Chapter 13-Viruses

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Chapter 13-Viruses
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General Characteristics of all viruses

• Contain a single type of nucleic acid
• Contain a protein coat
• Obligate intracellular parasites
• Are viruses the only obligate intracellular
  parasites?

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History began with the Tobacco Mosaic Virus (TMV)

• 1886 Aldolf Mayer showed that a virus was
  transmissable between plants
• 1892 Iwanowski tried to isolate it by filtering
  with porcelain filter

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Sizes of viruses
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Polyhedral virus

• Capsid coat made of capsomeres
• Nucleic acid inside

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Helical virus with an envelope

• The shape is a long rod
• Rabies and Ebola are helical viruses
• Influenzae virus is helical with an envelope

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Bacteriophage Complex virus
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Bacterial viruses

• Known as bacteriophages or phages
• Two different life cycles
• Lytic cycle-results in lysis of the cell
• Lysogenic cycle-may result in lysis of the cell
  or the virus becomes a permanent part of the
  chromosome by integrating

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Lytic Cycle
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Lytic Cycle
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Growth curve of bacteriophage
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Lysogenic Cycle
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How can you study bacteriophages?

• Plaque assay
• -pour agar with bacteria and phage on top of an
  agar plate
• -plaque develops where virus infected
  bacterial cell
• -each plaque is counted as one virus

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How do animal viruses differ from bacterial
viruses?

• Attachment
• Replication of nucleic acid
• Penetration
• Uncoating

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DNA Animal Viruses

• Adenoviridae
• dsDNA, non-enveloped
• First isolated in the adenoids
• Cause upper respiratory infections

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DNA Animal Viruses

• Poxviridae
• dsDNA, enveloped
• Cause small pox (variola)

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DNA Animal Viruses

• Poxviridae
• dsDNA, enveloped
• small pox virus (variola)

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DNA Animal Viruses

• Herpesviridae (dsDNA, enveloped virus)
• -simplex 1(cold sores)
• -simplex 2 (genital herpes)
• -chicken pox, shingles
• -epstein barr

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Herpes simplex-1

• HHV-1 causes fever blisters, HHV-2 genital herpes
• Symptoms fluid filled skin lesions
• Treatment Acyclovir

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Varicella (chickenpox) and Herpes Zoster
(Shingles)

• HHV-3 causes chicken pox and latent activation
  known as shingles
• Acquired by respiratory route, 2 weeks later see
  vesicles on skin
• Vaccine established in 1995 for chickenpox

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Epstein Barr

• Causes infectious mononucleosis
• Acquire by saliva, incubation period is 4-7 weeks
• Identify by
• -lobed lymphocytes
• -heterophile antibodies
• -fluorescent antibody tests

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Hepadnaviridae

• dsDNA, enveloped
• Hepatitis B
• -passes through intermediate stage (RNA)
• -three particles in blood
• Dane
• filamentous
• sphericle
• -exposure through blood/body fluids

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Hepatitis B

• Incubation period is 12 weeks
• 10 of cases become chronic, mortality rate is
  less than 1
• About 40 of the chronic cases die of liver
  cirrhosis

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RNA animal viruses

• Is there an enzyme in animal cells to replicate
  RNA?
• What does RNA polymerase do?

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RNA animal viruses

• () single stranded RNA viruses
• RNA serves as mRNA
• (-) single stranded RNA viruses
• RNA does not code for proteins

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Picornaviridae () ssRNA

• Poliovirus
• Virus ingested then travels throughout the body
• In some cases it impairs the upper motor neurons,
  less than 1 of all cases
• Vaccines
• Salk vaccine (IPV)
• Enhanced-inactivated polio (E-IPV)
• Sabin vaccine

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Cases of Poliomyelitis in US
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Picornaviridae () ssRNA

• Rhinovirus
• -causes the common cold
• -100 or more serological types
• -virus grows best in the nose and conjunctiva

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Picornaviridae () ssRNA

• Enterovirus responsible for 90 of viral
  gastroenteritis
• Rotavirus
• Most common cause of viral gastroenteritis
• Norwalk-like virus
• Responsible for local epidemics

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Rotavirus

• Note the shape which gave it the name
• rotawheel

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Picornaviridae () ssRNA

• Hepatitis A
• -obtain through fecal-oral route, enters GI
  tract and multiplies
• -incubation period is 4 weeks
• -symptoms include anorexia, malaise, nausea,
  diarrhea, abdominal discomfort, fever, and chills
  lasting 2-21 days

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Flaviviridae () ssRNA, enveloped

• Hepatitis C virus
• Obtain from blood/body fluids
• Incubation period averages 6 weeks
• Hard to screen blood for the virus
• 85 of all cases become chronic

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Rhabdoviridae (-)ssRNA, enveloped

• Rabies virus
• -enters the skin and multiplies in skeletal
  muscle and connective tissue
• -virus travels along nerves to the CNS causing
  encephalitis

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Pathology of rabies
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Orthomyxoviridae-multiple strands of (-)RNA

• Influenza virus
• Consists of 8 segments of RNA
• Envelope has H spikes (hemagglutinin) and N
  spikes (neuraminidase)
• Incubation is 1-3 days
• Symptoms include chills, fever, headache, muscle
  aches, may lead to cold-like symptoms

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Influenza virus
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Retroviruses convert RNA to DNA

• HIV, Hepatitis B

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Retroviridae-multiple strands of (-)RNA

• HIV
• -infects Helper T cells
• -requires the enzyme reverse transcriptase
• -integrates as a provirus
• -is released by budding, or lyses the cell

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Proteinaceous infectious particles PRIONS

• 1982 Stanley Prusiner proposed that there were
  infectious proteins
• Caused the disease scrapie in sheep
• Caused the mad-cowdisease in 1987
• Human forms suggest a genetic component

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Prions How do they replicate?
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Can viruses cause cancer?

• Michael Bishop and Harold Varmus received the
  1989 Nobel Prize for cancer-causing genes carried
  by a virus from animal cells
• Oncogenesgenes that can be transformed to cause
  cancer
• 10 of cancers have been found to be due to
  oncogenic viruses