notes

Pathogenesis of Viral Infections

• Understanding viral infections is crucial for:

  • Implications for transmission, treatment, and prevention of diseases.

  • Developing treatments and preventive measures based on replication and disease course.

• Response to infections can vary:

  • No apparent effect.

  • Significant cellular damage leading to cell death.

  • Hyperplasia.

  • Cancer.

Course Objectives

• Topics covered include:

  • Acute lytic viral infections.

  • Persisting viral infections.

  • Oncogenic viruses.

  • Host response to viruses.

  • Viral evasion strategies.

  • Transmission methods:

    • Airborne.

    • Faecal-oral.

    • Body fluids.

    • Vectors.

    • Foetal and neonatal transmission.

  • Inactivation methods for viruses.

  • Diagnostic techniques:

    • Immunofluorescence.

    • Nucleic Acid Analysis.

    • Serology.

    • Cell Culture.

  • Treatment with antiviral drugs.

Viral Diseases and Affected Systems

• Summary of various diseases and their associated viruses:

  • Bronchiolitis: Respiratory syncytial virus (Paramyxoviridae).

  • Chickenpox: Varicella zoster (Herpesviridae).

  • Common cold: Rhinovirus, Adenovirus (Picornaviridae, Adenoviridae).

  • CMV: Cytomegalovirus (Herpesviridae).

  • Encephalitis: Various viruses, primarily arthropod-borne.

  • Hepatitis: A, B, C, D, and E associated with various virus families.

  • HIV/AIDS: Human immunodeficiency virus (Retroviridae).

  • Measles: Rubeola (Paramyxoviridae).

  • Rabies: Rabies virus (Rhabdoviridae).

  • Teratogenic effects: Some viruses affect unborn foetuses leading to congenital defects.

Overview of Human Viruses

• 219 known virus species can infect humans, with new species discovered annually.

  • First virus: Yellow fever virus in 1901.

• Viral diseases can vary in signs and symptoms, influenced by the type of virus and host susceptibility.

  • Example: Measles can present as mild or severe.

Types of Viral Infections

• Viral diseases classified into categories:

  • Acute: Well-defined and recognizable symptoms.

  • Subclinical: No symptoms, recovery without awareness.

  • Persistent: Chronic, latent, or oncogenic infections.

Acute Lytic Infections

• Recognizable symptoms, virus attaches to specific cells, takes over host cell, causes cell death through lysis.

• Immune response typically eliminates the virus, providing long-lasting immunity.

Subclinical Infections

• Infections with mild, unnoticed symptoms, identified only through serological evidence.

Viral Persistence

1. Latent Viral Infections:

   • Dormant viruses reactivated under stress, commonly caused by herpes viruses.

   • Example: Herpes simplex and varicella zoster.

2. Slow Infections:

   • Long-term virus presence leading to new symptoms many years later.

   • Example: Subacute sclerosing panencephalitis (SSPE) from measles.

3. Chronic Viral Infections (Carriers):

   • Continuous low-level viral production without symptoms.

   • Example: Hepatitis B.

Oncogenic Viruses

• Transform host cells into cancerous states, resulting in uncontrolled growth and tumours.

• Viruses associated with specific cancers include:

  • Human T-cell leukemia-lymphoma virus (HTLV).

  • Human Papillomavirus (HPV).

  • Hepatitis B and C leading to liver cancer.

  • Epstein-Barr Virus (EBV) related to Burkitt’s lymphoma.

Host Response to Viral Infection

• Response depends on virus type and host susceptibility:

  • Early stages characterized by high viral particles (viraemia).

  • Immune response involves:

    • Production of interferons.

    • Activation of T lymphocytes and B lymphocytes.

    • Symptoms of viral disease due to immune activation.

Treatment and Diagnosis of Viral Infections

• Treat symptoms as few effective antiviral drugs exist:

  • Target viral replication without harming host cells.

• Diagnostic techniques include:

  • Immunofluorescence, Nucleic Acid Analysis (e.g., COVID-19 testing).

  • Serology, detecting antibodies to determine past infections.

  • Cell culture, although it is complex and time-consuming.

Transmission of Viral Diseases

• Involved methods include:

  • Airborne Transmission: Coughing/sneezing can release virions.

  • Faecal-Oral Transmission: Contact with contaminated food or water.

  • Body Fluids Transmission: Via blood or bodily fluids for viruses like HIV, HBV, and HCV.

  • Vector Transmission: Insects like mosquitoes transmitting arboviruses.

Faecal-Oral Transmission

• Common in gastroenteritis, notably with rotavirus, norovirus. Importance of hygiene emphasized.

Transmission in Body Fluids

• Vertically transmissible viruses (e.g., HIV, HBV, HCV) pose risks during pregnancy.

Inactivation and Diagnosis of Viruses

• Viruses vary in their survival outside hosts. Most are susceptible to heat or certain pH levels.

• Diagnosis may involve serological assays or PCR-based techniques based on clinical presentation and prior immune response.