notes

Chapter Focus

• Unique characteristics of viruses

• Mechanism of viral replication inside host cells

• Clinical outcomes of viral infections in humans

• Major transmission routes of viral diseases

• Diagnosis, treatment, and prevention of viral diseases

• Emerging viral diseases

Characteristics of Viruses

• Size: Among the smallest infectious agents, ranging from 20 nm (polio) to 400 nm (pox virus).

• Non-living: Viruses are not considered living cells as they cannot perform metabolic functions without a host.

• Structure:

  • Nucleic Acid: Single type, either RNA or DNA.

  • Capsid: Protein coat that encloses the nucleic acid, made up of capsomeres.

  • Envelope: Some viruses have an outer lipid layer derived from the host cell.

• Infection Mechanism: Obligate intracellular parasites relying on host machinery for replication. They damage host cells, causing lysis or gradual death.

Structure of Viruses

• Virion: Complete virus particle; can be enveloped (lipid bilayer) or naked (no envelope).

• Capsid Functions:

  • Protects genetic material.

  • Responsible for attachment and entry into host cells.

• Envelope: Contains glycoproteins that assist in binding to host cell receptors.

Classification of Viruses

• Based on nucleic acid type, morphology, and replication method. Key classifications include:

  • DNA Viruses (e.g., Parvoviridae, Adenoviridae)

  • RNA Viruses (e.g., Picornaviridae, Retroviridae)

• Host specificity: Tendency to infect particular species or tissues based on viral attachment sites.

Viral Replication Steps

1. Adsorption: Virus attaches to host cell.

2. Penetration: Virus enters the cell via various methods.

3. Uncoating: Viral capsid is removed, releasing nucleic acid.

4. Synthesis: Host cell enzymes and ribosomes synthesize viral components.

5. Assembly: New virions are formed from synthesized components.

6. Release: Viruses exit the host cell by lysis or budding.

Pathogenesis of Viral Infections

• Viral infections lead to a variety of symptoms depending on the virus type and host responses. Common diseases include:

  • Common cold, chickenpox, hepatitis, HIV/AIDS.

• Severity and outcomes may vary significantly among individuals.

Host Response to Viral Infection

• Immune response includes:

  • Production of interferon.

  • Activation of T lymphocytes and B lymphocytes that produce antibodies.

  • Development of immunity leads to symptoms such as fever and malaise.

Viral Evasion Mechanisms

• Strategies include:

  • Hiding in host cells.

  • Mutation of surface proteins to evade immune detection.

  • Latency where viruses remain dormant then reactivate under stress.

Outcomes of Viral Infection

• Acute lytic infections: Clear symptoms; full recovery often provides long-lasting immunity.

• Subclinical infections: Mild or no symptoms, but may still provoke an immune response.

• Persistent infections: Chronic carrier states with ongoing viral replication (e.g., hepatitis B).

• Latent infections: Dormant viruses that may reactivate (e.g., herpes).

• Oncogenic effects: Some viruses can integrate into host DNA, potentially leading to cancer (e.g., HPV).

Transmission of Viral Diseases

• Transmission is mainly through body fluids, respiratory droplets, or direct contact. Types include:

  • Airborne: Virus shed in droplets.

  • Faecal-oral: Contaminated food/water.

  • Vector-borne: Insect bites transmitting viruses.

  • Vertical transmission: From mother to offspring during pregnancy or childbirth.

• Reservoirs: Many viruses originate from animal reservoirs (e.g., HIV, Ebola).

Diagnosis and Treatment of Viral Infections

• Diagnosis often based on clinical symptoms and serological tests for antibodies.

• Treatments primarily are symptomatic; antivirals exist but are limited. Understanding viral replication aids in drug development.

• Vaccines remain a crucial preventive measure against many viral diseases (e.g., influenza, HPV).

Future Directions in Virus Research

• Emerging viral infections continue to be identified, highlighting the need for ongoing research and adaptation of public health measures.

• Development of new vaccines and treatments in response to evolving viruses and changing conditions.