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- Epidemiology is the study if frequency, distribution, and determinants of health and disease in populations
- (what is happening in a population, how its affecting the population, uses statistics)
What kind of relationships do the statistics of epidemiology study?
- discover and establisj causal relationships
- assosiations between ecposures to health outcomes uses many statistical tools
- but remember: correlation does not equal causation- just because it looks like here is a connection does not mean that exposure actually caused the disease (may just occur at same time)
- Occurrence of events (disease, impaired productivityu) in a population, clustered in time and/or space, in which incidence is significantly higher than usual
- -single case vs large # of cases
- -frequency of reporting varies according to owner's perception
- outbreak is a singe to 2 cases small outbreaks might break into and epidemic and then are recorded
Define Epidemic vs Endemic vs Pendemic
- Epidemic: affecting or tending to affect an atypically large number of individuals within a population, community, or region at the same time
- change =unexpected problem/ exceeded expected threshold
- -based on time sequence or temporal pattern of disease
- -large number of individuals, defined by the individuals in a population, in a place, in time
- **bell shaped curve Endemic: a disease present in a community, or population at all timesrestricted or peculiar to a locality or region
- in a population certain number of cases every year usually restricted to a region
- Example: Bovine upper respiratory diseases (shipping fever)- important endemic disease complex iof livestock in North America
- **So if there is suddenly a huge increase in cases and if it exceeds the expected threshold then it becomes epidemic
- Pandemic: around the world
What is the term used instead of "demic"
"zootic" so ie epizootic enzootoc
What did Dr. John Snow do?
- In europe people dumped chamber pots into earthern basements which created issues of disease outbreaks
- ie Collera- dehydrate to death
- Dr. Snow dotted maps of outbreaks and crossed were there were water lines on broad street and found that 87% of the deaths were clustered around broad st. pump. So the handle was removed
What are the Procedures for investigating and outbreak? What are the questions asked?
- Who, what, where, when , why, how
- WHERE: Examine the spatial pattern
- -detailed maps of the farm/region
- -herd in separate management groups
- -location at a specific point in time but avoid a bias sampling!
How are when and where related?
- temporal and spatial pattetrns of disease
- Temporal patterns- hiow the disease occurs over time, broadly categorized into epidemic and endemic disease
- Spatial patterns- dieases transmitted in clusters, indicaing transmission of infectious disease among animals in proximity or presence of risk factors at the location of the cluster
- look at number of cases of diseases by date of onset patterns over time
What are the two sources of data that are used by epidemiologists?
- Data already produced: gov or industry statistics, census, data reporting systems, medical records/ scientific publications, retrospective studies
- Active generation of data: surveys/ questionaires, descriptive or analytic studies, clinical trials/research, surveillance
- compare equivalant populations and follow up!
Frequency of Occruance: Define Incidence of Risk
- the number of NEW cases of disease occuring during a FIXED PERIOD of time divided by the number of animals at risk of developing the disease during that time ]
- (# of new cases/ # of animals at risk) at a fixed period of time
- deals only with NEW cases of disease (in previously disease-free animals)
- Only AT RISK animals are considered in the denominator (may be excluded if not at risk, not exposed, or already affected)
- ie after 12 months 121 dogs tested for worms and 25 test postive for worms = 25/121 = 21% so each dog has a 21% chance of being infected over a 12 month period in time
- The proportion of animals with a diseas at a point in time# of affected animals/ # animals in the population at that time
- -a "snapshot" that ID those that have the disease and those that do not
- NOT a measure of risk- since numerator includes those with a mix of durations (both recent and long-standing disease)
- ie. 15 out of 173 cows test positive for neospora so 15/173 = 9% so a randomly selected dairy cow form this her has a 9% chance of being infected at this point in time
- Cause: an event, condition, or characteristic which plays an essential role in producing an occurrence of the disease
- sometimes there are multiple knowledge about cause-effect relationships underlie every therapeutic maneuver in clinical medicine
- many methods for determining cause: personal experience, intuition, recommendations from others, scientific evidence (best way)
Criteria for establishing cause:
What are the Henle-Koch Postulates
- 1. The proposed "cause" has to be present in every case of the disease
- 2. Has to be isolated and grown in pure culture (ie rabies take brain samples and look in tissue culture and then grow that in mice) 3. Has to cause specific disease when inoculated into a susceptible animal
- 4. has to be subsequently recovered from the animal and once again identified
Criteria for establishing cause:
What are Hill and Evans criteria
- exposure should be more common in infected than uninfected animals
- time course- disease should follow exposure (not precede)
- experimental exposure should cause disease more often than it occurs in those not exposed (this means its not spontaneous)
- eliminating proposed cause should reduce incidence
- relationship should be biologically and epidemiologically plausable
What are the Aplication and Functions of Epidemiology?*****
- Prevention and Control of disease (life style related) Field of research for advanced scientific knowledge
- Public health and zoonosis
- Food safety
- Disease surveillance: important when going to do trade, need to know if actively looking International health standards do tests for trade standards
What is the iceberg concept of the probability of detection through surveillance of disease?
Disease is harder to find without aggressive surveillance (ie tip = death. then severe disease, mild illness, infection without disease, and exposure without infection is at the bottom of the ice berg in the water hard to see/ find)
What are some examples of applied epidemiological studies: Multifactorial nature of disease causation?
- contaminated water as a cause of cholera (John Snow 30 yrs before cholera discovered)
- Ticks associated with cattle disease 1890s (texas fever)
- -causual agent discoverd in 1984
- -ticks and disease had similar geog distributions
- *Disease CAN be prevented without knowing the causal agent
Give examples of applied epidemiological studies in Humans, Zoonotic disease, animals
- -use of flouride to prevent dental cavities
- -association between lung cancer and smoking
- -breast cancer cure- gave diff cancer
- Zoonotic disease:
- -association between intake of contaminated ecoli water and seven fatalities in ontario
- -determination of association of vaccines with feline sarcomas
- -trace backs to determine BSE + in birth cohorrts in Canada
What are the forms of epidemiology (approaches, focuses etc)
Clinical, molecular, genetic, social, global, industrial epidemiology etc
What are the two types of epidemiology?
What is Descriptive epidemiology?
- Describes population attributes
- -freq of disease (# of cases)
- -level of production
- -uses proportions, averages, means
- -basis to develop hypothesis, design studies, interpret results
- **Descriptive epidemiology deals with the questions Who, What, When, Where- describes the occurence of diseas in a population
- (here is what is happening, not sure whats going on yet)
What is analytic epidemiology?
- systematic evaluation of suspected association between exposure and outcome ie comparison group exists
- tests hypothesis about association between outcomes and exposure factors in the population using various approaches:
- 1. case control
- 2 . cross sectional
- 3. cohort studies
- 4. clinical trials
- **analytic epidemiology deals with the remaining questions of why and how- compares one group to another group to help determine the cause of a disease
- (determines what is going on)
A study (hypothesis) considers which two main factors?
- exposure: refers to factors that might influence one's risk of disease
- -exposure refers to any factor that might influence one's risk of disease- this could be a virus, a pollutant, genetics, or many other things
- outcome: refers to case definitions (the disease in question)
Define: Case Definition
- A set of standard diagnostic criteria that must be fulfilled in order to ID a person or animal as a "case" pf a particular disease
- Ensures that all animals who are counted as cases actually have the same disease
- Typically includes clincial criteria (lab results, symptoms, signs) and sometimes restrictions on identity, place, and time
How is a hypothesis developed?
- A hypothesis is an educated guess about an association that is testable in a scientific investigation
- descriptive data provide info to develop hypotheses
- hypotheses tend to be broad initially and are then refined to have a narrower focus
In sampling what are the hierarchy of populations?
- Target population: population to which the study results will be extrapolated (ie swine herds in North Am)
- Study population: individuals selected to participate in the study (ie 10 pig herds in NA)
What is a Cross-sectional study?
- one dimensional data
- data collected by observing many subjects (such as indivs, herds, or countries/ regions) at the same point of time, or without regard to differences in time
- analysis of cross-sectional data usually consists of comparing the differences among the subjects
- ie current obesity levels in a population of pet dogs
What is a cohort study?
- A cohort is a group of individuals who share a common characteristic or experience within a defined period (ie are born, are exposed to a drug or a vaccine)
- cohort studies are largely about the life histories of segments of populationsand the indivs who consitute these segments
- study involves and analysis of risk factors and follows a group of animals or ppl who do not have the disease at the start and uses correlations to determine the absolute risk of subject contracting the disease
- ie changing obesity levels in a population of pet dogs
Explain why correlation is not necessarily causation
- Cause: seems simple but confounding variables are there
- ie smoking causes cancer
- what about non-smoker with cancer?
- what about smokers who never get cancer?
- (look at all 4 parts of the equation disease with exposure, disease with no exposure, no disease with exposure, no diease with no exposure)
Correlation is not causation
What are the confounders?
- Reversed Time-order: a study may suggest that boy who play violent video games are more likely to engage in violent behaviour... but is is also possibelt that boys who engage in violent behaviors are more likely to play violent video games
- Chance: and association can be found in a single study due to chance, even if no true association exists
- •A confounder is another exposure that is associated with the exposure under study and is also a cause of the outcome under study.
- •So it looks like the association is between the exposure and disease under study, but the causal association is actually between the confounder and the disease under study. (look at chart in notes)
Explaine sampling error and measurment bias as ERRORS
- Sampling error: poor or wrong sampling design
- -Sample size: the lower the prevalence, the more samples are needed to detect disease
- -selection bias: (need to sample right population) study population not representative of the population-sampling some how over or under represents segments
- Measurement bias; the test used to detect disease may lead to a systematic error- test may not pick up all positives (false negatives) or incorrectly Id negatives as positives (false positive)
- -sensitivity: correctly IDing actual positives as positive (might not pick up all positives if not sensitive enough we dont want it to pick out only 70% of the positives)
- -vs specificity: correctly ID actual negatives as negatives (dont want false +s we want accurate test so somebody who doesnt actually have hepatitis is not diagnosed with it)
- There have been trade disputes over this becaue countries tweak which test they use so not all of the cases will be picked up
What questions are asked to critically weigh the evidence?
- 1. is the evidence consistent?
- 2. How strong is the association?
- 3. Is there a dose-response relationship? (more pathogen should cause higher rate of disease)
- 4. is the epidemiologic evidence cohernt with what was previosuly known? (study size large enough to be accurate)
- 5. experimental evidence- experiments in lab animals, or clinic trials (same/ the right condition?