Concepts of infectious disease epidemiology

Question 1

Unique features of infectious diseases

Answer 1

• dynamic system
  ~ Probability of new infections changes over time
  ~ infectious disease agents continually evolving (viral mutations, antibiotic resistance, pathogenicity change)
• infection dynamics are non-linear
  ~ E.g., vaccinating 50% of the population with an effective vaccine will reduce incidence of new infections by >50%
  ~ Interventions targeted at a proportion of the population will have an effect on those not targeted as well
• Threshold effects are present.
  ~ If 1 infected individual infects <1 other individual (on average) the infection will die out
  ~ The basic reproductive number = R0

Question 2

Important disease peramiters

• s
• i
• c
• p
• cp
• λ
• d

Answer 2

• S = the number of susceptible individuals in the population
• I = the number of infectious individuals in the population
• c = rate of contacts in a given time period (e.g., 10 fish per day)
• p = probability of transmission between a susceptible and an infectious individual (e.g., 15%)
• cp = rate of effective contacts (e.g., 1.5 fish per day)
• λ = “force of infection” (i.e., the rate a which susceptible animals become infectious)
• d = duration of infectious period

course of disease depends on these

Question 3

Basic reproductive number

Answer 3

The number of new infectious which arise on average from 1 infected individual
R0 = cpd

Question 4

Types of transmission

- verticle

Answer 4

transplacental or perinatal transmission from a mother to offspring

Question 5

Types of transmission
- horrizontal
~ indirect
~ direct

Answer 5

• Indirect transmission
  ~ E.g, vector borne (insects, milking machine, contaminated needle, etc)
• Direct transmission
  ~ Close contact (e.g., bovine tuberculosis)
  ~ Casual contact (e.g., many respiratory diseases)
  ~ Sexual transmission (AI falls into this category)

Question 6

Modes of causeation

- nessassry cause

Answer 6

= the disease does not occur without the factor

• Ness cause e.g. dog infected with canine influnza needs to be infected with the canine influenza virus
• Not ness cause e.g. Steph aureus not necessary cause of mastitis = lots of othe bac that can cause mastitis

Question 7

Modes of causation

- sufficient cause

Answer 7

= the factor always produces the disease

• However, very few factors are sufficient by themselves; rather, different groupings of factors can combine and become sufficient causes
• Suff cause e.g. not turning up to exam wiich 100% of grade = cause fail module always
• Not suff cause e.G genetics for ceoliac disease = other factors needed, enviro etc

Question 8

Modes of causation

- competent-cause

Answer 8

= one of several factors that, in combination, constitute a sufficient cause

Question 9

Time course of infection (basic)

Answer 9

• Susceptable
• time of infection
• latent period = infected but not capable of transmission
• infectious period
• non infectious
  ~ removed (dead, culled, sold)
  ~ recovered (immune, carrier, susceptable)

Question 10

Time course of disease (basic)

- if latent period shorter/same length as incubation

Answer 10

• susceptable
• time of infection
• incubation period
• symptomatic period
• non-infectious
  ~ removed (dead, culled, sold)
  ~ recovered
• shorter = period of time in incubation where animal is infectious
  = problem for disease control
  = asymptomatic contagiousness individuals shedding agent
• same = as soon as animal symptomatic, removed from herd

Question 11

Why are basic time courses unreliable?

Answer 11

= some infection dynamics more complx

e. g. johnes disease infection with Mycobacterium avium subsp. paratuberculosis (MAP)
- huge variation between individuals

Question 12

Johnes disease 2 cases how can differ in time courses

Answer 12

• incubation periods can be different
  ~ normally between 1.5 and 2 years
• transient phase times difffer = faecal shedding of MAP occurs in small quantities
• latent phase times differ = no shedding
• low shedding phase (some have some dont) and vary times
• high shedding phase (some have some dont) and vary times

Question 13

Progressors vs non progressors

Answer 13

Progressor = infection progressed into clinical disease

Non progressor = even though infected, never got clinical disease

Question 14

Why is the R* value not 1

Answer 14

= vaccines not useually 100% effective

• reduce the susceptibility of an individual and the probability of transmission from p to z1p
• reduce the duration of infectious period from d to z2d
• reduce the infectiousness of an infectious individual by a factor of z3

Question 15

R0 and vaccination

Answer 15

• For a vaccine with 100% efficacy, the effective reproduction number (R) is ~ R = R0 (1-f)
  ~ where f is the proportion of a population vaccinated
• Can use this equation estimate the critical percentage of the population that needs to be vaccinated to achieve herd immunity
  ~ we want to get the (effective) reproduction number below 1.
  ~ So if R0 is 5, what percentage of the population needs to be vaccinated to stop the outbreak?
  ~ R* = R0 (1-f)  1= 5(1-x)  x= 0.8
  ~ So we need to vaccinate at least 80% of the population