Viruses: Structure, Genome, and Infections

• Viruses are the smallest microorganisms capable of infecting animals, plants, and bacteria
• Viruses can cause severe acute oral and orofacial diseases
• Viruses can produce oral signs of systemic infection and be transmitted to patients and dental staff
• Viruses range in size from 10-100 nm, about one-tenth the size of a bacterium
• The viral genome contains either DNA or RNA, but not both, and can be single- or double-stranded, linear or circular
• The viral genome encodes all the genetic information for the virus
• RNA viruses are unique and rare in biology
• Viruses are metabolically inactive and are obligate intracellular parasites

Viral Structure

• Viruses consist of a nucleic acid core containing the viral genome surrounded by a protein shell, or capsid
• The entire structure is referred to as the nucleocapsid, which may be 'naked' or 'enveloped' within a lipoprotein sheath derived from the host cell membrane
• Some viruses (e.g., orthomyxoviruses, paramyxoviruses) start ensheathment via a budding process at the plasma membrane of the host cell
• Others, like herpesviruses, ensheath at the membrane of the nucleus or endoplasmic reticulum
• Protein units may go into prostructural units, which may be visualized as morphological units called capsomeres using electron microscopy
• The major bulk of the virion is protein, offering a protective sheath for the nucleic acid

Genetic Economy and Viral Nucleic Acid

• The variety of viral proteins is kept to a minimum, and viral genomes lack sufficient genetic information to code for a large array of different proteins
• In enveloped viruses, protein units that comprise the envelopes are visualized electron microscopically and are called peplomers
• Viral nucleic acid may be either DNA or RNA which may be single-stranded or double-stranded
• The RNA may be segmented or linked together

Viral Protein

• The viral protein offers a protective sheath and is made up of 2 or 3 different polypeptide chains although in some only 1 kind of polypeptide chain may be
• Viral protein may offer a special affinity to receptors and may bear antigenic determinants and epitopes

Viral Metabolism

• Viruses have a structural function, some have enzymatic activity
• Many viruses, such as HIV, contain reverse transcriptase
• In general, lipids and carbohydrates are only found in viral envelopes and are mostly derived from the host cells
• Forty to fifty percent of the viruses’ lipid content is cholesterol

Virus Symmetry

• Viral symmetry refers to the way the nucleocapsids of viruses are arranged
• The protein molecules are symmetrically arranged in the shape of an icosahedron
• The capsomeres surround the viral nucleic acid in the form of a helix or spiral to form a tubular nucleocapsid
• RNA viruses have this form of symmetry, and the nucleocapsid is arranged in the form of a coil and enclosed within a lipoprotein envelope.

Taxonomy

• Vertebrate viruses are classified into families, genera, and species
• Symmetry, protein units arranged, presence or absence of an envelope, nucleic acid composition, and the number of nucleic acid strands and their polarity, guide taxonomic classification
• 2 classes of infectious diseases and agents have been discovered: viroids, and prions

Viroids

• Viroids cause diseases in plants and comprise naked, linked, closed circles of single-stranded RNA, less than 300-400 nucleotides in length
• Despite their minute size, they replicate using host cell enzymes
• Viroids are not associated with human disease up till now

Prions

• They are generally discussed at the end of a textbook chapter

Viral Replication

• Viral replication processes in the replication cycle include
  • Adsorption
  • Penetration
  • Uncoating
  • Transcription
  • Synthesis of viral components
  • Assembly
  • Release of virions
• The period between infection and production is the eclipse or latent period

RNA Viruses

• The period between infection and the production of a new virion is about 3 hours or as long as several months or years
• Viral genome may be RNA or DNA and may use proteins, structural proteins, capsid spikes, and nonstructural protein enzymes

Viral Replication Details

• General viral replication steps may overlap (e.g., penetration and uncoating)
• Certain events occur in the cell nucleus, while others occur in the cytoplasm
• Orthomyxoviridae and Herpesviridae multiply in nucleus, while the Picornaviridae multiply in the cytoplasm
• The virion is the fully infectious, complete virus

Adsorption to the Host Cell

• Binding of a virus to specific receptors on the host cell surface
• The presence of receptors for the virus is required on the host cells plasma membrane

Replication and Nucleic Acid Attachment

• The RNA is of positive or negative polarity, there is a complementary difference
• Hemagglutinin and neraminidase are envelope proteins that attach certain viruses to host cells

Viral Penetration

• The process by which the virus or its genome enters the host cell and can occur through 3 mechanisms:
  • Endocytosis: most of the virions taken up by endocytosis appear to be degraded by lysosomal enzymes
  • Fusion: direct fusion of the viral envelope with the plasma membrane of cells, allows the nucleocapsid of some viruses to be released directly into the cytoplasm without an intervening phagocytic process
  • Translocation: non-enveloped viruses have th

Uncoating and Eclipse

• After penetration, the viral DNA is free to act as a template for the synthesis of viral mRNA
• After penetration, with no intact infectious virus, the 'eclipse phase' begins with uncoating of the lipid membrane and protein capsid surrounding the nucleic acid viral core
• When a virus initiates its reproductive cycle within the host cell, the host cell RNA is halted

Synthesis of Viral Components

• Viral proteins are synthesized at ribosomes
• Viral assembly is accomplished by incorporating viral nucleic acid into putative capsomeres-procapsids in the cell nucleus or cytoplasm
• Release may occur via budding

Prions and Prion Diseases

• Prions (proteinaceous infectious particles) are unique elements in nature and are the agents of a group of chronic diseases called prions or transmissible spongiform encephalopathies with long incubation periods
• Prions’ diseases infect nerve tissues of animals and humans, and manifest with long incubation periods that last up to decades
• Its infectious agent is difficult to destroy and is neither viruses nor viroids
• Prions do not have DNA or RNA, are of native/normal form and are constituent of normal healthy tissues designated as PRPc whereas disease related isoform derived from the latter is designated PRPsc
• The abnormal prion resists destruction, occurs in neural tissues, and is spongiform
• Prion diseases can be caused by a prototype prion agent; or the protein is high resistant to heat and chemical agents.
• They require special autoclaving and surgical procedures must also use disposable tools
• The prion agent can be transmitted to cows, minks, and cats

Details on Prion Diseases

• Latrogenic transmission of prion are found by medical personnel and even found on specific foods fed to mink and cat
• Kuru is typically called the Fatal Neurologic disease (1st).
• Acquired Prion are 10-90% familial in patients
• Creutzjeldt-Jakob disease has similar traits
• Variant Creutzjeldt-Jakob disease is usually localized to 1 country in europe
• Gerstmann-Straussler-Scheimker syndrome is also something to analyze
• In the area of transmissions they have very similar traits as well (medical facilities mostly)

Prevention of The Disease

• In this certain event that most or all of these circumstances occur we highly persuade a patient to use material to incinerate after doing dental precautions.
• In facilities the patient’s are usually a high risk towards family and friends Either use disposable instrumen or clean instrumen 18 in at 124C in
• Keep all instruments clean and sanitized, especially since tissue can dry fast
• Make sure the water supply is separate and only using one source
• Ensure the treatment will always need to be the last of the day/evening routine
• Know the standards and precaution’s in and out to a tee.