L7 Transmission

Question 1

how can co evolution of host and pathogen be studied

Answer 1

studied in terms of paired phenotypic traits, such as resistance and infectivity

Question 2

what determines a virulent pathogen

Answer 2

Pathogens that reduce the survival/reproduction of their host

Question 3

what is the R0 number (basic reproduction number)

Answer 3

new infections

Question 4

what does r0 measure

Answer 4

• Fitness of a virulent pathogen is proportional to the # hosts infected

Question 5

what is virulence measured by

Answer 5

case fatality ratio

Question 6

why is extreme virulence selected against

Answer 6

susceptible population may die

Question 7

what is infectivity

Answer 7

o Propensity for transmission (modes of transmission)

Question 8

transmission routes

Answer 8

• Aerosol (via droplets)
• Direct contact (venereal, in-utero)
• Oral (via consumption)
• Vector-borne (via insect)
• Zoonotic (via another animal)

Question 9

How have changes in human populations over time affected frequency of different infectious diseases?

Answer 9

• HG > Agriculture
o Increased pop density (waste buildup), some farming methods (stagnant water), vectors (rodents, mosquitoes)
o Cross-species transmission via livestock
• Increase propensity of disease
• Increased pathogen exposure ‘crowd diseases’, transmission from animal reservoirs, changes to vector populations

Question 10

formula for R0

Answer 10

R0 = bN/(v+d+r)
Numerator (those likely to be infected)
• N= susceptible population
• b= prob. Infection (propensity of transmission)
Denominator (those no longer able to be infected)
• v= mortality due to pathogen (function of pathogen virulence)
• d= mortality not due to pathogen
• r = recovery and immunity

Question 11

explain use of isolation procedures in minimising epidemics

Answer 11

• most infectious post symptoms usually
• • HIV: isolation redundant as most infectious prior to symptoms
  • SARS: isolation prior to peak

Question 12

example of isolation: ebola

Answer 12

• Ebola = 2-21 day incubation time and mostly not infectious until symptoms appear
• Prevention strategy could incorporate isolation of likely affected or exposed individuals

Question 13

stages of disease origins and transmissibility

Answer 13

stage 1: agent only animals 
stage 2: primary infection (only from animals)
stage 3: limited outbreak 
(few human cycles)
stage 4: long outbreak (many human) 
stage 5: only human

Question 14

explain stages 1-2 movement

Answer 14

prob inc with freq. encounters

prob dec w/ inc phyla distance

inc pathogen variation help overcome species barrier

Question 15

explain stages 2-3 or 4 movement

Answer 15

change in behaviours= evolve adaption to new host , pop size, transmission efficiency

Question 16

what increases risk of movement from stages 2-3

Answer 16

phylogenetic relatnedness

Question 17

explain the Cross-species transmissions – influenza (cat 4)

Answer 17

• Cell entry
• Viruses use host molecules to enter cell – affects host range
o Eg Influenza A uses sialic acid α2-3 in aquatic birds and α2-6 in humans
o Partially utilise same protein as bird // less risk but still apparent

Question 18

can ebola sustain long term epidemic

Answer 18

imited cycling // usually dies out after variable time

Question 19

explain stage 5 of disease origins

Answer 19

lacs animals or environmental reservoir
persistence depends on duration of hosts infectivity, rate of new infection, rate of host protective immunity, population density