Virology: Virus Morphology and Capsid Shapes

• Acellular particles contain either DNA or RNA and are obligate intracellular parasites.
• Viral replication does not occur through cell division.
• Viruses infect both eukaryotes and prokaryotes.
• Bacteriophages (phages) infect bacteria.
• Viruses are classified based on morphology, capsid shape, nucleic acid type, and viral infection.

Morphology of Viruses

• Size ranges from very small (parvovirus) to large (poxvirus); most are not visible by light microscopy.
• Viruses consist of genetic material within a viral coat or capsid, made of proteins called capsomeres.
• The association of viral capsid proteins with viral nucleic acid is the nucleocapsid.
• Spikes are long projections extending from the nucleocapsid.

Capsid Shapes

• 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 (e.g., bacteriophages).
• Bacteriophages have a head (capsid) containing nucleic acid, a tail that is a hollow tube surrounded by a sheath, and a base plate with attached fibers.

Nucleic Acid Types

• Viruses have either DNA or RNA, which can be linear or a closed loop.
• Reverse transcribing viruses include:
• Cytomegalovirus (a DNA virus that replicates through an RNA intermediate).
• Retroviruses (like 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 cell through irreversible binding.
• Penetration: Phage injects nucleic acid into the bacterium; only the nucleic acid enters the cell.
• Replication: Bacterial metabolism produces viral components.
• Assembly: Intracellular accumulation of viral components occurs.
• Maturation.
• Release.

Multiplication of Animal Viruses

• Stages are similar to bacteriophage multiplication:
• Adsorption.
• Penetration.
• Uncoating: Viral nucleic acid is released.
• Replication.
• Assembly.
• Release.

Viral Infections

• Abortive infections do not result in viral production.
• Lytic or cytocidal infections kill the host cell.
• Persistent infections include:
• Chronic infections: Not lytic but productive.
• Latent infections: 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, potentially causing oncogenic changes.

Host Cell Damage

• Morphological Effects (Cytopathic): Altered shape, lysis, membrane fusion, programmed cell death (apoptosis).
• Physiological/Biochemical Effects: Changes in ion movement or the host cell’s chemical molecules.
• Genotoxic/Mutation Effects: Damage to host cell DNA, potentially initiating cancer.
• Oncogenic viruses can lead to cancer.

DNA Viruses

• Adenovirus: Causes mostly respiratory illness, gastroenteritis, conjunctivitis, cystitis, and rashes.
• Hepadnavirus: Causes Hepatitis B.
• Poxvirus: Includes Smallpox (Variola virus), which WHO declared eradicated in 1981.
• Parvovirus: B19 causes 5th disease (erythema infectiosum).
• Herpesvirus:
• Herpes Simplex 1 (HHV-1): Causes cold sores or fever blisters and herpes whitlow; remains latent in the trigeminal nerve.
• Herpes Simplex 2 (HHV-2): Causes genital herpes and encephalitis; 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; more common above 50 years.
• Epstein-Barr Virus (HHV-4, EBV): Causes infectious mononucleosis; carcinogenic; common in young adults.
• Cytomegalovirus (CMV, HHV-5): Often asymptomatic; severe in immunocompromised patients; most common virus transmitted during pregnancy; transmitted sexually or via body fluids; carcinogenic.
• Human herpes virus 6 (HHV-6): Causes 6th disease.
• HHV-7 and HHV-8: Carcinogenic.
• Papillomavirus & Polyomavirus: Includes more than 90 viruses; causes warts; some are sexually transmitted and carcinogenic.

RNA Viruses

• Bunyavirus: Arthropod-borne (mosquitoes), except for Hantavirus (carried by rodents).
• Coronavirus: Causes respiratory (SARS, COVID-19) and enteric diseases.
• Orthomyxovirus: Includes influenza A (most virulent), B, and C viruses. Specific serotypes include H1N1 (Spanish), H2N2 (Asian), and H5N2 (Avian). Hemagglutinin (H) allows the virus to bind to respiratory cells (16 subtypes); Neuraminidase (N) breaks mucus (9 subtypes), allowing deeper penetration.
• Paramyxovirus: Causes Measles (Rubeola) and Mumps.
• Picornavirus: Includes Enterovirus (e.g., poliovirus), Rhinovirus, and Hepatovirus (HVA); smallest RNA viruses.
• Rhabdovirus: Infects plants, insects, fish, birds, mammals, and humans; includes Rabies virus.
• Reovirus: "Respiratory, Enteric, Orphan" (REO); Rotavirus causes traveler’s diarrhea and is the most common cause of viral diarrhea in infants and children < 5 years.
• Retrovirus: Includes HIV (human immunodeficiency virus).
• Togavirus: Includes Rubivirus (rubella virus), causing Rubella (German Measles).
• Flavivirus: Causes Yellow fever, encephalitis, dengue fever, hepatitis C, and West Nile virus.
• Calicivirus: Includes Norovirus (gastroenteritis) and Hepevirus (HEV).
• Filovirus: Includes Ebola virus.

Subviral Agents

• Viroids: Cause plant diseases and do not require a helper virus.
• Virusoids: Require a helper virus and are generally associated with plant infections. Hepatitis D virus (Delta agent) is a virusoid that can infect humans, requiring simultaneous infection with hepatitis B.
• Prions (proteinaceous infectious particles)
• Normal animal proteins that become infectious when abnormally folded.
• Lack nucleic acid and are not affected by autoclaves.
• Diseases caused by prions are called transmissible spongiform encephalopathies, including mad cow disease, kuru, and Creutzfeldt-Jacob disease (C-J disease).