Gen Viro Chapter 3: RNA Viruses, Baltimore Classification & Medical Virology

RNA-Containing Virus

A virus utilizing ribonucleic acid as its genetic material, occurring as single- or double-stranded RNA, characterized by high mutation rates.

Baltimore Classification System

A virus classification framework created in 1971 by Nobel laureate David Baltimore based on genome type, strandedness, and mRNA synthesis pathway.

Genome Polarity (Sense)

The orientation of a single-stranded viral nucleic acid relative to the positive-sense (+) mRNA required for ribosome translation.

Ambisense Genome

A viral single-stranded genome that contains both positive-sense and negative-sense coding regions.

Baltimore Class I

Double-stranded DNA (dsDNA) viruses (e.g., Adenovirus, Herpesvirus, Poxvirus) replicating via classical semiconservative replication.

Baltimore Class II

Single-stranded (+) DNA (ssDNA) viruses (e.g., Parvovirus) replicating via a dsDNA intermediate, discarding the (-) strand.

Baltimore Class III

Double-stranded RNA (dsRNA) viruses (e.g., Reovirus) that transcribe (+) ssRNA from the dsRNA template.

Baltimore Class IV

Positive-sense single-stranded RNA (+ssRNA) viruses (e.g., Picornaviridae, Togaviridae) whose genome functions directly as mRNA.

Baltimore Class V

Negative-sense single-stranded RNA (-ssRNA) viruses (e.g., Orthomyxovirus, Rhabdovirus, Ebola) requiring virion transcriptase to synthesize (+) mRNA.

Baltimore Class VI

Positive-sense single-stranded RNA with DNA intermediate (Retroviruses, e.g., HIV) replicating via reverse transcriptase and proviral integration.

Baltimore Class VII

Gapped double-stranded DNA viruses with RNA intermediate (dsDNA-RT, e.g., Hepatitis B / Hepadnaviridae) replicating via reverse transcription.

Reovirus Genome Structure (Group III)

Contains 1 to 12 distinct double-stranded RNA segments, each coding for one or more viral proteins.

RNA Replicase (RNA-dependent RNA Polymerase)

A viral enzyme synthesized by Class IV (+ssRNA) viruses that copies viral RNA into a double-stranded replicative form (dsRNA).

dsRNA Replicative Form

A double-stranded RNA intermediate formed during (+) ssRNA replication that directs the synthesis of new (+) viral RNA strands.

Picornaviruses (Picornaviridae)

Small ($10^{-12}$), non-enveloped RNA viruses with an icosahedral capsid belonging to Baltimore Group IV, infecting vertebrates.

Picornavirus Genome Length

6.7 to 10.1 kilobases (kb) in length.

Picornavirus 5' End Structure

Positive-sense (+) ssRNA lacking a 5' methylated cap.

Picornavirus Naked RNA Infectivity

The purified naked (+) ssRNA genome alone is infectious.

Picornavirus (Acronym Etymology)

Pico (small, $10^{-12}$) + P (Polio) + I (Insensitivity to ether) + C (Coxsackievirus) + O (Orphan virus) + R (Ribonucleic acid) + NA (Rhinovirus/NA).

Diseases Associated with Picornaviruses

Common colds, non-septic (aseptic) meningitis, flaccid paralysis, and hepatitis.

Poliomyelitis

An acute, contagious viral infection caused by poliovirus leading to muscle weakness and irreversible, lifelong flaccid paralysis.

Inactivated Polio Vaccine (IPV)

The Salk injectable polio vaccine containing formalin-inactivated poliovirus strains.

Oral Polio Vaccine (OPV)

The Sabin oral polio vaccine containing live-attenuated poliovirus strains.

Poliovirus Transmission Route

Person-to-person spread primarily via the fecal-oral route, and less frequently through contaminated food and water (common vehicle).

Poliovirus Pathogenesis

Multiplies initially in the gastrointestinal tract (intestine) before invading the central nervous system to destroy motor neurons.

Global Polio Eradication Initiative (1988)

Global campaign launched in 1988 spearheaded by national governments, WHO, Rotary International, CDC, UNICEF, Bill & Melinda Gates Foundation, and Gavi.

Polio Global Reduction Milestone

Wild poliovirus cases decreased by >99% since 1988, falling from ~350,000 cases in >125 countries to single-digit figures.

Wild Poliovirus Type 2 Eradication Year

Certified globally eradicated in 1999.

Wild Poliovirus Type 3 Eradication Year

Certified globally eradicated in 2020.

Endemic Wild Poliovirus Type 1 Countries

As of 2022, endemic transmission of wild poliovirus type 1 remains restricted to Pakistan and Afghanistan.

Endemic

A disease or pathogen constantly present at a baseline, predictable level within a specific geographic area or population group.

Epidemic

A sudden, unexpected increase in the number of disease cases above baseline in a specific town, city, or region.

Pandemic

An epidemic that crosses international borders and spreads across multiple countries or continents.

Enterovirus Genus

A genus of non-enveloped (+) ssRNA viruses (Picornaviridae) classified into Polioviruses, Coxsackie A, Coxsackie B, and Echoviruses.

Coxsackie A Viruses (CA)

Enteroviruses frequently associated with hand, foot, and mouth disease (HFMD) and herpangina.

Coxsackie B Viruses (CB)

Enteroviruses associated with pleurodynia, myocarditis, and pericarditis.

Echoviruses

Enteric Cytopathic Human Orphan viruses; non-polio enteroviruses associated with aseptic meningitis and febrile illnesses.

ECHOvirus (Acronym Meaning)

Enteric Cytopathic Human Orphan virus.

Non-Polio Enteroviruses

Group A coxsackieviruses, Group B coxsackieviruses, echoviruses, and numbered enteroviruses.

Rhinovirus

The primary cause of the common cold, (+) ssRNA non-enveloped virus spread via respiratory droplets and contaminated fomites.

Rhinovirus Vaccine Status

No vaccine exists due to extensive antigenic diversity and numerous serotypes.

Foot and Mouth Disease (FMD) in Animals

The first identified animal virus, caused by Aphthovirus (Picornaviridae); infects cloven-hoofed animals and is rarely transmitted to humans.

Aphthovirus

The Picornaviridae genus responsible for Foot and Mouth Disease in livestock.

Hand, Foot, and Mouth Disease (HFMD)

A common, mild, self-limiting enterovirus infection occurring mostly in children, causing fever, oral sores, and blister rashes on hands, feet, and buttocks.

Severe Complications of HFMD

Aseptic meningitis, encephalitis, and acute polio-like flaccid paralysis.

Negative-Stranded RNA Viruses (NSVs)

Enveloped viruses with antisense (-) RNA genomes and virion polymerases, including Influenza, Measles, Mumps, RSV, and Ebola.

Antisense Strand (NSVs)

Complementary RNA that requires transcription into (+) mRNA; virions feature an outer lipid envelope derived from host assembly membranes.

Ebola Disease

A severe, often fatal viral hemorrhagic fever caused by Orthoebolaviruses, characterized by acute fever, intense myalgia, and internal/external bleeding.

Ebola Outbreak Pathogen: EBOV

Ebola virus (EBOV), causing Ebola virus disease (EVD).

Ebola Outbreak Pathogen: SUDV

Sudan virus (SUDV), causing Sudan virus disease (SVD).

Ebola Outbreak Pathogen: BDBV

Bundibugyo virus (BDBV), causing Bundibugyo virus disease (BVD).

Ebola Case Fatality Rate

Approximately 50% average fatality rate, historically ranging from 25% to 90% in past outbreaks.

Ebola Outbreak Control Package

Comprehensive intervention package including intensive clinical supportive care, infection prevention and control (IPC), disease surveillance and contact tracing, laboratory services, safe and dignified burials, targeted vaccination, and social mobilization.