U

• Viruses


  Energy-less basic life forms composed of a protein coat called capsid that surrounds the genetic material
• Viruses do not have organelles or ribosomes
• Viral genetic material is either DNA or RNA
• Viral replication


  Replication of genetic material occurs when the virus takes control of the host cell's synthetic machinery
• Virus contains the genetic information but not the enzyme
• Viral Morphology


  • Nucleic acid (single, double stranded, linear or looped, separate segments or one continuous strand)
  • Capsids (capsomere, icosahedral, helical)
  • Envelope (lipid bilayer)
• Classification of viruses


  • Nucleic acid type (DNA or RNA, double or single stranded, (+) or (-) stranded)
  • Capsid (icosahedral or helical)
  • Envelope (naked or enveloped)
  • Size (diameter of helical capsid, number of capsomeres in icosahedral capsids)
• DNA Viruses


  • Parvoviridae
  • Hepadnaviridae
  • Papovaviridae
  • Adenoviridae
  • Herpesviridae
  • Poxviridae
• Most DNA viruses are double stranded and show icosahedral symmetry, replicating in the nucleus
• Parvoviridae and Poxviridae are exceptions to the typical DNA virus structure and replication
• RNA Viruses


  • Togaviridae
  • Coronaviridae
  • Picornaviridae
  • Retroviridae
  • Caliciviridae
  • Reoviridae
  • Orthomyxoviridae
  • Paramyxoviridae
  • Rhabdoviridae
  • Bunyaviridae
  • Arenaviridae
  • Filoviridae
• Most RNA viruses are single stranded, enveloped, and replicate in the cytoplasm
• Reoviridae is an exception as it is double stranded
• Picornaviridae, Caliciviridae, and Reoviridae are non-enveloped RNA viruses
• Rhabdoviridae has helical symmetry but a bullet-shaped capsid
• Retroviridae and Orthomyxoviridae undergo replication in the nucleus
• Viral Replication


  1. Adsorption and penetration

  2. Uncoating of the virus

  3. Synthesis and assembly of viral products (while inhibiting host cell DNA, RNA, and protein)

  4. Release of virions from the host cell (by lysis or budding)
• Host Cell Outcome


  • Death
  • Transformation (activation of oncogenes)
  • Latent infection
  • Chronic slow infection
• Orthomyxoviridae (Influenza virus)


  Causes flu and pneumonia, spherical virions with two types of glycoprotein spikes (hemagglutinin and neuraminidase)
• Hemagglutinin allows the virus to attach to host sialic acid receptors, while neuraminidase disrupts the mucin barrier
• Types of Influenza Virus

  • Type A (infects humans and animals)
  • Type B and C (only infect humans)
• Antigenic drift (mutation in HA and NA)


  Causes epidemics of flu despite antibodies
• Antigenic shift (complete change in HA and NA)


  Causes more severe disease like the 1918 Spanish flu pandemic
• Influenza complications include secondary bacterial pneumonia and Reye's syndrome
• Influenza is controlled by vaccines and antivirals like amantadine
• Paramyxoviridae


  Similar structure to Orthomyxoviridae but with a single-stranded (-) RNA genome and a fusion protein instead of separate HA and NA
• Paramyxoviruses that cause human disease

  • Parainfluenza virus
  • Respiratory syncytial virus
  • Mumps virus
  • Measles virus
• Paramyxoviruses


  • Adsorb to and replicate in the upper respiratory tract
  • Most infections occur in children
  • Cause viremia and spread to distant sites
• Parainfluenza virus


  Causes upper and lower respiratory tract infections, including croup in children
• Respiratory Syncytial Virus (RSV)


  Major cause of pneumonia in young children, highly contagious with limited immunity
• Mumps virus


  Replicates in upper respiratory tract and spreads to cause parotitis, orchitis, meningitis, and encephalitis
• Measles virus (Rubeola)


  Highly contagious, causes prodromal illness followed by characteristic rash, complications include pneumonia, encephalitis, and SSPE
• Measles, mumps, and rubella are prevented by the MMR vaccine
• Hepatitis Viruses


  • Hepatitis A virus (HAV)
  • Hepatitis B virus (HBV)
  • Hepatitis C virus (HCV)
  • Hepatitis D virus (HDV)
  • Hepatitis E virus (HEV)
  • Hepatitis G virus
• Hepatitis A and E are transmitted via the fecal-oral route, while the others are transmitted through blood-to-blood contact
• Acute viral hepatitis


  Sudden illness with variable incubation period, systemic symptoms, and eventual jaundice as liver cells are damaged and release enzymes
• Subacute Sclerosing Panencephalitis (SSPE)

  Slow form of encephalitis caused by measles virus; occurs many years after measles infection (child or adolescent) may have slowly progressing CNS disease with mental retardation and incoordination
• Prevention of SSPE


  MMR (live attenuated vaccine)
• Hepatitis viruses


  • Hepatitis A virus (HAV)
  • Hepatitis C virus (HCV)
  • Hepatitis D virus (HDV)
  • Hepatitis E virus (HEV)
  • Hepatitis G virus
  • Hepatitis B virus (HBV)
• Transmission of hepatitis viruses

  Hepatitis A and E are both transmitted via fecal-oral route while the rest are transmitted via blood to blood (parenteral contact)