Detailed Notes on Viral Diseases and Influenza

Infectious Diseases: Viral Diseases Notes

Overview of Viruses

  • Virus Definition: A small infectious agent that can multiply only within living cells of animals, plants, or bacteria. It cannot reproduce or conduct metabolic processes without a host cell.
  • Viral Structure: A virion is a complete virus particle that includes:
    • Capsid (outer protein shell)
    • RNA or DNA core
    • External envelopes (sometimes)
  • Vector-borne viruses: These viruses multiply in both an invertebrate vector (like mosquitoes) and the vertebrate host (like humans).

Virus Classification

  • Genomic Composition: Viruses can have either a DNA or RNA genome:
    • RNA can include retroviruses like HIV.
    • RNA viruses are prone to mutations and can be single- or double-stranded, linear or circular, and vary in length.
  • Baltimore Classification System: Classifies viruses based on their replication method and type of nucleic acid genome:
    • Example: Retroviruses convert their RNA into DNA after infecting a cell, integrating the viral DNA into the host DNA for replication.

Viral Interaction with Hosts

  • Cell-specific Infection: Viruses typically target specific cell types (e.g., common cold viruses target the upper respiratory tract).
  • Mechanism of Infection:
    1. Viruses penetrate the host cell membrane or wall and inject their genome.
    2. The host's cellular machinery is hijacked to replicate the viral genome.
    3. New viruses are released, usually killing the host cell in the process.

Types of Viral Infections

  • Latent Infections: Viral DNA/RNA remains in host cells without replication, allowing for transmission even when asymptomatic (e.g., herpes).
  • Chronic Viral Infections: Continuous shedding of virus, as seen in hepatitis B and C.
  • Cancer: Some viruses alter the function of infected cells and can promote uncontrollable cell division, leading to cancer.

Common Viral Infections

Infections can affect various body systems:

  • Respiratory Tract: E.g., common cold, flu, pneumonia.
  • Gastrointestinal Tract: E.g., norovirus.
  • Liver: E.g., hepatitis types A, B, C.
  • Nervous System: E.g., rabies, West Nile virus, polio.
  • Skin: E.g., warts, chickenpox.
  • Placenta & Fetus: E.g., Zika virus, rubella, cytomegalovirus.
  • Multiple Body Systems: E.g., enteroviruses.

Disease Management

  • Viral Diseases: Not treatable with antibiotics.
  • Vaccines available for viral infections include:
    • Hepatitis A and B
    • Human papillomavirus (HPV)
    • Influenza
    • Japanese encephalitis
    • Measles, mumps, rubella (MMR)
    • Polio
    • Rabies
    • Shingles
    • Yellow fever
    • COVID-19

Use of Fetal Cells in Vaccine Production

  • Some vaccines are produced using fetal cells, such as:
    • Chickenpox (varicella)
    • Rubella
    • Hepatitis A
    • Certain versions of shingles and rabies vaccines.
  • Ethical Consideration: Cells used descend from two fetuses whose mothers chose to have abortions about 40 years ago.
    • These cells are not directly from the fetuses, and their use is grounded in the need for effective viral growth.

Influenza Overview

Risk Factors for Complications
  • Higher Risk Populations:
    • Age under 2 or over 65
    • Individuals in nursing homes
    • People with underlying health conditions, e.g., asthma, diabetes, heart disease, weakened immune systems.
  • Transmission: Mainly through inhaling droplets; can also spread via contaminated surfaces.
Types of Influenza Viruses
  • Four Major Types:
    • Influenza A: Causes seasonal flu and can lead to pandemics.
    • Influenza B: Causes seasonal flu.
    • Influenza C: Generally causes mild illness.
    • Influenza D: Primarily affects cattle; not known to infect humans.

Seasonal Flu vs. Pandemic Flu

  • Seasonal Flu: Annual outbreaks caused by circulating Influenza A and B viruses. Vaccine recommended annually for most individuals.
  • Pandemic Flu: Global outbreaks of new Influenza A viruses, differing significantly from current seasonal strains. Vaccines may not be readily available at the onset of a pandemic.
Risk Factors and Complications
  • Complications are more likely in young children, the elderly, pregnant women, and individuals with chronic health issues.
  • In pandemics, healthy young adults may also be at risk for severe complications.

Pandemic Flu Strategies

Viral Host Jump
  • When viruses move from animals to humans, new strains may emerge due to reassortment, posing risks of pandemics (e.g., the historical pandemics of 1957, 1968, and 2009).
Addressing Infections Globally
  • Strategies include managing logistics for vaccination campaigns, maintaining a cold chain, and fostering cooperation among government and community stakeholders.
Questions to Address
  • Data collection challenges for managing pandemics.
  • Structural weaknesses in health systems.
  • Ways to support successful pandemic responses in countries with insufficient capacity.