The Viruses: Introduction and Major Groups

Question 1

Viruses

Answer 1

• nucleic acids surrounded by protein coat
• only able to replicate inside cells
• not living organisms on their own

Question 2

Origin of Viruses

Answer 2

• likely existed since beginning of life
• escaped nucleic acids
• co-evolved with cellular life

Question 3

Viruses in Nature

Answer 3

• found everywhere, millions in just one ml of seawater
• 8% of human genome is from viruses
• expansion of early human viruses dependent on agricultural revolution, allowed easier transmission of first viruses
• many have arisen through contact with animals

Question 4

Role of Viruses in Human History

Answer 4

• small pox
  
  • defeat of Aztecs and Incas
  • retreat of American troops from Quebec City
• influenza pandemics in 1918 and recently
• HIV currently a major viral ix in world

Question 5

Viral Morphology

Answer 5

Icosahedral - 20 sided shape of equilateral triangles
Helical - linear nucleocapsid surrounds genome
Cone-shaped
Complex

Question 6

Viral Genomes

Answer 6

• RNA vs. DNA
• ss vs. ds
• segmented vs continuous

Question 7

DNA Viruses

Answer 7

Adenovirus
Herpesvirus
Hep B
Smallpox
Parvovirus B19

Question 8

RNA Viruses

Answer 8

Polio
HIV
Influenza
Rabies
West Nile

Question 9

Viral Replication

Answer 9

1. Attachment - via receptors, determines specificity of virus
2. Entry - vis endocytosis, fusion, direct penetration
3. Uncoating - release of genome into cytoplasm
4. Replication - using host cell machinery to make viral proteins
5. Assembly - capsomeres assemble around genome
6. Release - non-enveloped do so by host cell lysis, enveloped viruses bud out of cell

Question 10

Viral Pathogenesis Determinants

Answer 10

• route of entry and viral tropism
• size of inoculum
• viral effects on host cells
• virulence of virus

Question 11

Host Factors Determining Pathogenesis

Answer 11

• host immune system
• host genetics - expressing right receptors
• physiological state - age, metabolic state

Question 12

Types of Viral Infections (6)

Answer 12

• Acute Ix eg. rhinovirus
• Acute Ix. w/ late complication eg. Measles and SSPE
• Latent Ix eg. Varicella and Zoster later in life
• Chronic Ix w/ shedding eg. HSV
• Chronic ix w/ late complication eg. Hep B and cancer
• Slow ix eg. CJD

Question 13

Host Immune Response to Viruses

Answer 13

• physical barriers
• innate immune system - macrophages, NK cells
• adaptive immune system - B cells produce neutralizing antibodies, T cells produce cell mediated immunity against virally infected cells

Question 14

Evasion of host immune system (5)

Answer 14

• modulate viral antigen expression, eg. downregulate expression or antigenic variation
• downregulate MHC expression
• integrate into genome
• infect cells that are not monitored by immune system (latent ix)
• infect cells of immune system

Question 15

Persistent Viral Infections

Answer 15

Chronic ix’s - balance between viral replication and host immune response, persisted low level infection

Latent ix’s - viral genome replicated with daughter cells, either in genome or extrachromosomal, reactivates only when permissive environment

Question 16

Norovirus

Answer 16

• non-enveloped
• icosahedral
• incubation 12-24 hr
• sx: watery diarrhea, vomiting, low grade fever, abdo cramps
• resolves in 12-72 hr
• viral shedding for 3 wks

Question 17

Norovirus epidemiology

Answer 17

• # 2 most common viral ix in developed world
• # 1 most common non-bacterial gastroenteritis
• often occurs in long term care facilities and hospitals, cruise ships

Question 18

Norovirus Transmission

Answer 18

• infectious dose 100 virions
• fecal oral transmission
• stable in environment for 24-48 hr
  
  • resistant to alcohol and low levels of chlorine

Question 19

Norovirus Pathogenesis

Answer 19

• asymptomatic and post-infectious shedders
• short term immunity to specific strains
• strong levels of antigenic drift and antigenic shift
• blood groups B and AB somewhat resistance to ix
• mutation in FUT2-alpha can confer resistance

Question 20

Norovirus Diagnosis

Answer 20

• collect stool/vomitus for diagnostics
• RT-PCR is best, see highest viral load within 48-72 hrs of sx onset
• also EIA and electron microscopy (low sensitivity)

Question 21

Varicella Zoster Virus

Answer 21

• Herpesviridae (HHV-3)
• dsDNA enveloped icosahedral virus
• causes both Varicella (chicken pox) and Herpes Zoster (shingles)
• highly infectious (most infectious Herpes virus)
• transmission via aerosols, droplets, contact
• Herpes Zoster is less infectious
• goes latent in nervous system (dorsal root ganglia)

Question 22

Chickenpox Pathogenesis

Answer 22

• ix of URT, viremia, ix of reticuloendothelial system, second virema, rash appears
• rash is due to viral damage to host cells and host immune system (macules, papules, vesicles, pustules, crusting)
• rash spreads in centripetal pattern (head to trunk outward)

Question 23

Pathogenesis of Shingles

Answer 23

• reactivation of VZV from sensory ganglia
• travel down neurons to epithelium, cause rash in single dermatome, resolves in 7-10 days
• associated with host factors (age, trauma, stress)
• can cause post herpetic neuralgia

Question 24

VZV Diagnosis

Answer 24

• important to differentiate VZV and HSV
• aspirate fluid from vesicles
• PCR is best
• use serology to determine immune status
• important to know immune status for: health care workers, pregnant women, immunocompromised individuals who have been exposed

Question 25

Chickenpox Management

Answer 25

• treatment: acyclovir, valacyclovir, famcyclovir
• post-exposure prophylaxis: vaccine within 3-5 days of exposure can reduce sx, immunoglobulin tx for newborns/high risk individuals
• live attenuated vaccine for children

Question 26

Shingles Management

Answer 26

• treatment: antivirals reduce sx and post-herpetic neuralgia if given within 48-72 hrs of onset
• VZV vaccine for over 60