and cytokine-induced damage.
  • Chronic inflammation leads to fibrosis, cirrhosis, and HCC.
  • High viral load and persistent inflammation are risk factors for progression.
• Hepatocellular Carcinoma (HCC)
  • HBV is a major risk factor for HCC.
  • Mechanisms:
    • Chronic inflammation and regeneration.
    • Integration of HBV DNA into the host genome.
    • Expression of HBx protein – oncogenic effects.
  • HBV-related HCC can occur even without cirrhosis.

Epidemiology of HBV

• Approximately 296 million people are living with chronic HBV (WHO, 2023).
• High prevalence in sub-Saharan Africa and East Asia.
• In Palestine (2023), 22 cases were reported in the West Bank, with 339 carriers in the West Bank and 63 carriers in Gaza Strip.
• Transmission routes:
  • Perinatal (most common in high-prevalence areas).
  • Sexual contact.
  • Blood and body fluid exposure (e.g., IV drug use, transfusions).
  • Horizontal transmission in children.

Risk Factors and Prevention

• High-risk groups: Infants of HBV-positive mothers, healthcare workers, people with multiple sexual partners, and IV drug users.
• Prevention:
  • Universal vaccination (birth dose + 2–3 additional doses).
  • Screening blood products.
  • Safe sex and injection practices.
  • Post-exposure prophylaxis with vaccine + HBIG.
• HBV Vaccine
  • Recombinant HBsAg-based vaccine.
  • Given as a 3-dose series (0, 1, 6 months).
  • 95% protective efficacy.
  • Induces the production of anti-HBs antibodies.
  • WHO recommends universal infant vaccination.

HBV Infection Outcomes

• Acute infection:
  • 90–95% of adults clear the infection.
  • <5% become chronic.
• Chronic infection:
  • 90% in neonates.
  • 25–50% in young children.
• Chronic HBV can lead to cirrhosis and HCC.

Clinical Syndromes

• Acute hepatitis B: Often asymptomatic; can present with jaundice, fatigue, nausea.
• Fulminant hepatitis: Rare, severe, rapid liver failure.
• Chronic hepatitis B: Persistence of HBsAg >6 months; may progress to cirrhosis.
• Cirrhosis: Fibrosis leading to liver dysfunction.
• Hepatocellular carcinoma (HCC): Major long-term complication.

Extrahepatic Manifestations of HBV

• Polyarteritis nodosa (PAN).
• Membranous glomerulonephritis.
• Essential mixed cryoglobulinemia.
• Aplastic anemia.
• Caused by immune complex deposition or autoimmune reactions.

Laboratory Diagnosis of HBV

• Markers and Molecular Tools
  • HBsAg: Current infection (acute/chronic).
  • Anti-HBs: Recovery or vaccine-induced immunity.
  • Anti-HBc IgM: Acute/recent infection.
  • Anti-HBc total: Past or current infection.
  • HBeAg: Active replication, high infectivity.
  • Anti-HBe: Lower infectivity, seroconversion.
  • HBV DNA (PCR): Viral load for treatment decisions.

Serologic Diagnosis

Treatment

• Acute: Supportive treatment.
• Chronic:
  • Tenofovir: Nucleotide analogue reverse transcriptase inhibitors (NtRTIs).
  • Entecavir: Nucleotide analog DNA polymerase inhibitors.

Hepatitis D virus

Structure and Replication

• HDV has a small, circular, single-stranded RNA genome (~1700 nt).
• Forms a rod shape due to extensive base pairing.
• The genome is surrounded by a delta antigen core.
• The envelope contains HBsAg (borrowed from HBV).
• Delta antigen exists in two forms:
  • Small (24 kDa).
  • Large (27 kDa).

Entry and Replication

• HDV uses HBsAg for hepatocyte entry, like HBV.
• Host RNA polymerase II replicates HDV RNA.
• HDV RNA forms a ribozyme structure to cleave RNA for mRNA production.
• Mutation by ADAR enzyme produces large delta antigen.
• Large delta antigen:
  • Inhibits replication.
  • Facilitates virion assembly with HBsAg.

Pathogenesis

• HDV needs active HBV infection to replicate.
• Co-infection (HBV + HDV) vs. Superinfection (HDV in chronic HBV carrier).
• Superinfection = more severe disease.
• HDV has a direct cytopathic effect.
• Exacerbates HBV-related liver damage.
• Chronic infection is possible in HBV carriers.
• Immunity against HBsAg (via vaccination or past HBV infection) protects against HDV.

Epidemiology

• Requires co-infection with HBV.
• Affects ~5% of 300 million HBV carriers.
• Endemic in:
  • Southern Italy.
  • Amazon Basin.
  • Africa.
  • Middle East.
• Epidemics in North America and Western Europe, mainly among intravenous drug users.
• High-risk groups:
  • IV drug users.
  • Hemophiliacs.
  • Recipients of blood products.

Clinical Syndromes

• Increases the severity of HBV disease.
• Fulminant hepatitis is more likely with HDV.
  • Symptoms: hepatic encephalopathy, jaundice, massive hepatic necrosis.
  • 80% fatality in fulminant cases.

Laboratory Diagnosis

• Detect:
  • HDV RNA genome (via RT-PCR).
  • Delta antigen (acute phase).
  • Anti-HDV antibodies (ELISA, radioimmunoassay).
• Serum sample should be detergent-treated for delta antigen detection.

Hepatitis B Risk Factors in Palestine: A Study

• Hepatitis B (HB) infection is a notable health issue in Palestine, classified as a moderately endemic area with a carrier rate of 2-6%.
• The study aimed to identify the risk factors of Hepatitis B transmission in northern Palestine to aid in prevention and control.
• The study used a case-control design with 100 HB virus seropositive cases and 100 seronegative controls.
• Univariate analysis showed that HB case-patients were more likely to report a history of blood transfusion, dental visits, hospitalization, Hejamat, sharing shaving equipment, intravenous drug use, or living abroad, compared to controls.
• Logistic regression identified dental visits as the most significant risk factor (P value <0.001, OR 5.6; 95% CI 2.8-11.1).
• The study highlights the need for increased HB vaccination uptake and targeted public health education.
• The development and enforcement of infection control guidelines for dental care services are crucial for preventing HB virus transmission.

Case Study: 25-Year-Old Male with Fever, Anorexia, and Fatigue

A 25-year-old male is hospitalized with fever, anorexia, and fatigue. He admits to IV drug abuse and many episodes of unprotected sex over the past few years. Based on serologic markers (not provided in text):

• Differential Diagnosis includes:
  • Had acute hepatitis B and successfully recovered
  • Had an exacerbation of preexisting hepatitis B
  • Had acute hepatitis B that progressed to chronic hepatitis with low infectivity
  • Had acute hepatitis B that progressed to chronic hepatitis with high infectivity
  • Was vaccinated against HBV during the hospitalization

Parvoviruses

Key Points

• Parvoviruses, including B19, are the smallest viruses known to infect and cause human disease.
• B19 replicates in erythroid precursor cells, causing cell death.
• Spread is by respiratory, blood, and transplacental routes.
• In healthy individuals, B19 causes immune-mediated rashes (fifth disease, slapped cheek syndrome) and less common polyarthropathy syndrome in adults.
• In patients with underlying hematological disease, B19 may result in anemia or a transient aplastic crisis.
• In immunocompromised patients, chronic anemia or pure red cell aplasia may ensue.
• In pregnancy, maternal infection at 0–20 weeks’ gestation may lead to fetal loss or nonimmunological fetal hydrops.
• There is no specific antiviral therapy, and no available vaccine for B19.

Introduction

• Smallest DNA viruses (18–26 nm).
• Nonenveloped, icosahedral capsid.
• Single-stranded linear DNA genome (~5500 bases).
• Depend on host cell machinery or helper virus (e.g., adenovirus).
• Viremia may persist beyond respiratory symptoms.
• Encodes 3 structural and 2 nonstructural proteins.

Human-Relevant Parvoviruses

• B19 virus: erythema infectiosum, aplastic crisis, arthritis, hydrops fetalis.
• Bocavirus: acute respiratory disease in children.
• AAVs (Dependovirus): nonpathogenic, used in gene therapy.

Replication Requirements

• Infect mitotically active cells.
• Do not encode polymerases or growth stimulators.
• B19 targets erythroid precursor cells.
• Binds to P antigen (globoside) on red cell precursors.
• Replication Cycle
  • Enters via P antigen, uncoats in the nucleus.
  • Host DNA polymerase converts ssDNA to dsDNA.
  • Inverted repeats help form primer structures.
  • Capsid proteins assembled in the nucleus.
  • Cell lysis releases new virions.

Pathogenesis

• B19
  • Direct killing of erythroid precursors.
  • Immune complex formation (rash, arthralgia).
  • Biphasic disease course:
    • Febrile phase: high viremia, infectious.
    • Symptomatic phase: immune-mediated symptoms.
• Bocavirus
  • Infects respiratory epithelium.
  • Can lead to bronchiolitis in young children.
  • Viremia may persist beyond respiratory symptoms.

Clinical Manifestations

• B19
  • Erythema infectiosum (fifth disease):
    • Slapped cheek rash.
    • Spreads to limbs.
  • Polyarthritis in adults.
  • Asymptomatic or mild illness in healthy individuals.
  • Risk of chronic infection (Immunocompromised).
  • Risk of aplastic crisis (Chronic hemolytic anemia).
  • Risk of hydrops fetalis (Pregnant women (seronegative)).
• Bocavirus
  • Children <2 years.
  • Symptoms: bronchiolitis, wheezing.
  • May cause prolonged viremia.
  • Rare fatal cases.

Epidemiology

• B19
  • 65% seroprevalence by age 40.
  • Common in children aged 4–15.
  • Spread via respiratory droplets, oral secretions.
  • Occurs mostly in late winter and spring.
• Bocavirus
  • Worldwide distribution.
  • Primarily affects children <2 years.
  • Transmitted via respiratory secretions, stool.

Laboratory Diagnosis

Treatment and Prevention

• No specific antiviral treatment.
• Supportive care.
• Dog/cat parvovirus vaccines are available.
• Gene therapy uses AAVs.
• Topical anesthetic or antihistamine for itching.
• Intravenous Immunoglobulin (IVIG) in chronic parvovirus.
• Aplastic crisis may require packed RBC transfusion.
• A vaccine is in trials.

Parvovirus B19 Vaccine Development

• Parvovirus B19 (B19) causes the fifth disease in children.
• Primary infection in pregnant women poses a high risk of hydrops fetalis and stillbirth due to severe fetal anemia.
• Virus-like particle (VLP) vaccine candidates for B19 have been developed, but none are yet approved.
• The B19 phospholipase A2 domain (B19 PLA2) may cause adverse inflammatory reactions; previous attempts to improve the vaccine involved mutations to impair PLA2 activity.
• VLPs lacking PLA2 activity (⊿PLA2 B19 VLP) were designed and found to be immunogenic and safe in vivo.
• These VLPs did not cause histological inflammatory reactions or IL-6 production in mice.
• CD4+ T cells from vaccinated mice and B19-seropositive individuals were not activated by B19 PLA2 stimulation.
• ⊿PLA2 B19 VLPs induced neutralizing antibodies against B19, suggesting their potential as a safe and effective vaccine candidate.