Flashcards in Epidemiology and Disease Deck (82)
What are the steps in disease investigation (Thrusfield 2007)?
- Observation and recording of natural occurrence of disease (basic parameters e.g. morbidity, mortality)
- Presentation and analysis of the observations
- Determine the cause of the disease (causal principles and approaches)
- Understand transmission, clinical signs, diagnosis ("disease ecology")
- Efficiency of different techniques to combat disease
Commonly found in, or restricted to a particular region, community or group of people. R0<1
Level of disease in a population significantly greater than usual R0>1
What is R0?
The average number of cases infected by a primary case i.e. R0>1 means that for every infected person, 1 or more people will get infected
How can R0 be used to assess degree of control over a disease?
When R0 reaches 1 or lower, this is a sign that we are gaining control over the disease and controlling its spread
What factors are considered in the observation and recording of the natural occurrence of disease?
- Infectious or contagious disease
- Prevalence and incidence
- Morbidity and mortality
- Sensitivity, specificity, positive predictive value, negative predictive value of diagnostic tests
- Gold standard of testing, accuracy
- Potential methods of disease control, prevention and eradication
What is included in the presentation and analysis of the observations in a disease outbreak?
- Number of cases and how this relates to the normal number seen
- Number/proportion of individuals at risk
- Distribution of the disease
- Rate of disease spread, timing of infection (calves vs adults)
What are Hill's criteria for establishing cause of disease?
- Time sequence (does cause precede effect?)
- Strength of association (apply percentages to potential causative factors)
- Biological gradient (dose-response relationship?)
- Coherence and plausibility
What approaches can be used to determine the cause of a disease in an outbreak?
- Case control studies
- Cohort studies
What are case control studies?
Comparison between cases and non-cases, looking retrospectively at the differences in exposure to risk factors
What are cohort studies?
Retrospective or prospective analysis of individuals exposed and not-exposed to risk factors, and see if they develop disease
What is an advantage of cohort studies over case control studies?
Larger population of all individuals that have potentially been exposed to the cause are assessed, providing a denominator to work with
How are cohort studies carried out?
- Start with a population, identify all those that have had contact with risk factors
- Observe outcome
How are case control studies carried out?
- Start with the outcome (and control group)
- Work backwards to identify possible risk factors
- Often then go onto cohort studies to get more precise information
What methods can be used to identify the strength of association of a risk factor, and this identify who is at risk in a disease outbreak?
- Odds ratio
- Relative risk ratio
What is an Odds Ratio (OR)? When is this mostly used?
- Odds of disease in an exposed group vs odds of disease in an unexposed group
- Commonly used in case-control studies
What is a Relative Risk ratio (RF)? When is this mostly used?
- Ratio of risk of disease in exposed group vs the risk of disease in unexposed group
- Commonly used in cohort studies
What is signified by an OR or RF >1?
Risk factor is associated with disease
What is signified by an OR or RF <1?
Risk factor is not associated with disease
What is the advantage of RF over OR?
Can be used to narrow down potential risk factors to the most likely
What information needs to be gathered initially in a disease outbreak scenario?
- Number of individuals affected
- Clinical signs
- Time to onset of signs
- Common activities
- Potential causes
Why is sub-typing of pathogenic microorgansism for investigation of disease outbreaks important?
- To be able to determine the source of the disease
- Confirmation if strain of the suspected cause and strain causing the disease are the same and therefore whether the suspected cause is the true cause
What methods can be used for subtyping genetic material for pathogenic identification?
Outline fingerprinting in subtyping of genetic material for pathogenic identification
- Comparison of "fingerprint" from case and potential source
- If they are the same, then this identifies the cause
- Done by trace comparison
How do molecular methods aid identification of pathogen?
- Amplify target pieces of pathogen DNA from case and source
- Can determine speciation (if strain unknown)
- Or strain identification (if known)
What are the advantages of sub-typing for pathogen identification in disease outbreaks?
- Increased sensitivity and specificity of diagnosis
- Good for organisms typically difficult to culture
- Outbreaks detected and controlled earlier
- Can provide evidence for causation in combination with epidemiological evidence
- Influence prevention or control programmes
Describe the framework for disease control
- Triad of agent, host and environment
- Knowledge of all factors is required in order to prevent , control and eradicate diseases
- Can identify actions in each part to control disease
Define disease prevention
- Avoiding disease occurrence
- Reduces the incidence and prevalence of disease
- Applies to all diseases
Define disease control
- Reducing the disease frequency to a tolerable level
- Reduces the prevalence of disease
- Mainly applies to diseases that are commonly found or seen