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Experimental studies provide the strongest evidence for the etiology of disease, but they must also be designed carefully to eliminate subtle effects of bias . Typically, experimental studies with humans are conducted as double-blind studies , meaning neither the subjects nor the researchers know who is a treatment case and who is not. This design removes a well-known cause of bias in research called the placebo effect , in which knowledge of the treatment by either the subject or the researcher can influence the outcomes.

  • Describe the advantages and disadvantages of observational studies and experimental studies.
  • Explain the ways that groups of subjects can be selected for analytical studies.

Part 3

Since laboratory tests had confirmed Salmonella , a common foodborne pathogen, as the etiologic agent, epidemiologists suspected that the outbreak was caused by contamination at a food processing facility serving the region. Interviews with patients focused on food consumption during and after the Thanksgiving holiday, corresponding with the timing of the outbreak. During the interviews, patients were asked to list items consumed at holiday gatherings and describe how widely each item was consumed among family members and relatives. They were also asked about the sources of food items (e.g., brand, location of purchase, date of purchase). By asking such questions, health officials hoped to identify patterns that would lead back to the source of the outbreak.

Analysis of the interview responses eventually linked almost all of the cases to consumption of a holiday dish known as the turducken —a chicken stuffed inside a duck stuffed inside a turkey. Turducken is a dish not generally consumed year-round, which would explain the spike in cases just after the Thanksgiving holiday. Additional analysis revealed that the turduckens consumed by the affected patients were purchased already stuffed and ready to be cooked. Moreover, the pre-stuffed turduckens were all sold at the same regional grocery chain under two different brand names. Upon further investigation, officials traced both brands to a single processing plant that supplied stores throughout the Florida panhandle.

  • Is this an example of common source spread or propagated spread?
  • What next steps would the public health office likely take after identifying the source of the outbreak?

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Key concepts and summary

  • Early pioneers of epidemiology such as John Snow, Florence Nightingale, and Joseph Lister, studied disease at the population level and used data to disrupt disease transmission.
  • Descriptive epidemiology studies rely on case analysis and patient histories to gain information about outbreaks, frequently while they are still occurring.
  • Retrospective epidemiology studies use historical data to identify associations with the disease state of present cases. Prospective epidemiology studies gather data and follow cases to find associations with future disease states.
  • Analytical epidemiology studies are observational studies that are carefully designed to compare groups and uncover associations between environmental or genetic factors and disease.
  • Experimental epidemiology studies generate strong evidence of causation in disease or treatment by manipulating subjects and comparing them with control subjects.

Matching

Match each type of epidemiology study with its description.

___experimental A. examination of past case histories and medical test results conducted on patients in an outbreak
___analytical B. examination of current case histories, interviews with patients and their contacts, interpretation of medical test results; frequently conducted while outbreak is still in progress
___prospective C. use of a set of test subjects (human or animal) and control subjects that are treated the same as the test subjects except for the specific treatment being studied
___descriptive D. observing groups of individuals to look for associations with disease
___retrospective E. a comparison of a cohort of individuals through the course of the study

C, D, E, B, A

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Match each pioneer of epidemiology with his or her contribution.

___Florence Nightingale A. determined the source of a cholera outbreak in London
___Robert Koch B. showed that surgical wound infection rates could be dramatically reduced by using carbolic acid to disinfect surgical tools, bandages, and surgical sites
___Joseph Lister C. compiled data on causes of mortality in soldiers, leading to innovations in military medical care
___John Snow D. developed a methodology for conclusively determining the etiology of disease

C, D, B, A

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Fill in the blank

________occurs when an infected individual passes the infection on to other individuals, who pass it on to still others, increasing the penetration of the infection into the susceptible population.

Propagated spread

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A batch of food contaminated with botulism exotoxin, consumed at a family reunion by most of the members of a family, would be an example of a ________ outbreak.

point source

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Short answer

What activity did John Snow conduct, other than mapping, that contemporary epidemiologists also use when trying to understand how to control a disease?

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Questions & Answers

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Source:  OpenStax, Microbiology. OpenStax CNX. Nov 01, 2016 Download for free at http://cnx.org/content/col12087/1.4
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