Unit 4: Viral Virulence

Question 1

what is pathogenicity?

Answer 1

the ability to cause disease or not

Question 2

what is virulence?

Answer 2

the capacity of a pathogen to cause disease (symptoms within the host)
is a measure of pathogenicity

Question 3

what are some host factors for virulence?

Answer 3

• productivity
• age
• gender
• nutritional status
• species/ breed resistance
• immunity
• geographical location
• physiological stress

Question 4

what are some environmental factors for virulence?

Answer 4

• climate
• exposure to insect vectors
• pasture/ feed quality
• stocking density
• management and policies

Question 5

why are comparative assays used to assess viral virulence?

Answer 5

to determine if what is seen is due to differences in v.virulence factors/ genetic modifications rather than host differences or the experimental protocol

Question 6

what is used to quantify virulence?

Answer 6

LD50 - the dose of virus which kills 50% of the animals

Question 7

how would you assess viral induced damage?

Answer 7

• pathological lesions
• reduction in circulating CD4+ lymphocytes
• levels of liver enzymes

Question 8

what are the 4 classes of determinants of virulence?

Answer 8

1. affect the ability of the virus to replicate
2. affect host defence mechanisms
3. affect tropism, body spread and transmissibility
4. encode directly toxic products

Question 9

give an example of a virus whose virulence determinant affects the virus’ ability to replicate (class 1) and how this is achieved

Answer 9

alpha-herpesvirus
- encodes TK genes - associated with nucleoside salvage pathway
- TK mutation = effects growth in neurons = decrease in neurovirulence
- expressed abundantly in early infection (rapid proliferation)

Question 10

give an example of a virus whose virulence determinant affects host defence mechanisms (class 2) and how this is achieved

Answer 10

orf
- production of virokines = mimic cytokines etc = homology to IL-10 = immunosuppression
- production of viroceptors = homologs of host cell receptors

Question 11

give an example of a virus whose virulence determinant affects tropism, body spread and transmissibility (class 3) and how this is achieved

Answer 11

newcastle disease virus (paramyxoviridae)
- virulence determined by cleavage of precursor F0 into F1/F2 and recognition of monobasic or multi-basic cleavage sites
- viscerotropic velogenic = 100% mortality (multi-basic + F117 intracellularly), mesogenic = intermediate virulence, lentogenic = low virulence (monobasic + F117 intracellularly)

Question 12

give an example of a virus whose virulence determinant encodes directly toxic products (class 4) and how this is achieved

Answer 12

rarely seen, but rotaviruses
- NSP4 = enterotoxin = inhibits Na+-glucose lumenal co-transporter
- increase intracellular Ca by phospholipase C-dependent calcium signalling pathway induction

Question 13

what are the 4 modes of molecular variation?

Answer 13

1. point mutation
2. recombination
3. reassortment
4. selection

Question 14

what are some features of DNA viral replication?

Answer 14

• not as error prone
• hijack host DNA polymerases
• less diversity, evolve slower

Question 15

what are some features of RNA viral replication?

Answer 15

• error prone
• use of RDRP, lack proofreading + 3’-5’ exonuclease domain
• error rate of 1 in 104 - 105 per nucleotide
• coronavirus = virally encoded proofreading repair ability = 15-fold accuracy increase

Question 16

what general assumption could be made about the size of the genome and the mutation rate?

Answer 16

the larger the genome, the lesser the mutation rate

Question 17

what is a quasispecies?

Answer 17

highly dynamic, non-identical but related viral species. High variation seen in a species of virus due to high mutation rate

Question 18

what is the error threshold?

Answer 18

the point at which accumulated mutations reduce fitness

Question 19

what does LI50 help define?

Answer 19

the mutation frequency at which 50% of the viral genomes are lethally mutated

Question 20

what are the implications of quasispecies?

Answer 20

• unique challenges to the host
• at any given time, viral population reservoir of both genotypic and phenotypic variants
• variant differences in virulence, immune evasion, antiviral resistance and tissue tropism
• variant replication favoured with certain selective pressures

Question 21

give an example of a virus which shows intra-host tropism as an adaptation

Answer 21

HCV (hep C)
- found in many tissues and fluids eg liver, brain, semen…
- tissue-specific mutations in HCV IRES = alter translation efficiency
- variants adapt for certain cell/tissue

Question 22

describe the haemagglutination (HA) test

Answer 22

viruses receptor binding to sialic acid on RBCs
round button of RBCs = neg (no virus)
‘shield’/ lining of RBCs = pos (virus)

Question 23

describe the haemagglutination inhibition (HAI) test

Answer 23

virus mixed + inoculated with virus-specific antibody before RBCs added, antibody sticks + masks virus receptors
round button of RBCs = no HA = pos
‘shield’/lining of RBCs = HA = neg

Question 24

what are some common methods used to quantitate the number of infectious viral particles?

Answer 24

• end point dilution assay
• plaque assay

Question 25

what does a plaque assay determine?

Answer 25

the number of plaques formed relative to dilution of virus to determine the number of infectious units per ml of sample
pfu/ml

Question 26

what does an end point dilution assay determine?

Answer 26

the end point at which 50% of cultures are infected
TCID50/ml