Summary Study Epi Flashcards
Attributable Risk (AR)- what is the question it answers? What is AR useful for?
How much disease amongst those exposed is due to the risk factor?
This is good for policy making because it gives an estimate of the absolute amount of disease associated with exposure. i.e. asbestos and dogs- reducing the exposure will not have a big impact on animal health in Australia because there are very few cases every year
Attributable Fraction of the exposed (AFexp)- what is the question it answers? When is AF useful?
What percent of disease amongst the exposed is due to the risk factor?
AF is useful because it conveys a sense of how much disease in the exposed population can be prevented by blocking the effect of the exposure or eliminating exposure.
Population Attributable Risk (PAR)- what is the question it answers?
How much disease in the population is due to the risk factor?
Population Attributable Fraction (PAF)- what is the question it answers? What does the magnitude of PAF for a given exposure depend on (2 things)?
What percent of disease in the population is due to the risk factor?
The magnitude of the PAF for a given exposure depends on:
1. the prevalence of exposure
2. The strength of association between the exposure and the outcome
** Even if we have an exposure that is strongly associated with an outcome, removing it will have little effect on disease risk in the population if its prevalence is very low
Pathogenicity
The ability of an agent to produce disease in an infected host
Virulence
The ability of an agent to produce severe disease
Incubation period
The interval between effective exposure to an infectious agent and the appearance of the first sign of disease in question
Latent period, also called? What is it?
Prepatent period. The interval between infection to shedding of the infectious organism.
Host
An animal capable of being infected with an infectious agent. Replication or development typically occurs in the host.
Carrier
An infected animal that harbours a specific infectious agent in the absence of discernible clinical disease and serves as a potential source of infection for other animals.
Induction period
The time from exposure to the agent to the first appearance of disease with a non-infectious agent e.g. cancer
What are drivers of disease spread?
Agent, host, environment
What are factors that affect a propagating epidemic curve?
The incubation period (can cause waves, delay start, prolong outbreak), infectious period, infectivity of the agent, the proportion of susceptible animals in the population, animal density, surveillance efficiency, reporting practices and the validity of diagnostic tests
What is antigenic drift?
An agent (such as influenza) that can mutate rapidly, evading humoral immune response, leading to antigenic drift- which produces new strains which may be more pathogenic or highly virulent. “Vaccine arms race”
What is antigenic shift?
Different strains of an agent (such as influenza) if they infect the same host can reassort and swap parts of its segmented genome, leading to new strains. This can lead to pandemics since the population will be naive and can also lead to a species jump.
What is the effective reproductive ratio (R)?
Represents the average number of secondary infections produced by each infected individual that enters a population that contains non-susceptible animals or is subject to disease control measures (i.e. as an outbreak progresses).
What are two factors that cause epidemics to die out?
A low rate of effective contact (c x p) and a reduced proportion of susceptible animals (St/N)
What effect does herd immunity have on an epidemic?
An increase in herd immunity leads to the peak of an epidemic can be delayed and /or the magnitude and duration can be greatly reduced
What is herd immunity?
The resistance of a population to attack by disease in which a large proportion of members are immune, thus lessening the likelihood of an animal with disease coming into contact with a susceptible individual.
What is the herd immunity threshold?
The proportion of the population that needs to be immune in order to reduce the incidence of disease
If herd immunity is greater than 75%, what does this mean for an infectious disease?
The infectious disease is often not able to propagate unless R naught is greater than 4.
What are the two parts of a good case definition?
- It specifies the characteristics of the population at risk
- It specifies what distinguishes cases from other members of the population
Why are rates preferred to ratios?
Because they are measures of risk and there is a time period specified.
Is incidence or prevalence the preferred measure of morbidity?
Incidence because prevalence is affected by both duration and incidence rate of disease, so direct interpretation of prevalence is difficult.
What is incidence risk? What type of population can incidence risk be applied to?
The probability that an individual animal will become a case (at least once) during a defined time period. This should only be applied to CLOSED populations.
(number of newly infected animals/ population at risk)
What is incidence rate? What type of population can incidence rate be applied to?
The number of new cases in a population per unit of animal-time during a given time period. (IR= number of cases in a defined time period/ animal units at risk during the time period).
Can be applied to either open or closed populations.
What is a crude rate vs. a factor specific rate?
A crude rate calculates for the whole population, however it is very rare that all animals in a population are equally likely of becoming a case. Usually there is some factor that changes the risk such as age, sex, or breed.
A factor specific rate is much more informative than a crude rate because it is a rate that is restricted to a specific factor, such as age or breed.
What questions does Case Fatality Rate answer?
How many of those that get the disease will eventually die because of it during a given time period?
What question does proportional mortality rate answer?
Given that an animal has died, what is the probability that it died of a specific cause?
What is a cause?
The presence of a combination of exposures that alone or in combination and in the correct sequence and timing during an individual’s life will inevitably result in an outcome (such as clinical disease).
What is association?
A quantitative measure of strength of the relationship between an exposure and outcome.
What is an example of a single factor disease?
Anthrax- Bacillus anthracis
Koch’s postulates
- The agent has to be present in every case
- The agent has to be isolated from the affected individual and grown in pure culture
- The agent has to cause disease when inoculated into a susceptible animal and the agent must then be able to be recovered from that animal and identified.
What are some limitations to Koch’s postulates?
What about multifactorial diseases?
What if the agent can’t be cultured in vitro?
Agents may have effects that are difficult to appreciate because of temporal delay, carrier states, and non-agent factor such as age, sex, immunity, etc.
What are Evan’s eight criteria?
E- comparing exposed and unexposed- proportion of those with disease should be higher in exposed
D- compared diseased and non- diseased- proportion of diseased should have a higher degree of exposure
S- comparing exposed and unexposed- studies should show that new cases are higher in the exposed
T- temporally, the disease should follow exposure to the cause
H- measurable biologic spectrum of host response
R- disease should be Reproducible experimentally
M- Preventing or Modifying host response should decrease or eliminate expression of disease
E- Elimination of cause should decrease incidence of disease
Comparing frequency of disease in groups of the population exposed and unexposed to potential risk factors does what?
It does NOT prove causation, however we can estimate causal effects of risk factors by contracting the association between exposure and the outcome (disease state)- as long as the comparison of groups is “fair” and any observed difference of risk between groups is considered an association, not necessarily causation
What is the 2 x 2 table for measures of association comparing?
Diseased and non- diseased (top)/ exposed and non-exposed (side)
What is the risk ratio or relative risk?
Comparing incidence rates. The incidence of the outcome in the exposed group divided by the incidence of the outcome in the unexposed group. A high risk ratio means that there is 10 times (for example) as much risk of disease in the exposed group vs. unexposed. It does not mean that the exposure is the CAUSE of disease, just that the ASSOCIATION has been demonstrated.
A risk ratio of greater than 1 indicates an association between exposure and disease.
A risk ratio of 1 means no association between exposure and disease has been demonstrated.
A risk ratio of less than 1 indicates a negative association (protection) between the factor and the disease
When would you use Odds Ratio?
An approximation of risk. You would use OR if only prevalence estimates are available. We cannot use incidence rates, but can compare odds of disease in groups with different exposures.
How are reference ranges estimated? What does it indicate if an animal falls outside of this range?
From the mean and standard deviation of the population. In normally distributed populations, 95% of the observations lie within 1.96 standard deviations of the mean. A measurement outside of this range only indicates UNUSUALNESS, not a definitive sign of disease.
If one test is performed on a healthy animal, what chance is there that it will fall outside the reference range? What if 5 tests are performed, what is the chance now? 8?
What does this tell us about testing animals?
If 1 test is performed on a healthy animal there is a 5% chance that it will fall outside the reference range. Five tests, 23% chance. 8 tests- 34% chance. Therefore tests need to be undertaken in a discriminating manner.
What is precision?
The ability of a test to give consisent results in repeated tests. The test may be wrong, but if it turns up repeatedly, the test has a high precision. A precise test is highly repeatable. A test can be precise without being accurate, but it cannot be accurate without being precise.
What is accuracy?
The ability of a test to give a true measure of the condition that is being tested or. A test can be precise without being accurate, but it cannot be accurate without being precise.
What are pathognomic vs. surrogate tests? What is an example?
Pathognomic tests are those for which the detection of a sign, substance, response, or tissue change is an absolute predictor of the presence of a disease or disease agent.
A surrogate test detect some secondary changes which is hoped will predict the presence or absence of a disease or the disease agent. For example, a positive culture of Brucella abortus from a cow’s milk sample is pathognomic for Brucella infection. Testing for antibodies to Brucella, however, is a surrogate test since it is not measuring the presence of Brucella abortus per se, but rather the body’s reaction to Brucella organisms or cross-reacting antigens.
Diagnostic sensitivity
The probability of a test correctly identifying a diseased or infected animal. Few false NEGATIVES.
Diagnostic specificity
The probability of a test correctly identifying a non-diseased or not infected animal. Few false POSITIVES.
How are DSe and DSp related?
Inversely
What does a DSe and DSp 2x2 table compare?
Diseased and non diseased (top)/ positive and negative (side)- + and - refer to test results. The four squares will end up with True Positives, False Positives, False Negatives, and True Negatives. So the diseased column will work out the DSe, the non-diseased column will work out the DSp, the positive row will work out PPV%, and the negative row will work out NPV %
What is true prevalence vs. apparent prevalence?
TP is the actual prevalence of disease in a population. Apparent prevalence is the estimate of disease prevalence worked out on the basis of an imperfect test.
How do we work out the DSe and DSp of a test?
By using animals in a test with known status.
When do you use a highly sensitive test? Example?
When you want to find infected animals. There is a high cost involved with calling a diseased animal negative. “Don’t let the killer out of jail.” Another way to remember ruling out- SNout. e.g. Quarantine.
When do you use a highly specific test? Example?
When you want to make sure a positive test is truly positive. There is a high cost involved with calling a non-diseased animal positive. Ruling-IN SPin- you want a low chance of a false positive. e.g. culling animals if diseased
What testing techniques can improve DSe and DSp?
Serial and parallel testing. Serial testing is multiple tests of the same kind, this will increase Diagnostic Specificity- fewer false positives. But it will decrease DSe therefore increasing the chance of a false negative.
Parallel testing is running different types of tests at the same time. This will increase Diagnostic Sensitivity- fewer false negatives. But it will increase DSp therefore increasing the chance of false positives.
Since no test can be taken as absolutely correct, what does the PPV and the NPV answers what questions?
The PPV is the positive predictive value. PPV answers the question, “If my patient returns a positive test, what is the probability that it really has the disease?”
And NPV, “If my patient returns a negative test, what is the probability that my patient really is free of the disease?”
What 3 values do you need to calculate the PPV?
DSe, DSp, and the probability that the animal had the condition prior to testing
When are tests worth doing?
When there is a greater than 50% chance that the animal has the condition prior to testing.
What kind of DSe and DSp do you want when screening?
Screening is used for early detection of disease. When you first start screening and disease prevalence is higher, you want to use a test with high sensitivity. In the later stages of the program, when prevalence has dropped, you want to change the test to a high specific test, so you get fewer false positives.
What is surveillance?
The systematic ongoing collection, collation,and analysis of information related to animal health and the timely dissemination of information for those who need to know so that action can be taken.
What are the main components of a surveillance system?
Design, implementation, analysis, interpretation, and action.
Objectives, hazard or health state under surveillance, case definition (who what when where why), target population(s), timing of sampling intervals, data management, methods for data analysis and triggers for action, feedback and dissemination of results
How much time do you have to report an exotic animal disease or anthrax?
Immediately calling DEPI
Serious but not exotic diseases- EHV1, Cattle Tick?
Notify DEPI within 12 hours
Less serious notifiable diseases- e.g. Johne’s disease, Salmonellosis?
Notify DEPI within 7 days
Passive vs. Active Surveillance
Passive surveillance is the secondary use of data that were generated for some other purpose. Usually feasible but subject to bias. Active surveillance is the generation od data for surveillance purposes to answer specific questions about disease on a population basis. Resource intensive.
General vs. Targeted Surveillance
General surveillance collects data for a range of diseases- known and unknown in populations. While targeted surveillance focuses on a single disease group and/or species.
Risk- Based Surveillance
Focuses on data collection in a high risk population and/or for a disease that is high risk. Conducting risk assessments to determine needs. This type is more efficient and effective allocation of resources.
Scanning Surveillance
General animal health surveillance- continuous watch over an endemic disease profile so that unexpected changes can be recognized.
Syndromic Surveillance
Tracks disease trends by syndromes based on clinical features rather than specific diagnosis. e.g. influenza tracking by looking for users that googled how to treat a runny nose
Sentinel Surveillance
Monitoring data from selected clinicians, institutions in the middle of population at risk or on the outskirts of endemic zones (e.g. sentinel chickens).
Rumour Surveillance
Rumours from media reports, professional groups, the public, etc. to follow up to confirm or refute them.
Participatory disease surveillance
Qualitative method of disease surveillance applying methods developed for working with rural farmers in regions of Africa and Indonesia, for example, lacking in vet and surveillance resources whereby a vet goes out and asks questions.
What code does Australia (or any country) have to follow to demonstrate freedom from infection to a specified level of confidence acceptable to trading partners?
OIE Terrestrial Animal Health Code
Surveillance trends include?
True disease trends + changes in reporting and diagnostic accuracy
What are the 3 types of bias in Epi?
Selection bias, Measurement bias (or information bias), and Confounding bias (alternate explanations for observed data- particularly when assessing associations between two variables, so not directly relevant when assessing crude incidence or surveillance data)
Why do outbreaks occur?
When the number of cases exceeds what is expected on the basis of past experience for a given population. e.g. Susceptible individuals travelling into an area that is endemic for an infectious disease, extension of endemic range of disease, new infectious disease introduced by humans or animals, contamination of food and/or water, mutation of an agent
Why do outbreaks end?
No more susceptible individuals (everyone at risk has been infected), exposure to the source has stopped (e.g. contaminated food has been consumed), vaccination has increased, agent mutates into a less pathogenic strain
What is Stevenson’s First Law of Vet Science? How does it apply to an outbreak?
Multiple clinical signs in an individual animal tend to arise from a single aetiology (single cause), while single syndromes in a population tend to have multiple aetiologies (multiple causes).
In a population, it is common for there to be more than one cause of a presenting syndrome. e.g. low milk yield in dairy cattle- genetics, mastitis, suboptimal nutrition, parasitism, etc.
What is an outbreak?
A series of disease events clustered in time.
What steps does an investigator and team take during an outbreak?
- Prepare for fieldwork
- Verify the outbreak (increase in public awareness, change in reporting procedures, change in case definition, etc)
- Verify the diagnosis (necropsies, clinical exams, samples and lab tests)
- Define, ID, and count cases (good case definition, consistency in case definition in order to allow the incidence of disease to be reliably measured, enhanced surveillance)
- Describe outbreak according to individual, place, and time (Map of area or pens with quarantined animals, calendar of vaccinations, policies for movement of animals, management practices that are relevant, collect history, plot epidemic curve by plotting cases and time)
- Formulate working hypotheses (a provisional diagnosis, type of outbreak, source of outbreak (single or multiple), possible modes of spread, possible risk factors that may be modified to prevent further cases)
- Evaluate and test your hypotheses (Attack rate tables, for example, to determine food that cause a point source outbreak, caculate population attributable fraction for each exposure- this will ID the percent of the risk of disease in the exposed group that is due to the exposure- the closer this value is to 100%, the more likely the exposure accounted for the outbreak)
- Conduct follow-up investigations (clinical, pathological, microbiological, and toxicological examination of tissues, feeds, objects, etc. Epidemiological follow up. Clinical trials on susceptible animals)
- Implement control and preventive measures
- Communicate findings and make recommendations
Eradication vs. control
Disease control implies doing something to reduce morbidity or mortality due to disease. Eradication implies doing something to cause an infectious agent to become extinct in a country, reduce prevalence of a disease so low that transmission cannot occur or is really rare, cause an infectious disease to become extinct.
What is elimination vs. eradication?
Eradication means no more disease globally. Elimination means no more disease in a defined geographic area as a result of deliberate efforts.
When containment is considered impossible or unfeasible, what is the only option?
Allowing a disease to become endemic.
Efforts to contain and control disease aim to do what? What do these efforts include generally?
Break the transmission cycle.
Preventing contact between agent and animals, stopping the production of the agent by infected or exposed animals, increasing disease resistance of susceptible animals to the agent
What are some specific measures that can be taken to contain and control a disease?
Movement controls, isolation and quarantine, vaccination (DIVA), humane slaughter and disposal, vector control, biosecurity measures (secure perimeter, personal hygiene, PPE, footbaths, vehicle dips, treatment of products and by products to reduce infectivity)
What are the criteria for assessing whether a disease is suitable for eradication?
Biological and technical feasibility (vaccine available, no animal reservoir, low ease of spread, demonstrated elsewhere)
AND political will and popular support (resources and will to get vaccine)
What do you need to know in order to control or eradicate a disease from the population?
amount of disease in the population, factors associated with its occurrence (sound epi understanding of the disease), facilities required to control the disease, costs and benefits involved (clear evidence of community benefits)
What made Rinderpest a good candidate for eradication?
Early diagnosed clinical signs, severe disease, close contact with infected animals- the main source of infection, recovered animals are not persistently infected, availability of suitable diagnostic tests, effective vaccine
What is the next disease for global eradication?
PRP or “goat plague”
What were the keys to successfully eliminating bTB from AUS?
Nat’l commitment and support from industry and govt, whole herd test and slaughter program, compensation to farm for culled stock, movement controls and quarantine, national bTB reference laboratory, database of cattle properties and lab testing results, prevention of establishment of bTB in potential wildlife reservoirs, ongoing surveillance at abattoirs to demonstrate regional and then national freedom
