Epidemiology: Surveillance and Outbreak Investigations: THE JON SNOW STORY (L20) – Flashcards
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How outbreaks are detected

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• When the incidence of disease in a given time period is above the normal range • Generally suspected when rates of occurrence (incidence) exceed limits defined by 95% confidence intervals around time averaged means
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Time series analysis

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detection description and measurement of periodicities from temoporal disease occurrence data; example: moving average
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Time series analysis: Trend Types

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• Secular trends: overall, long term increases or decreases in rate of occurrence (Height, obesity) • Seasonal fluctuations: regular changes in occurrence with periods shorter than a year (Flu season) • Cyclic changes: regular changes in occurrence with period longer than a year (Outbreak of dz every few years in mumps: due to children being born, getting it, herd immunity protects for a bit, then the cycle occurs again when herd immunity lost) • Irregular variation: random or unpredictable changes in occurrence over time
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Case Definition
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based on clinical signs, microbio and serologic features etc. Necessary to an outbreak investigation to accurately classify cases vs non- cases.
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Population at Risk
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must also be defined for an outbreak investigation. Who is at risk? = Denominator
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Index Case
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first case to be brought up to a public health official (not necessarily first case of outbreak)
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Primary Case
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A case that gets infected from exposure to the souce of the dz/agent (not first case either)
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Secondary Case
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Case that gets dz from exposure to a primary case
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Primary Attack Rate
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Recall from Exam 1. Measures initial speed of outbreak "How BAD the situation is"
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Secondary Attack Rate
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Recall from Exam 1. Measured *ongoing* speed of outbreak. = # individuals that become diseased in *subsequent* period/(initial PaR - primary cases)
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Exposure Specific Attack Rates
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compare attack rates from sick vs non sick animals to figure out if factor is associated with the dz.
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Multiple Exposure Specific Attack Rates: Identifying causal factors

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Causal factors will have: - higher attack rates in exposed groups than unexposed - greatest difference between attack rates - relative risk >1 for exposed groups - Confidence Intervals that don't cross 1 - nonexposed level=endemic dz level.
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Cross Tablulation

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used for single exposure outbreaks (one cause, like the potato salad!) - can be done by hand in the field, using exposure specific attack rates.
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Quarantine
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Quarante Giorno (40 days!) to prevent spread of black plague. *Quarantine period should be longer than incubation period.* • Quarantine individuals coming into the population to prevent reintroductions of the disease agent • Quarantine individuals leaving the population to prevent the spread of disease
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Common Intervention strategies
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• Limit access by people • Clean and disinfect equipment: beware fomites! Equipment includes vehicles • Call appropriate officials (state vet, especially for reportable dz) • Adopt a generous "down time" policy for health professionals serving multiple clients and herds (beware iatrogenic transmission)
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Disease prevention, control, eradication
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*Prevention*: exclude a disease from an unaffected popln -->Requires continuous effort and vigilance for as long as the possibility remains that a disease may occur *Control*: reduce the frequency or cost of dz to some socially or economically acceptable level *Eradication*: the complete elimination of an infectious agent from a geographic region. Ex. Rinderpest (smallpox)
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Common Tactic for Dz control/management
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• Mass treatment: treat all (food/water) • Mass immunization • Slaughter • Environmental hygein: prevent contact with contam environ (disinfection) • Biological control ("applied ecology"): reduce the vector or reservoir • Education: essential but can be ineffective
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Epidemic types displayed by Epidemic Curve

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Sporadic Point Source Propagating
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Epidemic Curve

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graph of the times of onset of cases - Y axis = raw number of cases; X axis = different time points Contains info about: o Avg # secondary infections causes by a primary infection (early curve) o Time between exposure and onset of dz (incubation period) o Types of epidemic (sporadic, point source, propagating)
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Case fatality rates
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Probability of dying if you get the disease • # deaths due to disease/total # of cases of dz • CFR = "honest" measure of severity of an outbreak
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