Epidemiology Flashcards
(27 cards)
Define incidence risk..
An individuals risk of developing disease within the period in question. Animal must be disease free at the beginning of the time period. Assumes a CLOSED population.
‘Apparent prevalence’ is a more appropriate interpretation of disease prevalence as opposed to ‘real prevalence’. Explain..
Real prevalence assumes results are based on tests that are 100% correct.. Most tests are not 100% correct - Apparent prevalence- can calculate true prevalence by knowing test characteristics and test results
Diagnostic tests: list some important factors to consider when evaluating a DT…
Evaluation of test, evidence (sufficient??), sensitivity/specificity.
Briefly explain the workings of a 2x2 table in relation to determining disease status..
Compares true disease status (diseased/non-diseased) with test results (+ve/-ve)… gives you insight into the sensitivity and specificity of the disease (i.e. true/ false -ve)
Explain sensitivity and specificity..
Measures test validity- how well does a test perform?
Sensitivity and specificity are inversely related – (Increasing Se decrease Sp) When do we need high Sensitivity or high Specificity?
High Se (high confidence in a negative result). High Sp (high confidence in a positive result).
Give an example of when you might need high sensitivity (high confidence in a negative result):
Diagnostic test for rabies – consequences of false-negative is very serious.
Give an example of when you might need high specificity (high confidence in a positive result):
Diseases affecting trade – need to make sure that positive results are truly positive!
Name two types of epidemiological studies and give an example of each:
Descriptive studies: case reports, case series and surveys
Analytical studies: experimental (intervention) and observational studies (comparison between groups of animals – statistical association of risk factors)
State the main components of a cohort study:
Basis for other designs. Disease-free at baseline. Longitudinal study. Quite expensive!!
State the main components of an observational study:
Natural environment of the animals. Measurement or observation. Prospective or retrospective
Briefly explain ‘experimental studies’..
Manipulation of the conditions of study (intervention)
•Laboratory study (what about environmental factors??)
•Field study
Briefly explain case-control studies:
Animals with disease (=cases) and without disease (=controls) and evaluate frequency of exposure to risk factors. Retrospective, good for rare diseases, inexpensive.
What is a cross-sectional study?
“Snapshot” of exposure and outcome. Looks at portion of the sample with outcome in comparison to portion of sample without outcome.
Basic reproduction number, Ro- (Infectiousness (Ro)= 1) This is a key concept in epidemiology and is defined as:
“the average number of secondary
cases caused by a single infected individual introduced into a naïve population”
What is the role of Ro?
The value of Ro determines whether an outbreak will take-off or not.. (i.e. Ro1, Ro=0)
If R0>1…..
the outbreak spreads.
If R0<1….
the outbreak declines
List five factors affecting Ro:
- Pathogen virulence
- Host susceptibility and genetics
- Population density
- Behaviour and contact rates
- Dose
We can calculate Ro directly and indirectly. Explain briefly how we directly calculate Ro:
- If we know who infected who
* eg contact tracing in FMD outbreak
We can calculate Ro directly and indirectly. Explain briefly how we indirectly calculate Ro:
- From time course of case numbers
- Statistical methods
- Modelling approaches
What are the benefits of calculating Ro?
Gives us a measure of the degree of control required, for either:
•outbreak control
•pre-emptive control such as vaccination strategies
State two factors that STOP an outbreak:
Reduced availability of susceptibles
Implementation of control
Control of disease does not require everyone to be vaccinated. True/False?
True!
