Viruses and Prions

Viruses

Definition

  • Viruses: Microscopic infectious agents that replicate only inside the living cells of an organism.

  • Virion: An individual virus.

  • Lack machinery to reproduce independently, relying entirely on the host's cellular machinery.

  • Can cause cell damage or death, resulting in diseases.

  • All viruses are considered parasites.

Historical Background

  • Martinus Beijerinck (1898): Dutch scientist who laid the foundation for virology, concept involving filtration technique to separate viruses from bacteria.

Taxonomic Hierarchy

  1. Realm: Highest rank (e.g., Kitrinoviricota for yellow viruses).

  2. Kingdom: Grouping within a realm (e.g., Kitrinoviricetes).

  3. Phylum: Based on genetic and structural characteristics.

  4. Class: Grouping of related orders.

  5. Order: Contains one or more families.

  6. Family: Group of related viruses (e.g., Herpesviridae).

  7. Genus: Related species within a family (e.g., Simplexvirus).

  8. Species: Specific viruses sharing common characteristics (e.g., Herpes simplex virus 1).

Naming Conventions

  • Family names typically end in -viridae (e.g., Retroviridae).

  • Genus names usually end in -virus (e.g., Lentivirus).

  • Species names often include common names associated with diseases.

Classification Criteria

  • Genetic Material: Can be DNA or RNA, single-stranded or double-stranded.

  • Capsid Structure: Influences virus classification based on shape and structure.

  • Envelope: Presence of a lipid envelope affects classification.

  • Replication Method: Different methods of how viruses replicate within host cells.

Key Characteristics

  1. Structure: Genetic material encased in a protein coat (capsid).

    • Capsid Composition: Made of protein subunits (capsomers).

  2. Size: Much smaller than bacteria (20 – 300 nm).

  3. Lack of Cellular Structure: Not made up of cells; no metabolic machinery.

  4. Reproduction: Requires host cell to replicate and produce new virions.

  5. Host Specificity: Certain viruses can only infect specific hosts.

  6. Transmission Methods: Spread through various means, including direct contact and respiratory droplets.

  7. Genetic Variation: High mutation rate can lead to variations impacting infectivity.

  8. Pathogenicity: Viruses can cause diverse diseases from mild to severe.

Structure of Viruses

Genetic Material

  • Can be either DNA or RNA (single-stranded/double-stranded).

Capsid

  1. Capsid Shapes:

    • Icosahedral (20 triangular faces, e.g., adenoviruses).

    • Helical (cylindrical shape, e.g., tobacco mosaic virus).

    • Complex (e.g., bacteriophages).

Envelope

  1. Presence: Some viruses have lipid envelopes, derived from host cell membranes.

  2. Function: Provides protection and aids in evasion of host immune responses.

  3. Surface Proteins: Crucial for viral infectivity and immune response triggering.

Viral Reproduction Process

  1. Attachment: Virus binds to specific receptors on host cell.

  2. Entry: Mechanisms include direct fusion with the membrane or endocytosis.

  3. Uncoating: Shedding of capsid to release genetic material.

  4. Replication and Transcription:

    • DNA viruses: Use host's DNA polymerase in the nucleus.

    • RNA viruses: Often use own polymerases; retroviruses convert RNA to DNA.

  5. Translation: Host ribosomes synthesize viral proteins.

  6. Assembly: New virions formed from genetic material and proteins.

  7. Release:

    • Budding: Enveloped viruses may retain host cell.

    • Lysis: Naked viruses can cause cell death.

Transmission of Viruses

  1. Direct Contact: Virus transferred through physical touch (e.g., herpes).

  2. Indirect Contact: Spread through contaminated surfaces (fomites).

  3. Respiratory Droplet: Spread via droplets during coughing/sneezing (e.g., influenza).

  4. Airborne Transmission: Smaller particles can remain airborne longer (e.g., tuberculosis).

  5. Vector-Borne: Transmission through insects (e.g., mosquitoes).

  6. Zoonotic: Viruses jumping from animals to humans (e.g., rabies).

  7. Vertical Transmission: From mother to fetus during pregnancy.

  8. Fecal-Oral Transmission: Through contaminated food/water (e.g., hepatitis A).

Virus Classification Examples

  • Adenoviridae: Icosahedral, non-enveloped, DNA, causes acute respiratory disease.

  • Herpesviridae: Icosahedral, enveloped, DNA, causes chickenpox.

  • Orthomyxoviridae: Helical, enveloped, RNA, causes influenza.

Prions

Definition

  • Infectious agents composed entirely of protein; no nucleic acids involved.

Structure and Characteristics

  1. Misfolded Proteins: Prions induce normal proteins to misfold, affecting brain function.

  2. Infectious Nature: Cause neurodegenerative diseases.

  3. Resistance: Resistant to conventional sterilization methods.

Prion Diseases

  • Bovine Spongiform Encephalopathy (BSE): Affects cattle; can infect humans.

  • Creutzfeldt-Jakob Disease (CJD): Rapid cognitive decline in humans.

  • Scrapie: Affects sheep and goats.

Transmission of Prions

  1. Consumption: Contaminated meat ingestion.

  2. Medical Procedures: Rare transmission through surgical instruments.

  3. Genetic Predisposition: Inherited mutations causing prion diseases.

Public Health Awareness

  1. Surveillance and Monitoring: Tracking diseases to control outbreaks.

  2. Vaccination Programs: Prevent infections and control outbreak spread.

  3. Infection Control Practices: Hygiene and sanitation to reduce infections.

  4. Education and Awareness: Public campaigns to inform on prevention.

  5. Research and Development: Ongoing vaccine and treatment research.

  6. Global Health Collaboration: International efforts to respond to outbreaks.

  7. Economic Impact: Healthcare strains due to outbreaks.

  8. Preparedness and Response: Emergency plans for effective public health responses.