Unit 10: Outbreak Investigations

steps in controlling an outbreak

1-control exposure to the source of infection

2- prevention of further cases

3-access the severity of the problem and the risk

4- oppertunity for research and training

5- reassure the public

6-minimise disruption both economical and social

7- make reccomendations

role of research and training

learn more about the disease to prevent future outbreaks

define outbreak

more cases of a particular disease than expected ina. given area, or among a specific populaiton, over a particular period of time

systematic approach to investiate an outbreak steps

1- confirm existance

2-establish a diagnosis

3- deterine the magnitude of the problem

4- analyse the problem, who, when , where

5-develop a working hypothesis

6- evaluate the hypothesis

7- further data colection and analysis

8- communicate findings

how would you analyse the problem (who)

animal patterns e.g. sex, age, breed other risk factors

how would you analayse the problem (where)

look at clustering of cases and identiying a source and draw a sketch map

how would you analayse the problem (when)

estiate the incubation and exposure period and draw an epidemic curve

epidemic curve

graphical representative of the numbers of new cases in an outbreak or epidemic plotted over time

what can an epidemic curve tell you

-where you are in the course of an epidemic

-time periods of exposure

-mode of transmission

common source epidemic curve

cases of disease arising from a single common source such as contaminated food or water source

classes of common source epidemic curves

-point source

-continouing source

-intermittent

contangious spread epidemic curve

the disease spreads via contact with individuals

classes of epidemic curves for contangious spread

-index case

-propagated spread

point source- common source epidemic curve

cases all appear within one incubation period suggesting a single brief exposure that did not persist over time e.g. outbreak of diarrhoea from a bbq

continouing source- common source epidemic curve

population is exposed to an infectious agent but the exposure continues over a longer time e.g. a contaminated water supply that isnt fixed

intermittent - common source epidemic curve

outbreaks reoccuring due to a poorly controlled source, often seasonal or weather related and peaks are conparable in size

index case- contagious spread epidemic curve

a single index case infects other indivividuals and further cases arise after an incubation period

propagated- contagious spread epidemic curve

starts with an infection from an index case but develops into an epidemic wher secondary cases infect new indivudals and so on, successively taller peaks are seen seperated by the incubation period

use of further data collection

mathematical disease models can be used to stimulate disease spread, ientify possible interventions and then identify the costs of these control options

possible sources of infections for index farms for avian influenza

-wild birds

-entrance of new chickens

-shared equiptment with close by farms

-visitors

sources of transmission for avain influenza between farms

-equiptment

-people

-same source of infection e.g. wild birds

-rodents/animal vectors

epidemiolgoical triad

-host

-agent

-environment

epidemiolgoical triad host

animals infected, clinical signs, case definition

case definition

A set of standard criteria for deciding whether a person has a particular disease or health-related condition, by specifying clinical criteria and limitations on time, place, and person.

epidemiolgoical triad agent

causative agent and incubation period

epidemiolgoical triad environment

affected animals and environmetal factors involved

how could you test a hypothesis for avian influenza outbreak causation

-surveillance of wild birds

-tracing back to find links between farms

-obervational studies

ideal study for investigating a rare disease

case control

ideal study for investigating a rare exposure

cohort

ideal study to investigate impact of intervention

randomised-controlled trial

ideal study to investigate multiple exposures

case control

ideal study to investigate multiple outcomes

cohort

ideal study to estimate prevalance of disease in a population

cross sectional

control measures for avian influenza

-movement restrictions

-culling

-inspections

-serological tests

-PCR swabs

define specificity

ability of a test to correctly identify individuals who do not have the disease or condition being tested for

secificity equation

true negatives/ (true negatives + false positives)

sensitivity definition

ability of a test to correctly identify individuals who have the disease or condition being tested for

prevalence of infections causes variation in...

PPV and NPV

positive predictive value

the probaility that a positive test result is correct

positive predictive value equation

true positives / (true positives + false positives)

Negative Predictive Value definition

the probability a negative test is correct

negative predicitve value equation

true negatives / (true negatives + false negatives)

principles of risk communication

-create and maintain trust

-acknowledge and comnunicate even in uncertainty

-coordinate

-transparent and fast communication

-proacitve in public communication

-involve and engage those affected

-build national capacity and support

systems thinking as a loop structure

A systems behaviour emerges from the structure of its feedback loops • Root causes are not individual nodes, they are forces emerging from particular feedback loops