Final Exam Epi Flashcards
Define epidemiology. What are epi’s two main uses?
The study of the frequency, distribution, and determinants of health- related states in populations, and the application of such knowledge to control health problems.
** Epi serves as an investigative tool and diagnostic tool to support preventive action against disease–> aimed to produce information for action
What is health?
A state of complete well being and not merely the absence of disease
What is One Health?
The collaborative effort of multiple disciplines locally, nationally, and globally to atain optimal health for people, animals, and the environment
How is the approach to epi slightly different than the traditional clinical approach?
The unit of concern is the population, as opposed to the individual. The “well” animals are just as important as the sick animal. The aim is prevention, not just treatment.
What disease has Australia successfully eradicated, making AUS the only major exporter in the world to do so? How?
Bovine Tuberculosis
*Recent history of ruminants on the continent
*Geographical isolation
*Strict biosecurity
* National commitment
* adequate compensation to farms for culled stock
* whole herd test and slaughter program
* database of cattle properties and laboratory test results
* prevention of establishment in potential reservoirs
* ongoing abattoir surveillance to demonstrate disease freedom
What does this indicate? Define the terminology.
Propagating Epidemic. An epidemic is the occurrence of a disease affecting a number of individuals in clear excess of what would be expected for the population in a specific region and period of time. An epidemic is about something changing that favours the spread of the disease.
** That is, the disease is clustered in SPACE AND TIME.
e.g. Ebola- movement of people meeting highly susceptible populations- so you get massive spikes.
When does this pattern occur?
Sporadic. The disease occurs infrequently and without a pattern in a population. E.g. Hendra
** The agent is usually maintained in:
- a reservoir host and only infrequently comes in contact with this host
OR
- carriers (hosts with inapparent infection)
** Clinical disease when factors tip the host-agent-environment balance
Endemic. Predictable pattern (long term trends, seasonal variation, cyclical variation). The disease is constantly present in a population in a given region, it is clustered in SPACE but NOT IN TIME. e.g. Ross River Virus- endemic mossie borne virus in Victoria- transmitted from native wildlife reservoirs to humans
** Stable longterm balance between host, agent, and the environment
** Source of major ongoing losses in animal production systems (parasites, mastitis, abortions)
** Environment important- disease in one region and not in another
Explain the importance of epi in veterinary practice
* Epi is about prevention
* Knowledge of disease transmission: Cat transmitting TB to humans and therefore disease control. Advising clients on how to ensure their family remains healthy, other animals in the home, neighbors/ friends animals are unaffected by their sick animal
* Herd immunity- encouraging vaccination in a non-compliant population therefore cutting down on parvo virus, for example
* Evidence based medicine- integrating the best research evidence with clinical expertise and each patients’ unique circumstances– epidemiology is the science underlying evidence based medicine
* Does a causal relationship exist? Is a key question we will answer everday when diagnosing, deciding on treatment, and of course prevention! Most infections have multiple causal components before the injury or disease can occur
What is an EID?
Emerging Infectious Disease
* newly recognized in a population
* known for some time but is rapidly increasing in incidence or range
* Mutation, mixing, and trafficking of pathogens
* Encroachment on wild animal habitats previously untouched– “spill over”
** 60% of EIDs are zoonoses and 72% of these are from wildlife reservoirs
Point source epidemic. Clustered in SPACE AND TIME. e.g. Cholera- one water point contaminated or food borne outbreak at a birthday party (poor food handling practices)
** steep up-slope and gradual down slope because no further susceptible animals at risk
What are the factors that tip the balance towards disease?
What is infectivity?
Ability of an agent to infect. Says nothing about the severity of disease.
What is pathogenicity?
The ability of an agent to produce disease in an infected host. (Virulence and infectivity).
What is virulence? What is the equation?
The ability of an agent to produce severe disease.
What is an EAD?
Emergency Animal Disease. An animal disease that requires an emergency response. It may be:
* Exotic- has penetrated quarantine barriers (e.g. Rabies)
* Emerging- start within Australia, previously unrecognized (e.g. Hendra)
* Re-emerging- known to occur in Australia, but now spreading more widely than previously (e.g. Bluetongue)
What is biosecurity?
Measures to protect populations of people and animals, plants, and ecological systems from infectious disease and other biological threats.
What is the incubation period?
The interval between effective exposure to an infectious agent and the appearance of the first sign of the disease in question.
What is the latent or prepatent period? What two periods is the animal in when in the “danger period?”
The interval between infection to shedding of the infectious organism.
* In the danger period in incubation period and latent period– shedding disease before clinical signs and symptoms
What is the iceberg effect?
Within a population, subclinical/asymptomatic disease is often the big problem. e.g. persistent carriers shedding disease constantly
What is the host?
Animal capable of being infected. Replication/ development of the agent typically occurs in the host.
What is the vector?
Animate object that transmits infection (insects with vector borne viruses)
What is a vehicle of transmission?
Inanmiate object that transmits infection e.g. fomites
What are carriers?
Infected host without clinical signs, potential source for others. The carrier state may be unapparent throughout infection (healthy or asymptomatic carrier) or may occur during the incubation period (an incubatory carrier in the danger period) or recovery phase for an animal with clinically recognizable disease (convalescent carrier).
What is a reservoir?
Source where agent normally lives (wildlife/soil/water)
What is a TAD? Examples?
Transboundary animal disease. Diseases that cross political or geographical borders.
* Serious socio-economic or public health consequences
* Major importance in trade of animals/animal products
e.g. FMD, Bluetongue, African Swine Fever, Newcastle Disease
What factors cause an epidemic to occur?
* A gross imbalance in favour of the agent:
* new strain (mutation)
* naive host population (viral trafficking)
* environmental shift (climatic change)
What factors shape a propagating epidemic curve? 6 of them!
* Incubation period: delays commencement and prolongs outbreak, waves may correspond to incubation period
* Infectious period: affects number of effective contacts by each infected host
* Herd immunity: > 75% of population resistant to infection, disease won’t propagate, cycling each time susceptible increase
* Density: influences number of contacts, effectiveness of contacts
* Infectivity of the agent
* Surveillance efficiency, reporting practices, and the validity of diagnostic tests
What can be a propagating epidemic that looks like a point source epidemic?
Short incubation period can mimic a point source epidemic
What is an epidemic curve? How is it useful?
A graph of the count of NEW CASES plotted against time on the X axis. It can provide useful info about the nature of causal agents.
Why do diseases often “cycle?” What do you always have to consider that has nothing to do with increase disease incidence?
Initially, the entire population can be susceptible, eventually when >75% of the population is no longer susceptible because they have had the disease, recovered, and are now immune due to vaccination, the disease can no longer propagate. However, eventually the population of susceptible animals MAY increase again and another outbreak can occur. In endemic situations, it is just maintaining itself in the population at a constant but relatively low frequency due to contact with susceptible animals… though there would be levels or resistance to disease present in the population due to antibodies, genetic resistance, etc.
** Always consider surveillance efficiency as it can falsely show an increase in number of cases
Does virulence make sense for an agent? What are two key factors? 2 examples of diseases on opposite ends of the spectrum.
* Over time the relationship between agent and host often moves from parasitic to commensal
* Balance between host and agent, in a given environment
* If illness impairs disease transmission: virulence does not favour the agent
* However, if severe illness or death does not impair transmission e.g. massive increase in shedding- Anthrax and “sit and wait” strategy then it does not impair transmission OR e.g. Adenovirus- causes mild URT infection in dogs- the ability of the host to move is critical because the virus labile in the environment – the dog requires close proximity to other dogs to transmit
** key factor is mode of transmission and resistance in the environment
What allows Influenza to cause pandemics?
Antigenic shift–> 2 different strains infecting the same animal. They can swap parts of their genome to produce a completely new SUBTYPE, therefore the population is now 100% naive.
* Antigenic drift allows the virus to create a new strain and jump species, but not as likely to cause a pandemic as there will be some cross over with immunityin the population from old strains likely
What are the millennium development goals?
- Eradicate extreme poverty and hunger
- Achieve universal primary education
- Promote gender equality and empower women
- Reduce child mortality
- Improve maternal health
- Combat HIV/AIDs, Malaria, and other diseases
- Ensure environmental sustainability
- Global partnership for development
In regards to descriptive studies, why are studies based on prevalence and studies based on incidence not equivalent?
Studies based on prevalent cases are less clear about causation then those based on incidence.
* Prevalence studies consider factors contributing not only to the disease development but also the survival and/or duration of disease.
*While incidence studies examine only factors that contribute to the development and therefore causes of disease.
What is R-naught?
Basic reproductive ratio. R-naught represents the number of new (secondary) infections that arise, on average, from the introduction of one infected individual into an entirely susceptible population (e.g. at the beginning of an epidemic), assuming random (non-preferential) mixing. IT is the MEASURE OF TRANSMISSION POTENTIAL OF AN INFECTION.
** It depends on the number of contacts between individuals in a population (in each time period), the probabily of transmission (per contact), and the duration of infectiousness (infectious period).
** If R-naught> 1 then an outbreak is expected to spread, if R-naught < 1 then, on average, each infected individual infects less than 1 other individual and the epidemic should theoretically die out
What are the drivers of spread?
* Basic reproductive ratio
* Herd immunity and vaccination
What are the measures of disease?
* Case definitions
* Proportions, rates, and ratios
* Prevalence and incidence
* Mortality rates
What is true about R-naught across a population?
It is not uniform across a population, it is influenced by:
What is the effective reproductive ratio? Why do epidemics die out?
Rt represents the average number of secondary infections produced by each infected individual that enters a population that contains non-susceptible individuals or is subject to disease control measures (e.g. as an outbreak progresses).
** Epidemics die out because of decreased effective contact (c x p) AND decreased susceptible proportion (St/N) – Not typically because of a change in virulence of the organism.
What is fcp? How do you determine fcp? What is the fcp of Equine Influenza?
Critical percentage for vaccination to prevent infection. It is the HERD IMMUNITY THRESHOLD. It is the proportion of the population that needs to be immune in order to reduce the incidence of disease.
What is herd immunity?
The resistance of a population to attack by disease to which a large proportion of members are immune, thus lessening the likelihood of an animal with disease coming in contact with a susceptible individual
** you do not need to vaccinate every individual in order to prevent an epidemic… if a population contains less than 25% susceptible animals, it is often not possible for an infectious disease to propagate UNLESS R0> 4. A new wave will only occur when the proportion of susceptible animals builds up again.
When do you use this equation?
If a proportion (f) of the population is vaccinated with a vaccine that is fully protective, then this will calculate the reproductive number (R).
Why do we measure health (disease events)?
To answer questions like: is there an unusual problem with disease? Which animals are at greatest risk?
What is PAR? What is important to keep in mind about PAR? Give an example.
Population at risk. Consider counts of cases in context of the size of the population at risk.
example: “I had 10 calves die of respiratory disease lask week”– if there are 20 calves in the group, that is terrible! If there are 40,000– that’s good!
What two parts does a case definition have?
What is an example of a case definition?
- It specifies the popuation at risk
- It specifies what distinguishes cases from the rest of the population
Proportion equation and example?
a/n
* e.g. 60 of 500 chickens were affected, proportion affected was 0.12 or 12%
Ratio equation and example?
a/ (a+n)
* 8 foetal deaths/ 100 live births= 8/108= 7.4%
What do rates measure? Equation and example?
Why are rates preferred to ratios?
Rates measure the number of occurrences of events in a specified population at risk during a given time period.
The denominator should be based on some form of animal-time units (defined periods of time that one animal is at risk)
* Example: 54 cases of Salmonellosis are reported per 100,000 people per year in Australia.
** Rates are preferred to ratios because they are a measure of risk due to the addition of the time dimension
What is prevalence? Is it a rate or a proportion? What can increase prevalence? What can decrease prevalence? And why can a decrease in prevalence be bad?
The proportion of cases (new and old) in a population at a specific time. It depends on the incidence and duration of infection, so measures of incidence are preferred.
** It is a proportion (no time dependence)
** example of increased prevalence: factors that increase the duration of disease such as an imporvement in a treatment regime for a chronic disease that cannot be cured Diabetes mellitus in dogs
** example of a decrease in prevalence- increase in virulence of a virus, more rapid death (a decrease in prevalence is not always good!)
What is incidence? Is it a proportion or a rate? What are two factors you need to specify? Why is incidence preferred over prevalence?
The frequency of new cases of disease observed in a specified population in a specified time period; must account for both population size and the time that each subject observed.
** a proportion
** Incidence is preferred over prevalence because you need to visit at least twice to determine the animlas that are free from disease and then the second time to assess the number of new cases during the trial period. Prevalence is easier to measure because you only need to visit once to see who appears well and who is sick, but since prevalance is affected by both duration and incidence rate of the disease, DIRECT INTERPRETATION IS OFTEN DIFFICULT.
How do you calculate prevalence? What affects prevalence?
Incidence and duration of disease
When would you use the following equation? Example?
Used for closed populations.
Example: If 20 cats in a cattery housing 100 cats develop respiratory disease with ulcers due to feline calicivirus over a week. Incidence risk after 1 weeks is 20%
- 2nd week (if risk is constant over time): 20% of the remaining 80 cats (16 cats) would have succumbed to infection and the total (cumulative) incidence risk would have risen to 36% (since now 36/100 cats have respiratory disease)
When would you use this equation? How can you be more specific?
In an open population– accounting for those that enter and leave throughout the followup period (births, purchases, sales and deaths)
** you can be more specific by including the mid-point population
When would you use the following equation? When would it be fairly accurate?
In an outbreak situation initial population may be all the data that is available. Attack rate is an approximation of the incidence rate.
* Accurate if the population is stable over the time period which is usually okay since outbreaks are often a short duration of time
How do you calculate the Mortality or Morbidity rate?
Use the incidence rate equation
How do you calculate crude mortality rate? What is it?
Incidence rate in which a case is a DEATH. As deaths occur, the population at risk goes down- you can only die once.
What are you determining when you use this equation?
How many of those that get the disease will die from it? VIRULENCE
When is this equation used? What question does this “rate” NOT answer?
Used when investigator lacks proper denominator data
Does not answer the question: What is the risk of an animal in a population dying of a specific cause?
How do you measure morbidity? Which is the most common measure of morbidity?
Prevalence, Incidence risk, Incidence rate, and attack rate.
* Incidence rate is the most common measure
How do you measure mortality? What is the main measure of mortality?
Mortality rate, Case Fatality Rate, Proportion Mortality Rate.
* Mortality rate is the main measure of mortality
How does epidemiology help distinguish cause from association? (3 main points)
* Provide information to help understand:
- what factors are involved in causal pathways to disease?
- the relative importance of each factor as a determinant of disease
- allows interventions to be targeted at the most important determinants (the most efficient use of resources!)
What are the conditions of a cause?
- Must precede the effect
- Can involve host or environmental factors
- Can be either positive (presence of an exposure causes disease) or negative (protective, e.g. vaccination)
What is confounding? Example?
An alternative explanation for the association between a factor and an outcome
e.g. ice cream consumption and drowning= summer time
Is identification of association quantitative or subjective? What about causality?
What is a component cause? Example?
Pieces of the pie.
A, B, C, or D.
e.g. Factors such as smoking, high cholesterol, lack of exercise, genetics and the presence of concurrent disease are all component causes of coronary heart disease in humans.
What are sufficient causes? Example?
The set of conditions wihtout any one of which disease would not have occurred.
A+B+C or A+ D
Example: Pasteurella spp., respiratory virus and stress are all component caues of respiratory disease in calves. Two of these factors are sufficient to cause disease.
What are necessary causes?
A is present in both (all) causal complexes
* a single component cause may necessary, sufficient, neither or both
* A component cause may be described as THE CAUSE when its removal renders the rest of the component causes insufficient
What does it mean for a cause to precipitate? Reinforce?
* When a cause precipitates: exposure to a specific disease agent tips you over the edge to disease
* When a cause reinforces: repeated exposure may aggravate an established disease state
How can you demonstrate the sufficient and necessary causes of disease?
Causal web diagram. It shows direct and indirect causes.
What are Koch’s postulates?
- the agent has to be present in every case of the disease
- the agent has to be isolated and grown in pure culture
- the agent has to cause disease when inoculated into a susceptible animal
- the agent must then be able to be recovered from that animal and identified
e. g. Anthrax= true, but not true for Leprosy as you cannot culture and grow the bacteria
What are risk factors? Examples?
Characteristics of some individuals that, on the basis of epidemiological evidence, are associated with increased risk of disease
What six things are 2 x 2 tables for association used for?
You can come up with the (1) Risk Ratio or the (2) Odds Ratio which measures HOW MUCH YOUR RISK IS INCREASED IN EXPOSED COMPARED WITH UNEXPOSED.
OR
You can come up with the (3)AR, (4)AF, (5)PAR, and (6)PAF. WE ARE MEASURING THE IMPACT HERE.
OR
You can use it to calculate PPV, NPV, DSe, and DSp to determine when a diagnostic test might be good to use- some tests are good with diseased animals but hopeless with non-diseased animals and the converse can be true.
When would you measure AR and AFexp? What is the questions you are answering?
Measuring effect with both.
AR= attributable risk. How much disease amongst the exposed is due to the risk factor?
AFexp= Attributable fraction. What percent of disease amongst the exposed is due to the risk factor?
When would you measure PAR and PAF? What question are you answering for both?
You are measuring effect with both.
PAR= Population attributable risk. How much disease in the population is due to the risk factor?
PAF= Population attributable fraction. What % of disease in the population is due to the risk factor?
Why would you use these equations?
What does it mean if you have a RR > 1? 1? <1?
State what it means if RR= 10?
You would use Risk Ratio or Relative Risk (RR) when comparing incidence rates. The incidence of the outcome in the exposed group divided by the incidence of the outcome in the unexposed group.
RR>1 indicates an association between exposure and disease
RR= 1 means no association between exposure and disease has been demonstrated
RR<1 indicates a negative association (protection) between the factor and the disease
** If RR = 10 then there is 10 times as much risk of disease in the exposed group than in the unexposed group. A high risk ratio by itself does not mean that exposure is the cause of disease, only that an association has been demonstrated.
Components of a surveillance system
* Clearly defined objectives
* The hazard or health state under surveillance
* Case definition
* Target population(s)- region, species, farms, animals
* Timing of sampling intervals
* Data management- capture, collation, and cleaning
* Methods for data analysis and triggers for action
* Feedback and dissemination of results
What are good questions to ask yourself when interpreting surveillance data? Surveillance cases are often an indicator of what??
Are these results likely to reflect the pattern of disease in the target population?
Has there been any change in the tendency to report the disease?
Has there been any change in disease diagnostics?
** Surveillance cases are often the tip of the iceberg, so likely there is a bigger problem e.g. for each notified case of salmonellosis there were a further 6 community cases not notified
Diagnostic windows for FMD
If it is past 4 days testing for serum antibodies would be okay, but the best test is really virus in oral/pharyngeal fluid because it starts the day before clinical signs and lasts for at least 24 days
How do you define the limits of normality? What do measurements outside of the range indicate? Example?
*Statistically: the range of values which capture 95% of all values observed in the population of healthy animals
* Measurements outside the range indicates UNUSUALNESS but is not a definitive sign of disease
* e.g. In 100 healthy dogs: 95 will have a White cell count within reference range but 5 HEALTHY dogs will have a value outside the interval
What is the standard deviation in a normal distribution- two-tailed?
95% of observations lie within 2 standard deviations of the mean.
Dog mean Temperature = 38.6 +/- 1.96
Ref range= 38.0-39.2
* if skewed use percentiles 2.5%
What happens with the standard deviation if enzymes are only abnormal if elevated, so one-tailed distribution?
* Within 5% (> mean +1.65)
e.g. Lipase, Amylase, AST, CK
What is the probability of a false positive?
= 1 - (0.95)^n
n= the number of tests
* this means that the more you test, the more likely you are to get a false positive
What does it mean if a test is fit for intended purpose?
- Accurate
- Precise (repeatability- measures closeness of repeated test results performed with same conditions, assuming a normal distribution. Repeatability= 1.96 x SD (SD= standard deviation of the observed differences))
Diagnostic Sensitivity
Low number of false Negatives
* The probability of correctly detecting infected animals (should include early/subclinical cases, not just experimentally infected animals tested late in disease)
Diagnostic Specificity
Low number of false Positives
* The probability of correctly detecting uninfected animals
What does high sensitivity imply?
High specificity?
High sensitivity implies a low number of false negatives
High specificity implies a low number of false positives
What happens if you move the cut off by increase sensitivity?
Specificity will decrease. Therefore, more diseased animals are classed as positive but more non-diseased animals are also classed as positive (FALSE POSITIVES).
** BUT increasing sensitivity means that those animals identified as negative are VERY LIKELY TO BE TRULY NEGATIVE
What happens if you move the cut off to decrease sensitivity?
Specificity will increase which means that less diseased animals are classed as positive. BUT less non-diseased animals are classed as positive.
* Therefore, increasing specificity means that those animals identified as positive are very likely to be TRULY DISEASED.
What do predictive values do for a test? What causes them to be variable? What 3 estimates do you need to calculate predictive values?
Evaluate a test’s ability to correctly identify the condition of interest. However, predictive values vary with probability (prevalence) of the condition!!
** To calculate them you need:
- DSe
- DSp
- the probability of the animal having the condition PRIOR to testing (prevalence can be used)
Common reasons for eradication failure
* wildlife reservoir
* survival of agent in the environment
* poor diagnostics
* expense of test and either poor DSp or DSe
** Diagnostic difficulties arise towards the end of eradication programs when prevalence is very low
Eradication of Rinderpest vs. Johne’s Disease
If there is severe observation bias, what does it mean for the study?
No analytical manipulation of the data will overcome the problem. The study is fundamentally flawed.
Define confounding. What does it mean if there is confounding for a study?
An alternative explanation for the association between a factor and an outcome
e.g. many smokers are also drinkers– so how do we know that laryngeal cancer is due to smoking and not drinking?
* Appropriate analysis, in most cases, will overcome the problem.
What is external validity?
- Can the results be applied to the eligible population?
- Can the results be applied to the source population?
- Can the results be applied other relevants populations?
** If the internal validity is poor- then no!
What is the difference between a descriptive and analytical epidemiological study? Give examples of each. What are the three main types of studies?
* Descriptive studies do not have a hypothesis– earliest studies done on a new disease in order to characterize it, quantify its frequency, and determine how it varies in relation to individual, place, and time
e.g. Looking at injuries in a ER in Philadelphia- authors found a high incidence of intentional interpersonal injury in that part of the city
* Analytical studies are undertaken to identify and test hypotheses about the association between an exposure of interest and a particular outcome. (the major difference is that in experimental studies the investigator has direct control over study conditions)
e.g. does cigarette smoking cause lung cancer?
* Three main types: Descriptive, Analytical, and Experimental
Major features of case control studies.
What is the achilles heel of case control studies?
Intended to provide the same answer in a much simpler way by studying all of the dogs who got bladder cancer and sample of dogs who did not.
e.g. 117 dogs with bladder cancer and 117 dogs who remained free of disease. We can work out the odds of cancer in the exposed vs. unexposed groups and compare them. Is Benzidene associated? We can use Odds Ratio because the outcome of interest (bladder cancer) was relatively rare.
** the key is to make sure the cases and controls are similar in every way except for the exposure factors hypothesised to be associated with disease of interest
*** The CONTROL group is the achilles heel because you first have to select appropriately. And then you have to break them up into positive and negative groups based on the exposure.
** As well as RECALL BIAS
What are randomized clinical trials?
* resembles a laboratory experiment
* objective is to test the possible effect of a therapeutic or preventive intervention
* the design’s key feature is that a formal chance mechanism is used to assign participants to either the treatment or control group
* subjects then followed over time to measure one or more outcomes, such as occurrence of disease
