Viruses, Viroids, and Prions

Viruses, Viroids, and Prions

Viruses Overview

  • Definition and Size:

    • Viruses are too small to be visualized with a light microscope and cannot be cultured outside a host cell.

    • Significant advancements in understanding viral diseases occurred only in the 20th century.

  • Historical Discoveries:

    • 1886: Adolf Mayer discovers tobacco mosaic disease (TMD) is transmissible.

    • 1892: Dimitri Iwanowski filters TMD sap and identifies an infectious agent passing through porcelain filters designed for bacteria.

    • First human disease associated with a filterable agent: yellow fever.

    • 1980s-1990s advancements in molecular biology led to recognizing viruses like HIV and SARS.

Types of Viral Diseases

  • Global infectious diseases include viral hepatitis, with various types (e.g., Hepatitis A, B, C).

  • The World Health Organization lists emerging viral pathogens including:

    • Human coronaviruses

    • Chikungunya virus

    • Zika virus

    • Thrombocytopenia syndrome virus

  • Human diseases caused by viruses will be discussed in Part Four of the study material.

Clinical Case Example

  • Case: An 8-month-old girl presented with fever (39°C) and runny nose.

  • The mother requests antibiotics, but the pediatrician denies prescribing them due to the viral nature of the symptoms.

  • Discussion required on why antibiotics are ineffective against viral infections.

  • Important References:

    • Virus structure (p. 363)

    • Multiplication of animal viruses (p. 376)

General Characteristics of Viruses

Learning Objective 13-1: Differentiate viruses from bacteria

  • Early concepts labeled viruses as contagium vivum fluidum (contagious fluid).

  • 1930s: Introduction of the term "virus" (Latin for poison).

  • 1935: Wendell Stanley isolates TMD virus leading to structural studies.

  • Characteristics Definitions:

    • Obligate Intracellular Parasites: Require living host cells to multiply.

    • Contrast with small bacteria (e.g., rickettsias) that may share properties with viruses.

Distinctive Features of Viruses

  • Structure and Function:

    • Contain either DNA or RNA (one type only).

    • Surrounded by a protein coat (capsid), which may have a lipid envelope.

    • Multiply by commandeering the host cell’s metabolics to synthesize new viral components.

    • Cause synthesis of structures to transfer viral nucleic acids.

  • Minimal enzyme presence means viruses depend entirely on host machinery, affecting antiviral drug development.

Host Range

  • Definition: Spectrum of host cells a virus can infect (invertebrates, vertebrates, plants, etc.).

  • Most viruses are specific to one host species;

    • Bacteriophages: Viruses that infect bacteria.

  • Host specificity is dependent on viral attachment mechanisms and cellular factors availability.

  • Complexities in using viruses for therapeutic benefits due to their narrow host range and potential for pathogen resistance.

Viral Structure
Learning Objective 13-2: Describe chemical and physical structures of viruses

  • A virion is the complete, fully developed infectious viral particle composed of nucleic acid surrounded by a protein coat.

  • Virus genetic material can be single or double-stranded; may be linear or circular, with widely varying lengths of nucleic acid (thousands of nucleotides possible).

  • Capsid and Envelope:

    • Capsid structure determined by nucleic acid type.

    • Capsomeres: subunits forming the capsid, visible via electron microscopy.

    • Envelopes consist of lipids, proteins, and carbohydrates; spikes may help in virus identification and host attachment.

Viral Size Specification

  • Viruses typically range from 20 nm to 1000 nm. Comparative sizes should be known:

    • Largest viruses comparable to small bacteria and sizes relevant to diseases in plant and animal hosts.

Morphological Classification

  • Classifications based on capsid architecture include:

    • Helical Viruses: Long rods (e.g., rabies, Ebola).

    • Polyhedral Viruses: Many-sided, commonly icosahedral shape (e.g., adenovirus, poliovirus).

    • Enveloped Viruses: Enveloped structures resulting from host membrane envelopment during release (e.g., Influenza).

    • Complex Viruses: Unique structures, particularly bacteriophages with double forms.

Taxonomy of Viruses

Learning Objectives 13-3 and 13-4: Define viral species, family, and genus

  • Classification systems have evolved from symptom-based naming to classifications by genomic methods. Families end in -viridae, and genera in -virus.

  • Viral species group share genetic information and their respective ecological niches (e.g., human immunodeficiency virus designated HIV).

Culturing Viruses

Isolation and Identification (Part II)

  • Respective challenges to cultivation include the need for living cells and methodologies for viral growth.

  • Animal Viruses: Cultured in live animals, embryonated eggs, or cell culture environments.

  • The method of establishing plaques for quantifying bacteriophage samples clarifies viral representations in isolated cultures and aids in diagnosis using serological tests and molecular methods.

Viral Multiplication Mechanisms

  • Viral replication chains, notably through the lytic and lysogenic cycles.

Specific Reproduction Processes

  • Multiplication primarily occurs in stages: attachment, penetration, biosynthesis, maturation, release.

  • Detailed stepwise processes defining the lytic and lysogenic cycles (e.g., T-even bacteriophages).

Viral Diseases and Oncogenesis

Oncoviruses and Cancer (Part III)

  • Cancer-causing mechanisms relate directly to viral infection and integration affecting oncogene activity in host cells.

  • Specific oncogenic viruses from families (e.g., Retroviridae, Papovaviridae) that modify host cellular machinery during the oncogenic process are significant.

Viroids and Prions

Definition and Differences

  • Viroids differ from viruses being simpler structures composed solely of RNA without a protective protein coat, affecting primarily plants.

  • Prions are infectious proteinaceous agents presenting distinct pathways for neurological damage in hosts.

This study guide details key characteristics of viruses, their lifecycle, implications for health, relevant clinical experiences, and their roles in viral diseases and cancer. Comprehensive understanding is structured in segments addressing definitions, classifications, and significant case studies inherent in virology.