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1

CPP - Communicable Disease Control

LO1 - Understand the mechanisms and risk factors for infectious agent transmission.

  1. What is an infectious disease?

  1. Illness due to a specific infectious agent, (or its toxic products) that arises from an infected person, animal or reservoir to a susceptible host.

2

CPP - Communicable Disease Control

LO1 - Understand the mechanisms and risk factors for infectious agent transmission.

  1. How do we study infectious disease?
  2. What are the two unique featres in the epidemiology of infectious diseases?
  3. What is distrbution influenced by?

  1. Epidemiology - “The study of the distribution and determinants of health-related states or events in specified populations and the application of this study to the control of health problems”.
  2. A case may be a risk factor & People may be immune 

  3. Specific infectious agent

    Characteristics of the host population - Contacts become important

    Relationship between the infectious agent and the host

3

LO1 - Understand the mechanisms and risk factors for infectious agent transmission.

  1. What is INFECTIVITY?
  2. What is INFECTIVE DOSE?
  3. What is INFECTIOUSNESS?
  4. What is VIRULENCE?
  5. What is TRANSMISSIBILITY?

  1. Ability to enter, survive & multiply in host once infective contact has occurred
  2. Quantity of infectious agent required to cause infection. Varies with immune status of the host.
  3. Ease with which it is transmitted to another host
  4. Amount of pathogenicity that an infectious agent causes
  5. Probability of transmission given specified exposure to an infectious agent

4

LO1 - Understand the mechanisms and risk factors for infectious agent transmission.

Modes of transmission

List the 'Direct and immediate transfer' modes of transmission, and give clinical examples.

Direct contact eg. touching, biting, kissing or sexual intercourse

  • E.g. N. gonorrhoeae (from mucus membranes during sexual intercourse)
  • Rabies virus; (from a rabid animal through a bite or scratch)

Droplet spread - sneezing, coughing, spitting, singing or talking (usually limited to about 1 metre or less)

  • E.g. Influenza virus (droplet spread or direct physical contact)

5

LO1 - Understand the mechanisms and risk factors for infectious agent transmission.

Modes of transmission

List the 'Indirect transmission via intermediate means' modes of transmission, and give clinical examples.

  • Vehicle-borne - contaminated inanimate materials or objects, water, food, surgical instrument
    • e.g.V. cholera (from contaminated water or food)
  • Vector-borne -  agent carried by an invertebrate vector
    • e.g. Entoamoeba histolytica (from houseflies)
  • Air-borne - microbial aerosols, which may remain suspended in the air for long periods of time
    • e.g.H. zoster virus (chickenpox)

6

LO2 - Explain the concept of herd immunity.

Host population characteristics

  1. What must happen for an agent to be transmitted?
  2. What is transmission dependent upon?
  3. What must there be for an infectious agent to persist?
  4. What happens if sufficient individuals are immune to an infectious agent?

  1. susceptible individuals must be exposed.
  2. The number of infective and susceptible individuals present and Effective contact between individuals
  3. an adequate no. of susceptible hosts
  4. the remaining susceptible are protected by this population characteristic called herd immunity

 

7

LO2 - Explain the concept of herd immunity.

Host population characteristics

  1. What is herd immunity?

Immunity of a specified population, where a high proportion of immune individuals prevent exposure of susceptible hosts.

8

LO2 - Explain the concept of herd immunity

Herd Immunity

  1. How can vaccination reduce transmission?
  2. The higher the herd immunity (the proportion immune....?
  3. What will happen at the herd immunity threshold?
  4. What is the herd immunity threshold?
    1. give an example

Herd immunity

  1. by reducing the proportion of susceptible individuals (x) in a population and increasing the proportion of immune (1 – x).
  2. the less likely the infected case will make contact and transmit
  3. each case will only transmit to one other case
  4. the proportion of the population that need to be immune (1-x) for the disease to become stable (R=1).
    1.  HIT = (R0-1)/R0. 
      1. E.g. If R0 for measles is 9; can calculate the proportion that would need to be immune for outbreaks of measles to be avoided i.e. (9-1)/9=0.89=89%

(RO is the basic reproduction number, or the average number of secondary infectious cases produced by a single index case in completely susceptible population

9

 

How is Herd Immunity Threshold calculated?

 

Give an example of a HIT calculation

HIT = (R0-1)/R0 

E.g. If R0 for measles is 9; can calculate the proportion that would need to be immune for outbreaks of measles to be avoided i.e. (9-1)/9=0.89=89%

10

LO3 - Understand how to measure the transmissibility of infections.

Measuring transmissibility of infections

  1. What is Basic Reproductive Number (RO)?
  2. What is it influenced by?
  3. What does a higher RO mean?
  4. What does RO assume?

  1. expected number of secondary cases produced when a single primary case is introduced into a 100% susceptible population. 
  2. the number of contacts made by the infectious case during the infectious period – NOT constant for a specific agent. 
  3. the higher the RO, the higher the proportion of the population that will have to be vaccinated to achieve herd immunity.
  4.  that all contacts are with susceptible persons

11

LO3 - Understand how to measure the transmissibility of infections.

Measuring transmissibility of infections

  1. What is Net reproductive number (R)?

  1. average number of secondary cases produced by an index case introduced into a given population where some individuals are not susceptible

12

LO3 - Understand how to measure the transmissibility of infections.

Measuring transmissibility of infections

What can RO be conceptualised as?

R0 can be conceptualised as a function of 3 factors :

R0 ≈ c p d

c = number of contacts per unit time

p = probability of transmission per contact

d = duration of communicability period

 

 

13

LO3 - Understand how to measure the transmissibility of infections.

Measuring transmissibility of infections

R0 can be conceptualised as a function of 3 factors :

R0 ≈ c p d

  1. If individuals mix at random so that infectious individuals are as likely to make contact with susceptible individuals as with immune, then what can R be conceptualised as?
  2. Therefore, What if R=1?
  3. What if R > 1?
  4. What if R < 1?

then R is the product of R0 times the proportion that are susceptible (x)R can be conceptualised as:

R(t) ≈  R0x

X = proportion of a given population susceptible to infection

2. each case will produce one infective secondary case, The infectious disease incidence and prevalence in the population remains stable over time.

3. each case produce more than one infective secondary case and Incidence will increase over time

4. the generation of secondary cases is insufficient to sustain transmission of the infection within the specified population. Incidence of disease will decrease

14

LO4 - Understand the steps in an outbreak investigation.

Define the following

  1. Epidemic 
  2. Outbreak
  3. Pandemic

  1. the occurrence in a community or region of cases of an illness.. Clearly in excess of normal expectancy (Last 1995)
  2. Local increase
  3. Wide geographical area 

15

LO4 - Understand the steps in an outbreak investigation.

Outbreak Investigations

Consider Epidemics, Outbreaks, Pandemics

  1. What are these always relative to?
  2. What is surveillance?
    1. What does it involve?

  1. the number of cases you would expect in a specific population and time period (use surveillance data.)
  2. information for action’. 
    1. systematic on-going process of collection, collation, analysis and interpretation of health event data and dissemination of the resulting information on communicable disease and associated risk factors to ensure an appropriate and timely response.

16

LO4 - Understand the steps in an outbreak investigation.

Epidemiological characteristics

What are the following characterisitics and what can they show?

  1. Time
  2. Place
  3. Person

  1. when the disease occursTemporal clustering

    Seasonality, Cyclical patterns, Long-term trends

  2. where the disease occursSpatial Clustering and Geographical Restriction
  3. who develops itOccupational, Behavioural, Socio-economic

17

LO4 - Understand the steps in an outbreak investigation.

Types of outbreak

What doe the following mean?

  1. Point source
  2. Propagated 
  3. Extended 

  1. everyone exposed more or less at the same time

  2. agent transmitted person to person

  3. exposed to a common source over a period of days/weeks 

18

LO4 - Understand the steps in an outbreak investigation.

 

What are the steps in investigating an outbreak?

  1. Establish investigative team
  2. Background (Lit review)
  3. Establish existence of an outbreak
  4. Identify additional cases
  5. Collect preliminary data
  6. Characterise outbreak in terms of time, person, place
  7. Formulate hypothesis as to the source/mode of transmission
  8. Test hypothesis, reformulate & retest
  9. Recommend interventions
  10. Communicate findings