Virology Lecture Notes

Introduction to Virology

• Viruses are non-living microorganisms and obligate intracellular parasites containing either RNA or DNA.
• Viruses are non-cellular (acellular) compared to other cellular microorganisms like bacteria, protozoa, and fungi.
• Smallest infectious agents, ranging from 20 to 400 nanometers.
• Visualization requires an electron microscope.

Structure of Viruses

• Basic structure: protein shell (capsid) surrounding nucleic acid core (RNA or DNA).
• Capsid: Protects genetic material.
• Naked (non-enveloped) viruses: Consist only of nucleocapsid.
• Enveloped viruses: Have a lipid-containing membrane.

Classification

• By nucleic acid type: RNA or DNA viruses.
• By capsid symmetry: helical, polyhedral (icosahedral), complex.

Examples

• RNA viruses: Influenza, coronavirus, paramyxoviruses (measles, mumps).
• DNA viruses: Adenovirus, hepatitis B, herpesvirus.

Viral Replication

• Requires living host cells; cannot replicate in extracellular environments.
• Replication stages: attachment, penetration, uncoating, replication, assembly, maturation, release.
• Replication Types:
  • Lytic cycle: Results in host cell lysis.
  • Lysogenic cycle: Viral DNA integrates into host DNA.

Baltimore Classification

• Based on viral replication and genome type.
• Seven groups:
  1. Double-stranded DNA (e.g., smallpox virus).
  2. Positive single-stranded DNA (e.g., parvoviruses).
  3. Double-stranded RNA (e.g., rotaviruses).
  4. Positive single-stranded RNA (e.g., coronaviruses).
  5. Negative single-stranded RNA (e.g., measles virus).
  6. Positive single-stranded RNA with reverse transcriptase (e.g., HIV).
  7. Double-stranded DNA with reverse transcriptase (e.g., hepatitis B).

Laboratory Diagnosis of Viral Infections

• Direct Examination:
  • Electron microscopy (expensive, labor-intensive).
  • Light microscopy for observing cytopathic effects.
• Serological Tests:
  • Immunofluorescent tests (direct and indirect).
  • ELISA (enzyme-linked immunosorbent assay).
  • Rapid tests for antigen detection (e.g., COVID-19).
• Molecular Techniques:
  • PCR (polymerase chain reaction).

Cultivation of Viruses

• Cannot use traditional nutrient media; require live cells.
• Methods:
  • Cell Culture: Primary, semi-continuous, continuous (tumor) cells.
  • Embryonated Eggs: Used for influenza virus vaccine production.

Antiviral Drugs

• Mode of Action:
  • Inhibit viral entry, replication, or assembly.
  • Examples: Tamiflu for influenza, AZT for HIV.

Types of Viral Infections

• Acute Infections: Rapid onset and resolution.
• Latent Infections: Virus remains dormant within host cell.
• Persistent Infections: Long-term infections that may become symptomatic later.

Conclusion

• Importance of understanding virus structure, classification, and replication for diagnosis and treatment.
• Key focus on laboratory methods for detecting and diagnosing viral infections.

Note: This lecture provided foundational knowledge on virology, useful for further studies in medical microbiology.