Microbiology: Virology

Introduction to Viruses

• A virus is best defined as an infectious, obligate intracellular parasite.
• Viral replication happens inside a host cell.
• The eclipse period in viral replication refers to the time after a virus infects a cell but before new virions are made.
• Viruses typically range in size from 20-300 nanometers.
• Transmission electron microscopy (TEM) is commonly used to visualize viruses.
• Anton von Leeuwenhoek first observed living microorganisms.
• Chamberland's porcelain filter removed bacteria from water.
• Dimitri Ivanovsky and Martinus Beijerinck discovered that the agent of tobacco mosaic disease passes through filters that retain bacteria.

Viral Structure and Classification

• The protein shell surrounding the viral genome is called a capsid.
• The viral envelope is composed of a lipid bilayer.
• A complete, infectious virus particle is called a virion.
• The size of the host cell is NOT a characteristic used to classify viruses.
• Nucleic acid polymerases synthesize nucleic acids.
• In the Baltimore classification system, mRNA is considered to be a (+) strand.
• Group I viruses contain dsDNA.
• Retroviruses use reverse transcriptase to convert their RNA to DNA.
• Helical symmetry is a type of capsid symmetry.
• The outer membrane of a virus is called an envelope.

Viral Infection Cycle

• The first step in the viral infection cycle is attachment and entry.
• Tropism is the term for a virus's ability to infect specific cells or tissues.
• Non-enveloped viruses typically enter a host cell through endocytosis.
• DNA viruses typically synthesize mRNA in the nucleus.
• Assembly is the process of assembling viral components into new virus particles.
• Enveloped viruses typically exit the host cell through both exocytosis and budding.
• The time between infection and when the host can spread the virus is called the latent period.

Viral Pathogenesis

• Pathogenicity is the term for the ability of a virus to cause disease.
• Tropism is the term for the capacity of a virus to infect selectively cells in particular organs.
• Increased cell division is NOT a mechanism of host cell damage by viruses
• Inclusion bodies are accumulations of viral proteins or nucleic acids within infected cells.
• Immunopathology is the term for tissue damage caused by the host's immune response to a viral infection.

Diagnosis of Viral Infections

• Methods for laboratory diagnosis of viral infections include virus culture, detection of viral nucleic acids, and detection of antiviral antibodies.
• An advantage of using electron microscopy for virus identification is that it is rapid and specific.
• Cytopathic effects (CPE) are changes in cells caused by viral infection.
• Serology methods detect antiviral antibodies.
• A four-fold rise in antibody titer indicates a current infection.

Specific Viral Families

• The African swine fever virus belongs to the Asfarviridae family.
• A notable characteristic of Asfarviridae is stability across a wide temperature and pH range.
• The Poxviridae family has a dsDNA genome.
• Poxviridae replicate in the cytoplasm.
• Contagious ecthyma (Orf) is caused by a poxvirus.
• A characteristic feature of Herpesviridae is latent infections.
• Bovine herpesvirus 1 causes abortion in cattle, encephalitis in cattle, and respiratory disease in cattle.
• Papillomaviridae have a circular dsDNA genome.
• Papillomaviruses are known to cause warts.
• Parvoviridae have a ssDNA genome.
• Canine parvovirus 2 causes enteritis in dogs.
• Parvoviruses are very stable in the environment.

Retroviridae and Other RNA Viruses

• A unique feature of Retroviridae is that they use reverse transcriptase.
• Reverse transcriptase is the characteristic enzyme of retroviruses.
• A provirus is viral RNA integrated into the host cell genome.
• Immunodeficiency and cancer are the main disease syndromes caused by retroviruses.
• A proto-oncogene is a gene that regulates normal cell growth.
• Cell transformation by retroviruses can occur through oncogene capture, insertional mutations, and oncogenesis mediated by essential retroviral proteins.
• Flaviviridae have a ssRNA genome.
• Bovine Viral Diarrhea (BVDV) is caused by a flavivirus.
• Pyrexia and oral ulcerations, diarrhea and decreased milk production, and immunosuppression are clinical signs of Bovine Viral Diarrhea (BVDV).
• A characteristic of Coronaviridae is tissue tropism switch.
• The tissue tropism switch observed in feline coronavirus (FCoV) is from intestinal epithelium to macrophages.
• Antibody-dependent enhancement (ADE) involves enhanced viral entry into immune cells.
• Feline infectious peritonitis (FIP) is caused by mutation of FCoV.

Prions and Microbial Control

• Prions cause neurodegenerative diseases.
• The key pathological change in Transmissible Spongiform Encephalopathies (TSEs) is a spongiform appearance of the brain.
• Scrapie is an animal TSE.
• The normal form of the prion protein is called PrPc.
• PrPsc differs from PrPc by its different folding structure.
• The most common mechanism of TSE transmission is ingestion of contaminated material.
• Sheep were affected by the first recognized TSE.
• Bovine Spongiform Encephalopathy (BSE) is also known as Mad Cow Disease.
• Variant Creutzfeldt-Jakob disease (vCJD) in humans is linked to BSE in cattle.
• Cervids (deer, elk, moose) are primarily affected by Chronic Wasting Disease (CWD).

General Virology

• A virus is an obligate intracellular parasite.
• Viruses replicate through the assembly of pre-formed components.
• The term "virion" refers to the infectious virus particle.
• Charles Edouard Chamberland developed the porcelain filter to remove bacteria from water.
• The protein shell that surrounds the viral genome is called the capsid.

Viral Infections and Treatment

• The first step in the viral infection cycle is attachment and entry.
• The primary purpose of viral vaccines is to prevent viral infections by inducing immunity.
• Live attenuated vaccines use a weakened form of the virus.
• Passive immunization is the use of preformed antibodies to provide immediate protection against a virus.
• Targets for antiviral drugs include viral attachment and entry, viral mRNA synthesis, and viral DNA/RNA replication.