Chapter 13: Viruses – Structure, Classification, and Human Diseases

Viruses: General Characteristics

Definition and Properties

Viruses are acellular infectious agents responsible for a wide range of diseases in humans, animals, and plants. They are unique in their structure and replication, requiring host cells for propagation.

• Acellular: Not composed of cells; lack cellular structure.

• Obligate intracellular parasites: Must infect and use host cell machinery to reproduce.

• Genetic material: Contain either DNA or RNA (never both).

• No metabolism: Do not carry out metabolic processes independently.

• No ribosomes: Cannot synthesize proteins without host cell.

• Antibiotics ineffective: Standard antibiotics do not affect viruses.

Common viral diseases:

1. Upper respiratory infections

2. Diarrheal diseases

3. Sexually transmitted infections (STIs)

Basic Virus Structure

Virion and Components

The virion is the complete, infectious virus particle. It consists of genetic material and a protective protein coat, with some viruses possessing an additional lipid envelope.

• Nucleic acid core: DNA or RNA (single- or double-stranded).

• Capsid: Protein coat made of subunits called capsomeres.

• Envelope (sometimes present): Lipid membrane derived from host cell, often with glycoprotein spikes for attachment.

Enveloped vs Non-enveloped Viruses

• Non-enveloped viruses: Only nucleic acid and capsid; more resistant to disinfectants.

• Enveloped viruses: Capsid plus lipid membrane; contain spikes for host cell attachment; more easily destroyed by drying, heat, and detergents.

Functions of Capsid and Envelope

• Protect viral genetic material

• Facilitate entry into host cells

• Stimulate immune response (antibody production)

Viral Classification and Terminology

Key Definitions

• Viral species: Group of viruses with similar genetic information and host range.

• Virion: Complete infectious virus particle.

• Plaques: Clear zones on cell cultures where viruses have lysed host cells.

• Latency: Virus remains dormant in host cells.

• Cytopathic effect (CPE): Visible damage to infected host cells.

Naming Conventions

• Family: Ends in -viridae (e.g., Herpesviridae).

• Genus: Ends in -virus (e.g., Herpesvirus).

• Species/common name: (e.g., Herpes Simplex Virus, HSV).

Virus Culturing and Identification

Bacteriophage Culturing

• Bacteriophages infect bacteria.

• Mixed with bacteria and grown on agar plates.

• Clear zones (plaques) indicate lysis of bacteria.

• Plaques measured as PFUs (Plaque Forming Units).

Animal Virus Culturing

• Require living cells for growth.

• Methods: Living animals, embryonated eggs, or cell/tissue culture.

• Cell culture allows continuous virus growth in laboratory conditions.

Identification Methods

• Cytopathic effects: Observing cell damage.

• Serological tests: Detect viral antigens or antibodies.

• PCR: Detects viral nucleic acids.

Viral Replication Cycles

Lytic Cycle

The lytic cycle results in the destruction of the host cell and release of new virions.

1. Attachment: Virus binds to host cell.

2. Penetration: Viral DNA enters host cell.

3. Biosynthesis: Viral DNA and proteins are synthesized.

4. Maturation: New virus particles are assembled.

5. Release: Host cell lyses, releasing new viruses.

Result: Host cell death.

Lysogenic Cycle

1. Viral DNA integrates into host DNA (prophage).

2. Host cell replicates normally, copying viral DNA.

3. Virus may later enter lytic cycle.

Key idea: Virus can remain latent for long periods.

Multiplication of Animal Viruses

• Attachment

• Penetration: By fusion or endocytosis.

• Uncoating: Enzymatic removal of capsid.

• Biosynthesis

• Maturation

• Release: Enveloped viruses by budding; non-enveloped by cell rupture.

DNA Viruses

• DNA enters nucleus; replication, transcription, and translation occur.

RNA Viruses

• RNA serves as mRNA or produces mRNA for protein synthesis.

Retroviruses (e.g., HIV)

• Use reverse transcriptase to convert RNA to DNA, which integrates into host genome.

Major Human Viral Diseases

Transmission, Symptoms, Diagnosis, and Prevention

Virus	Transmission	Symptoms	Diagnosis	Treatment	Prevention
Rhinovirus	Respiratory droplets	Common cold	PCR	Supportive care	Hygiene
RSV	Respiratory droplets	Bronchitis, pneumonia (infants)	PCR	Supportive, antiviral	Hygiene
Influenza	Respiratory droplets	Fever, chills, body aches	Rapid antigen, PCR	Antiviral (amantadine)	Vaccine
Mumps	Respiratory droplets	Swollen salivary glands	Antibodies, PCR	Supportive care	MMR vaccine
Rubella	Respiratory droplets	Rash, mild fever	Antibodies	Supportive care	MMR vaccine
Measles	Respiratory droplets	Rash, Koplik spots	Antibodies, PCR	Supportive care	MMR vaccine
Smallpox	Respiratory, contact	Pustules, fatigue, fever	PCR	Supportive care	Vaccine
Varicella zoster	Respiratory, contact	Chickenpox, shingles, rash	PCR (lesion sample)	Antiviral	Vaccine
Hepatitis A	Fecal-oral	Liver inflammation	IgM antibodies	Supportive care	Vaccine
Hepatitis B	Blood, sexual	Liver infection, jaundice	HBsAg antigen test	Antiviral, interferon	Vaccine
Hepatitis C	Blood	Chronic liver disease	ELISA antibodies	Antiviral, interferon	Avoid needle sharing
Hepatitis D	Blood	Liver infection	PCR	Antiviral	HBV vaccine
Hepatitis E	Fecal-oral	Acute hepatitis	Antibodies	Supportive care	Sanitation
Rotavirus	Fecal-oral	Severe diarrhea (children)	Stool antigen	Fluid replacement	Vaccine
Norovirus	Fecal-oral	Vomiting, diarrhea	PCR (stool test)	Fluid replacement	Hygiene
HSV-1	Contact	Cold sores (mouth/lips), latent	PCR (lesion)	Antiviral (Acyclovir)	Avoid contact
HSV-2	Sexual, perinatal	Genital herpes	PCR (lesion), CPE	Antiviral (Acyclovir)	Condoms
HPV	Sexual	Warts, cervical cancer risk	Pap smear, HPV test	Wart removal, vaccine	Vaccine
HIV	Blood, sexual	Immune suppression (AIDS)	ELISA, PCR	Antiviral	Safe sex
Polio virus	Fecal-oral	Paralysis	PCR (stool)	Supportive care	Vaccine
Rabies virus	Animal bite	Fatal encephalitis	Brain tissue exam	Post-exposure vaccine	Vaccine
Epstein-Barr Virus (EBV)	Saliva	Mononucleosis, fatigue	Monospot test	Supportive care	Avoid saliva contact
Cytomegalovirus (CMV)	Body fluids	Congenital infection, asymptomatic	PCR	Antiviral	Hygiene

Additional info: The table above summarizes key viruses, their transmission, symptoms, diagnosis, treatment, and prevention. Some details (e.g., specific diagnostic tests) are inferred from standard clinical practice.

Special Topics in Virology

Influenza: Antigenic Drift vs. Shift

• Antigenic drift: Small mutations in viral genes; causes seasonal flu changes.

• Antigenic shift: Major genetic changes; leads to new virus strains and pandemics.

Prions

• Prions: Infectious proteins lacking DNA or RNA.

• Diseases caused: Creutzfeldt-Jakob disease, mad cow disease, spongiform encephalopathy.

Key Enzymes and Viral Processes

Reverse Transcriptase (Retroviruses)

• Enzyme used by retroviruses (e.g., HIV) to synthesize DNA from RNA template.

• Allows integration of viral DNA into host genome.

Summary Table: Virus Structure Comparison

Feature	Non-enveloped Virus	Enveloped Virus
Structure	Nucleic acid + capsid	Nucleic acid + capsid + lipid envelope
Resistance	More resistant to disinfectants	Less resistant; destroyed by drying, heat, detergents
Attachment	Capsid proteins	Envelope spikes (glycoproteins)

Key Equations and Concepts

• Plaque Forming Units (PFU): Quantifies number of infectious virus particles.

• Reverse Transcription (Retroviruses):

$\text{RNA} \xrightarrow{\text{reverse transcriptase}} \text{DNA}$

Additional info: The above equation summarizes the process by which retroviruses convert their RNA genome into DNA for integration into the host genome.