PHPH Flashcards
What is epidemiology?
Study of the frequency, distribution and determinants of health-related states, and the application of such knowledge to control health problems
The occurrence of disease depends on an interplay between what factors?
Host, environment and agent
What is the difference between infection and infectivity?
Infection is when an agent enters a host and multiplies. Infectivity refers to its ability to infect an individual
What is the difference between prepatent period and incubation period?
Incubation= from infection to first signs of disease
Prepatent (latent)= infection to shedding
What is a common source epidemic? How about propagated epidemics?
Subjects exposed to common noxious influence and disease cases emerge over one incubation period
Propagated occurs when a case of disease serves as a source of infection for subsequent cases.
What is a sporadic disease?
Disease occurs infrequently and without a discernible pattern within population
What is an endemic disease? How about epidemic disease?
Endemic= diseases occurring at a predictable frequency within a population Epidemic= occurrence of disease affecting a number of individuals in clear excess of what would be expected for a population in a specific region and at a specific time.
What is the difference between prevalence and disease?
Prevalence is the proportion of cases measured once at a specific time point
Incidence is the frequency of new cases observed in a population in a specified time period
What is an attack rate and in what situations do we use it?
New cases amongst those exposed/ pop exposed at start of time period
Essentially it’s an estimate of incidence rate and is used in outbreak situations when you only have initial population data
What types of studies can incidence risk ratio not be estimated? Why?
Case-control studies because they don’t allow us to calculate risks
What is the difference between accuracy and precision?
Accuracy relates to the ability of a test to give a true measure.
Precision relates to how consistent the results are. A precise estimate means that your confidence interval is small.
What is the gold standard test for Johne’s disease?
Microbiological and histopathological examination of the small intestine
What is the difference between sensitivity and specificity?
Sensitivity= proportion of individuals with disease that test positive Specificity= proportion of individual without disease that test negative
What is the positive predictive value?
Proportion of individuals with a positive test that actually have the disease
What is the difference between apparent and true prevalence?
Apparent prevalence is the number of diseased individuals based on their test results (whether test be wrong or right)
What is the difference between parallel and serial interpretation of diagnostic test results?
Parallel= individual is declared positive if at least one of multiple tests come back positive. This increases sensitivity
Serial= individual is declared positive if all tests come back positive. Increases specificity
How may we increase the predictive value of a positive test?
Use the test on animals where prevalence is high
Use a more specific test
Use more than one test in series
What is the difference between screening and diagnostic tests?
Screening of healthy indivuals to detect disease.
Diagnostic tests used to confirm or classify disease status
How might different tests be used in disease control programs in terms of specificity and sensitivity?
Highly sensitive at start. As diseased animals die, true prevalence declines and so does the positive predictive value thereby increasing the proportion of false positives. Therefore, a highly specific test should be used in later stages
Briefly describe the two types of error that can occur in epidemiological research.
Random error. Occurs by chance. It is the inherent error that arises from using a sample to make a measurement of the situation.
Bias. Caused by systematic error (I.e. one that is inherent to the measurement technique being used). Can be divided into selection bias and mis classification bias.
What are the different types of selection bias?
Surveillance bias: disease is more likely to be detected in persons under frequent medical surveillance
Referral bias: referral patterns (for example in hospital)
Non-response bias: no response or refusal to participate
Length of stay bias:
Survival bias: ie. introduction of insulin leads to more diabetics surviving and an increased prevalence
What are the different types of misclassification bias?
Recall bias (better at recalling past exposures than non-events) Interviewer bias (leading questions) Prevarication bias (subjects may have ulterior motives for overestimating exposure (such as in compensation claims)) Improper analysis bias ( Obsequiousness bias (subjects alter their answers in the direction they perceive to be desired by the investigator)
How may misclassification bias be avoided?
- Ensure that exposure and disease are assessed independently (blindly)
- Use a rigorous and biologically valid method for determining presence of disease and exposure
- Use complete and detailed sources of info
- Use objective measures where available
What is confounding?
Distortion of the true association between an exposure and outcome due to the influence of a third factor
What three things must be considered when deciding on whether something is a confounding variable?
- Confounder must be causally associated with the outcome
- Confounder must be causally or non-causally associated with the exposure
- The confounder must not be an intermediate variable in the causal pathway between exposure and the outcome
What is effect modification?
Aka interaction. Present when the strength of association between an exposure and a given outcome varies with the level of a second exposure outcome (eg Tim tams are great, hot chocolates are great, Tim tams in hots chocolates are AMAZING)
What is the difference between a component cause, sufficient cause and necessary cause?
Component causes are those that are causally related to the outcome.
Sufficient causes are a set of conditions without any one of which the disease would not have occurred
A necessary cause is one that MUST be present
What are Hill’s criteria for causation?
- Strength of association (risk/odds in exposed vs that in unexp)
- Consistency
- Temporality
- One response relationship
- Plausibility and coherence
- Experimental evidence
- Specificity
- Analogy
What is an outbreak?
A series of disease events clustered in time
What are the four types of surveillance systems? Describe each, outing whether they are targeted or other.
▪️Risk based (targeted.involves risk assessment. Advantageous when there are limited resources)
▪️Syndromic (tracks disease trends by syndromes rather than diagnoses)
▪️Participatory (combination of local vet knowledge and conventional surveillance)
▪️Sentinel (carefully selected clinicians, institutions or sites that report in detail on selected diseases)
How may surveillance be classified?
Based on how data is collected (passive vs active)
Or classified by disease focus (general- collects info about a range of known and unknown diseases, vs targeted- focuses on a single disease group and/or species)
What are some typical activities carried out as part of an emergency response?
Movement restrictions, On-farm biosecurity, Quarantine
Enhancement of surveillance
Vector and reservoir control (where possible and appropriate)
Vaccination
Culling (test and slaughter)
In controlling disease outbreaks what do “biosecurity measures” entail?
Establishing a secure perimeter/ exclusion zone around infected or susceptible populations
Use of PPE
Property access issues (eg. footbath, disinfection etc)
Treatment to reduce clinical phase and restrict shedding
Improved husbandry and nutrition
What is the PICO approach?
Questions surrounding: Population intervention, Comparison/ control outcome
What type of study is most often done with the emergence of a new disease?
Descriptive studies (PO studies)
What are some advantages and disadvantages to doing descriptive studies?
Advantages: quick, cheap, fast turnaround, raises awareness, helps generate hypotheses
Disadvantages: no testing of hypotheses, limited evidence, low validity
What is an ecological study? What are some pros and cons?
Study where the unit of analysis is a group rather than an individual
Adv: quick and cheap, data is often readily available
Disadvantages: ecological fallacy (assuming that all animals on the farm are equal)
What is cross level inference?
Occurs when inferences are made at the individual level
In a cross-sectional study, what is the measure of health and association measured?
Health= prevalence Association= odds ratio or risk ratio
In a cohort study, what is the measure of health and association measured?
Health= incidence risk or rate Association= relative risk or odds ratio
In a case-control study, what is the measure of health and association measured?
Health= none. Not measuring a pop. Picking cases and non cases Association= odds ratio of exposure
What is surveillance?
The systematic ongoing collection, collation and analysis of information related to animal health and the timely dissemination of the information to those who need to know so that action can be taken
What is the difference between monitoring and surveilllance?
Monitoring- reg observing the health of a pop to identify trends
Surveillance- observing health/disease status of a pop and taking action when disease frequency measures a specified threshold
What are some reasons for why surveillance is important within a population? (6)
- Helps identify what diseases exist in a pop
- Estimation of prevalence (can set priorities for resource use)
- Planning, implementation and evaluation of control programs
- Detect emerging disease
- Meet international reporting reqs
- Demonstrate disease freedom
What components make up a surveillance system?
Clear objectives Hazard/ health state under surveillance Case def Target pop Timing of samples Data mngt Methods for data analysis Feedback and dissemination of results
What are the three major threats to internal validity of an epidemiological study?
Bias, confounding and chance
