Medical Important Viruses Overview

Medical Important Viruses

• Viruses are infectious agents, minimally constructed of a genome (RNA or DNA, not both) and a protein-containing structure (capsid).
• Many viruses have additional features, like an envelope, a protein-containing lipid bilayer. This distinguishes different virus groups.
• A complete virus particle is called a virion.
• Viruses only produce in host cells.
• A virion can be viewed as a delivery system to surround a nucleic acid payload.
• The virus payload is the viral genome and may also contain enzymes needed for viral replication, a process that is intracellular.

Virus Structure

• Viruses are much smaller than cells (100-1000 times smaller).
• The smallest viruses have a diameter of 20 nm, while the largest are about 300 nm.
• They often pass through filters used to catch bacteria.

Viral Classification

• Viruses are categorized into families, subfamilies, genera, and species.
• Factors used in classification include: type and structure of nucleic acid, replication strategy, capsid symmetry (helical or icosahedral), and presence/absence of an envelope.

Viral Genome

• Viral genomes can be either DNA or RNA, and single-stranded or double-stranded.
• Common viral genomes are often single-stranded RNA.
• Single-stranded RNA is further subdivided into positive-polarity (mRNA sense) and negative-polarity (antisense). Positive polarity can directly be used as a template for protein synthesis.

Capsid Symmetry

• The capsid, the protein shell enclosing the genome, has either a helical or icosahedral symmetry.
• Helical symmetry is rod-shaped, while icosahedral is spherical.
• These structures are composed of multiple copies of one or more polypeptides.

Envelopes

• Many viruses have a lipid-containing membrane surrounding the nucleocapsid, which is called the envelope (a significant way to categorize viruses).
• Viruses lacking an envelope are called naked viruses.
• Envelopes are derived from the host cell's membranes and contain virus-specific proteins.

Viral Replication

• Viral replication follows steps including attachment to host cells, penetration, uncoating, gene expression, replication, assembly, and release.
• Gene expression and replication are followed by progeny virion assembly and release.

Viral Classification (Baltimore)

• Viruses are grouped into 7 categories based on their genome's structure and strategy. These are based on the nature of the nucleic acid and their strategies of replication.

DNA Viruses

• Three naked, icosahedral groups - parvoviruses, papovaviruses, adenoviruses – are categorized by size, with increasing particle size.
• Hepadnaviruses and herpesviruses are enveloped, icosahedral.
• Poxviruses have a complex internal symmetry, being among the largest.

Papovaviruses

• These are naked, icosahedral viruses with double-stranded circular supercoiled DNA.
• They include papilloma, polyoma, and simian vacuolating viruses (SV40).
• There are clinical significances of this group.

Adenoviruses

• Naked, icosahedral viruses with double-stranded linear DNA.
• They cause pharyngitis, respiratory disease, and cause sarcomas in animals.

Hepadnaviruses

• These are enveloped viruses with icosahedral nucleocapsids, a double-stranded DNA, which is unusual since its other strand is missing ~25% of its length.
• Hepatitis B is the human pathogen associated with this family.

Herpesviruses

• These are enveloped viruses with an icosahedral nucleocapsid and double-stranded DNA; the envelope has a unique shape.
• They include herpes simplex virus types 1 and 2, varicella-zoster virus, cytomegalovirus, and Epstein-Barr virus.

Poxviruses

• Large, brick-shaped enveloped viruses with complex symmetry composed of double-stranded DNA.
• Some causes of skin pox include smallpox (vaccinia virus is used for vaccine)

RNA Viruses (General)

• Reoviruses, a group of RNA viruses, have a double-stranded RNA genome.
• Many other RNA virus families share single-stranded RNA as their genome, with some linear, some circular in arrangement.

Picornaviruses

• Smallest of the RNA viruses with a single-stranded, positive-polarity RNA, non-segmented genome.
• Naked, icosahedral capsid.
• Major human pathogens are enteroviruses (polio) and rhinoviruses (common cold).

Caliciviruses

• Naked, icosahedral RNA viruses with single-stranded, positive-polarity RNA.
• Cause diarrhea among various human lineages (Norwalk virus).

Reoviruses

• RNA viruses, have a double-stranded RNA genome with 10 segments, linear.
• Respiratory enteric orphan virus, primarily causes diarrhea (Rotavirus)

Flaviviruses

• Enveloped, icosahedral RNA Viruses with single-stranded positive-polarity RNA, causing yellow fever, dengue virus, West Nile virus, and Japanese encephalitis.
• The family is highly diverse, with clinical significance.

Togaviruses

• Enveloped, icosahedral RNA Viruses with single-stranded positive-polarity RNA, causing eastern and western encephalitis diseases and rubella.

Retroviruses

• Enveloped, icosahedral RNA Viruses with a single-stranded positive-polarity RNA, with a reverse transcription mechanism.
• Human immunodeficiency virus (HIV) is the most significant pathogen.

Orthomyxoviruses

• Enveloped, helical RNA viruses with negative-polarity RNA in eight segments.
• Primarily cause flu (influenza virus), significant pathogen in human health.

Paramyxoviruses

• Enveloped, helical RNA viruses with negative-polarity RNA, nonsegmented.
• Measles, mumps, parainfluenza, and respiratory syncytial viruses are the most important pathogens in this group, with significances in human health.

Rhabdoviruses

• Enveloped, bullet-shaped viruses with helical nucleocapsid, single-stranded, linear, non-segmented, negative-polarity RNA.
• Rabies virus is only significant pathogen in this viral family.

Filoviruses

• Enveloped RNA viruses with helical nucleocapsid and single-stranded, linear, nonsegmented, negative-polarity RNA.
• Very severe pathogens include Ebola virus and Marburg virus, with significances in human health.

Coronaviruses

• Enveloped, helical RNA viruses with single-stranded, linear, nonsegmented, positive-polarity RNA.
• Respiratory tract infections like the common cold and SARS, MERS, and COVID-19 highlight their importance for human health.

Viral Diseases

• Viruses cause numerous diseases categorized based on symptoms and tissue or organ systems affected.

Molecular Tests and COVID-19

• Molecular tests detect the presence of viral RNA.
• Antibody tests detect presence of antibodies in the patient. Both can aid in diagnosis of the COVID-19 infection.

Viral Vaccines

• Various COVID-19 vaccines are available. including mRNA vaccines, inactivated virus vaccines, viral vector vaccines, and protein based.

COVID-19 Variants

• Several versions of the SARS-CoV-2 virus have been identified, and these mutations/evolution are significant.

Summary of virus classification

• Viruses are primarily categorized by their genome (DNA or RNA) and their presence/absence of an envelope.