Evolutionary Dynamics of Infectious Disease

Question 1

SIR disease example

Answer 1

measles - oscillation, vaccine responsive

Question 2

SI disease examples

Answer 2

HIV, TB- reach equilibrium, can evade clearance so makes vaccination harder

Question 3

SIRS disease examples

Answer 3

covid, flu- antigenic diversity, requires this to be considered in vax strategy

Question 4

why doesn’t measles have issues with antigenic diversity?

Answer 4

the dominant target of immunity (hemagglutinin) is conserved due to structural constraints- it needs to bind to receptors

Question 5

why does influenza have antigenic diversity?

Answer 5

the major targets for the immune system are outside of the part which is required for binding

Question 6

what determines R0?

Answer 6

rate of new infections from an infected person * duration of infectiousness

Question 7

competitive exclusion in the context of R0

Answer 7

strain with a higher R0 will ‘win’, with no difference in virulence or antigenic type

Question 8

virulence impact on competitiveness

Answer 8

more virulent > less time infected as more likely to die > reduced competitiveness

Question 9

R0 and virulence

Answer 9

R0 may be maximised at intermediate levels of virulence, as it maximises both transmission rate and transmission duration

Question 10

example- how did myxoma virus evolve?

Answer 10

convergence towards intermediate levels of virulence (grade 3 on a 1-5 scale) after fully virulent strain was introduced

> same thing happened in australia and europe

Question 11

how can evolution towards virulence be encouraged?

Answer 11

imperfect vaccination- strains which can overcome the immune system have the advantage

Question 12

cross-immunity

Answer 12

extent to which immunity to one strain will help when exposed to a second strain- can encourage coexistence if it is low

Question 13

example of cross-immunity avoidance

Answer 13

B. parapertussis bacteria arising even though B. pertussis was endemic- O antigen on the LPS protects the new species from any immune responses to the old species

Question 14

what factors might determine if there is coexistence or one species only?

Answer 14

immune memory period, cross-protection, vaccine coverage

Question 15

example of cases where cross-immunity avoidance has caused issues with our vaccination strategy

Answer 15

strep. pneumoniae- a lot of differentiation of immune targets, so hard to create appropriate vaccines, 90+ capsule types in total with 15 causing most disease

Question 16

example of how virulence and transmissibility vary with serotype

Answer 16

in M. pneumoniae, can see significant differences in infection trends based on serotype

Question 17

standing diversity

Answer 17

fixed (ish) level of diversity of serotypes

Question 18

example of a bacterium avoiding physical competition

Answer 18

salmonella typhimurium- uses a carbon source that no other bacteria can

Question 19

type of diversity which can arise from resource competition

Answer 19

metabolic types- different strains using different sugars etc

Question 20

association between antigenic and metabolic types is often…

Answer 20

non-overlapping- idea that the strains need to differ on multiple levels to be in real competition

Question 21

how can vaccination alter non-vacine serotypes?

Answer 21

removing a strain from the population means others can take on the genes eliminated by the vaccine

Question 22

how can we determine what is causing lineage structures?

Answer 22

bioinformatics to predict what is associated with the different lineages, and see if it illicits antibodies etc

Question 23

how does immune selection act when there are a lot of antigens?

Answer 23

these antigens will self-organise into non-overlapping combinations, due to immune selection

Question 24

gamma- a parameter

Answer 24

measure of the degree of cross protection conferred by any related antigenic type- measures strength of immune selection

Question 25

types of strain structure

Answer 25

discrete- very easily a dominant strain
cyclical/chaotic- when there isn’t a single dominant strain- ‘flipping’, when immune selection is smaller