Virology: Virus Structure and Classification

• Acellular particles contain either DNA or RNA and are obligate intracellular parasites, meaning they cannot reproduce on their own.
• Replication does not occur through cell division.
• Viruses infect both eukaryotes and prokaryotes; those infecting bacteria are called bacteriophages or phages.

Virus Classification

• Viruses are classified based on morphology, capsid shape, nucleic acid type, and viral infection characteristics.

Morphology

• Virus size ranges from very small (parvovirus) to large (poxvirus).
• Light microscopy cannot visualize most viruses.
• A virus consists of genetic material inside a viral coat called a capsid.
• The capsid consists of proteins called capsomeres.
• The nucleocapsid is the viral capsid proteins' association with viral nucleic acid.
• Spikes are long projections extending from the nucleocapsid.

Capsid Shape

• Helical viruses have rod-shaped capsomeres and can be naked (e.g., tobacco mosaic virus) or enveloped (e.g., flu virus).
• Icosahedral viruses are three-dimensional with corners, triangular faces, and edges (e.g., herpesvirus, adenovirus, papovavirus, parvovirus).
• Complex viruses have capsids that are neither purely helical nor icosahedral, such as bacteriophages.
• Bacteriophages contain:
  • A head (capsid) containing nucleic acid
  • A tail consisting of a hollow tube surrounded by a sheath
  • A base plate with attached fibers

Nucleic Acid Type

• Nucleic acid can be DNA or RNA, and may be linear or a closed loop.
• Exceptions to standard replication:
  • Cytomegalovirus (Herpesvirus genus): A DNA virus that replicates through an RNA intermediate.
  • Retrovirus (HIV): An RNA virus that replicates through a DNA intermediate.

Multiplication of Bacteriophages

• Each phage is specific to a particular bacterium (host range).
• Stages include:
  • Adsorption: Phage attaches to a susceptible host via irreversible binding.
  • Penetration: Phage injects nucleic acid into the bacterium; the phage remains outside.
  • Replication: Bacterial metabolism produces viral components.
  • Assembly: Intracellular accumulation of viral components occurs.
  • Maturation: Further development into infectious virus.
  • Release: Virus exits the cell.

Multiplication of Animal Viruses

• Similar to bacteriophage multiplication, with the following stages:
  • Adsorption
  • Penetration
  • Uncoating: Viral nucleic acid is released.
  • Replication
  • Assembly
  • Release

Viral Infections

• Abortive: No viral production occurs.
• Lytic or Cytocidal: Kills the host cell.
• Persistent:
  • Chronic: Not lytic but productive.
  • Latent: Virus remains in an asymptomatic host for a long period (e.g., cold sores, shingles).
  • Slow infections: Prolonged incubation followed by disease.
  • Transforming infections: Viral nucleic acid remains indefinitely without virus production; potential oncogenic changes.

Host Cell Damage

• Morphological Effects (Cytopathic): Altered shape, lysis, membrane fusion, apoptosis.
• Physiological/Biochemical Effects: Changes in ion movement or host cell chemical molecule production.
• Genotoxic/Mutation Effects: Damages host cell DNA and may initiate cancer.
• Oncogenic viruses may cause cancer.

DNA Viruses

• Adenovirus: causes respiratory illness, gastroenteritis, conjunctivitis, cystitis, and rashes.
• Hepadnavirus: Includes Hepatitis B.
• Poxvirus (largest DNA virus): Includes smallpox (Variola virus), which the WHO declared eradicated in 1981.
• Parvovirus (smallest DNA virus): B19 causes fifth disease (erythema infectiosum).
• Herpesvirus:
  • Herpes Simplex 1 (HHV-1): Cold sores or fever blisters (vesicles on lips), herpes whitlow (vesicles on fingers); remains latent in the trigeminal nerve.
  • Herpes Simplex 2 (HHV-2): Causes genital herpes and encephalitis; is carcinogenic; remains latent in the sacral nerve.
  • Varicella-Zoster Virus (HHV 3, VZV): Causes varicella (chickenpox) and shingles (herpes zoster) upon reactivation; latent in the dorsal nerve root; shingles are more common above age 50.
  • Epstein-Barr Virus (HHV-4, EBV): Causes infectious mononucleosis (mono, glandular fever, kissing disease); more common in young adults; individuals should avoid contact sports; carcinogenic.
  • Cytomegalovirus (CMV, HHV 5): Asymptomatic or mild infection, but severe in immunocompromised patients; the most common virus transmitted during pregnancy; transmitted sexually, by body fluids, or transplanted tissues; carcinogenic.
  • Human herpes virus 6 (HHV 6): Causes sixth disease.
  • HHV 7: Carcinogenic.
  • HHV 8: Carcinogenic.
• Papillomavirus & Polyomavirus: Includes over 90 viruses; causes warts; some are sexually transmitted and carcinogenic.