Introduction to Virology

Question 1

What are the 3 types of capsid symmetry exhibited by viruses?

Answer 1

1. helical/tubular- most have an outer envelope (measles); RNA
2. Icosahedral– isometric/cubic; envelope (herpes) and no envelope (adenovirus)
3. Complex- doesn’t fit into other two groups (vaccinia/smallpox)

Question 2

What 4 factors is virus classification based on?

Answer 2

1. type and structure of the iral nucleic acid
2. strategy used in replication
3. type of symmetry of the virus capsid
4. presence or absence of lipid envelope

Question 3

What defines a viral:
family
genera
subtype

Answer 3

family: similar structure/genomic/replication properties; “-viridae’ ex. herpesviridae
genera: families breakdown into genera; “-virus” ex. HSV, CMV, Vericella Zoster virus

Subtypes: based on nucleotide sequence and anitgenic reactivities ex. HSV type 1, HSV type 2 etc.

Question 4

What are the common clinically encountered DNA viruses?

Answer 4

• Hep B
• HPV
• Parvovirus B19
• Adenovirus
• Herpesviridae
• Polyomaviruses

Question 5

What are the common clinically encountered RNA viruses?

Answer 5

• Influenza
• RSV
• Parainfluenza
• Hepatitis A,C,D,E
• Enteroviruses
• Encephalitis viruses
• Measles, mumps, rubella
• Norwalk, Rotavirus

*most others

Question 6

What are the 7 steps of viral replication?

Answer 6

1. adsorption
2. entry
3. uncoating
4. transcription
5. synthesis of virus components
6. assembly
7. release

Question 7

What virus is an exception in that it synthesizes its parts (nucleic acid + structural proteins) in the host cells cytoplasm instead of the nucleus?

Answer 7

Poxvirus

Question 8

What are 4 possible outcomes of virus-host cell interaction?

Answer 8

1. cell death (lytic)- due to cytophathic effect of virus
2. cell transformation - cell converted to malignant/cancerous cell
3. latent infection (occult)–persistent infection in quiescent state …can reactivate anytime to produce disease; continuous or intermittent shedding.
4. cell fusion to form multinucleated cells

Question 9

What are the 3 types of persistent viral infection?

Answer 9

1. chronic carrier- ex. Hep B, C and congenital Rubella, CMV—chronic illness characterized by continuous shedding for long periods
2. Latent infection- ex. herpesviridae—symptomatic or asymptomatic shedding
3. Slow virus infections- due to prolonged incubation period (measles, SSPE)

Question 10

Which viruses take the following amount of time for incubation:

hours to 1-2 days

1-3 weeks

weeks-months

months-years

Answer 10

hours to 1-2 d: respiratory viruses, GI viruses

1-3 wks: M/M/R, VZV, HSV, Chlamydia, Enteroviruses, Polio, WNV

weeks-months: Hepatitis viruses, HIV, EBV, Rabies

Months-yrs: Prions

Question 11

What 3 Igs are involved in viral infections?

Answer 11

IgM- earliest Ab produced, pentamer, formed 1 wk post infx, persists 4-6 wkes pi

IgG- months-yrs, immunity to re-infx bc of this

IgA- dimer, found in secretions, key for immunity post enteric infx, not measured for diagnosis usually

Question 12

How do the following act in fighting viral infx?

Th
Ts
Tc
Td
NK
IL-1, IL-2

Answer 12

Th- T helper cells–> stimulate CTL and activate B cells

Ts- T suppressor cells–> control and regulate the CTL by suppressing Th cells

Tc- cytotoxic T cells–> main effector cells that kill virus-infected cells

Td- delayed hypersensitivity T cells–> release macrophage activation factor

NK-direct killing of virus-infected cells

IL-1,IL-2: modulate immune response

Question 13

What are the 3 mainstays of diagnostic virology?

Answer 13

1. virus isolation (culture)
2. direct detection
3. serology (Ab/Ag)

Question 14

What is the diagnosis of acute (recent) infx based on wrt serology?

Answer 14

i) detection of IgM
ii) four-fold rise in Ab titre
iii) seroconversion
iv) very high single Ab titre (unreliable)