Introduction to Epidemiology: The Epidemiologic Approach

Escherichia coli O157:H7 was first identified as a human pathogen in 1982 in the United States of America, following an outbreak of bloody diarrhea associated with contaminated hamburger meat. Sporadic infections and outbreaks have since been reported from many parts of the world,including North America, Western Europe, Australia, Asia, and Africa. Although other animals are capable of carrying and transmitting theinfection, cattle are the primary reservoir for E. coli O157:H7. Implicated foods are typically those derived from cattle (e.g., beef,hamburger, raw milk); however, the infection has also been transmitted through contact with infected persons, contaminated water, and other contaminated food products.

Infection with E. coli O157:H7 is diagnosed by detecting the bacterium in the stool. Most laboratories that culture stool do not routinely test for E. coli O157:H7, but require a special request from the health care provider. Only recently has E. coli O157:H7 infection become nationally notifiable in the U.S. Outside the U.S., reporting is limited to a few but increasing number of countries.

In the last week of June 1997, a certain state department of community health in the US noticed an increase in laboratory reports of E. coli O157:H7 infection. Fifty-two infections had been reported that month, compared with 18 in June of 1996. In preliminary investigations, no obvious epidemiologic linkages between the patients were found. The increase in cases continued into July.

Laboratory subtyping can help determine if an increased number of isolates of the same bacterial species results from a common source outbreak. Subtyping methods are based on selected biologic and/or geneticcharacteristics of bacteria that tend to differ between isolates of the same species. In a common source outbreak, however, isolates typically arise from the same parent organism. These isolates will be similar to each other with respect to these biologic and genetic characteristics and have similar subtyping results.

One subtyping method is DNA “fingerprinting” by Pulsed Field Gel Electrophoresis (PFGE). In DNA fingerprinting, the bacterial DNA is cut into pieces. The pieces are separated by placing them in a jelly-like substance (i.e., the gel), acting as a sieve, to which a pulsing electric fieldis applied. The electric field drives the DNA pieces across the gel over a period of hours. The smaller pieces move through the gel more quickly and the larger pieces more slowly resulting in a separation of the DNA into distinct bands. The bands are made to fluoresce and are read under ultraviolet illumination. This DNA“fingerprint” resembles a bar code. (Figure 1)

Figure 1. Typical DNA banding pattern resulting from PFGE.

Different DNA composition will result in different PFGE banding patterns. Bacteria descended from the same original parent will have virtually identical DNA and their DNA fingerprints will be in distinguishable. Identification of a cluster of isolates with the same PFGE pattern suggests that they arose from the same parent and could be from the same source.

Similar DNA fingerprints alone, however, are insufficient to establish a linkage between isolates and a common source outbreak. An epidemiologic investigation is necessary to demonstrate that there is a common source and to identify it. To be most useful, PFGE subtyping needs to be performed on a routine basis, in realtime, so that results are available (and reviewed) soon after a case is first detected.

Figure2. PFGE results on E. coli O157:H7 isolates, June-July 1997.

Typically, a PFGE “pattern” is defined as having the same banding pattern but including up to one band difference. By this definition, isolates #2, 3, 4, 6, and 7 are indistinguishable by these PFGE results. (Isolate #4 differs by one band.)

DNA finger printing, performed during the second week of July showed that 17 of the first 19 E. coli O157:H7 isolates from June-July were indistinguishable. They did not match any fingerprints from a convenience sample of isolates frompatients with E. coli O157:H7 infection before May.

Based on the PFGE findings, it was suspected the cases of E. coli O157:H7 infection resulted from a common source. On July 15, an investigation was initiated.

Module 1 – Background

Introduction to Epidemiology: The Epidemiologic Approach

If you cannot locate an article in one set of databases (e.g., EBSCO), try to locate it in ProQuest.

Required Readings

Coggon, D., Rose, G., & Barker, D.J.P. (2007) Epidemiology for the Uninitiated (4th edition). British Medical Journal. Retrieved May 22, 2012 from http://www.bmj.com/about-bmj/resources-readers/publications/epidemiology-uninitiated

Centers for Disease Control and Prevention. PulseNet. Retrieved Retrieved May 22, 2012 from http://www.cdc.gov/pulsenet/

Centers for Disease Control and Prevention Timeline for Reporting of E. coli Cases. Retrieved February 26, 2011. http://www.cdc.gov/ecoli/reportingtimeline.htm

OMRAN, A. (2005). The Epidemiologic Transition: A Theory of the Epidemiology of Population Change. Milbank Quarterly, 83(4), 731-757. doi:10.1111/j.1468-0009.2005.00398.x. Retrieved Retrieved May 22, 2012 from: http://onlinelibrary.wiley.com/doi/10.1111/j.1468-0009.2005.00398.x/pdf

Principles of Epidemiology. Retrieved Retrieved May 22, 2012 from University of Illinois at Chicago Web site: http://www.uic.edu/sph/prepare/courses/ph490/resources/epilesson01.pdf

DNA Fingerprinting. Retrieved May 22, 2012 from cphp.sph.unc.edu/focus/vol4/issue4/4-4LabTechniques_slides.ppt

Module 1 – SLP

Introduction to Epidemiology: The Epidemiologic Approach

A Multistate Outbreak of E. coli 0157:H7

For your session-long project, you will conduct an epidemiologic investigation of an outbreak of E. coli 0157:H7. The grade for the SLP is based on the depth of your work and your ability to analyze the information, generate hypotheses about the source of the outbreak and mode of transmission. This means that the student is focusing on those factors of importance to disease transmission, control, and prevention. A public health perspective will be advanced within the assignment.

Click here to read the background information specific for the module 1 SLP

Your Task for this modular component

Please answer these questions

1. What could account for the increase in cases?

2. Compare the DNA fingerprints in Figure 2 from seven of the E. coli O157:H7 cases. Each isolate has its own vertical lane (i.e., column). Controls appear in lanes #1, 5, and 10. Which isolates appear similar? In your opinion was DNA typing necessary in this case? Why?

SLP Assignment Expectations

Length: SLP assignments should be at least 2 pages (500 words) in length.

References: At least two references must be included from academic sources (e.g. peer-reviewed journal articles). Required readings are included. Quoted material should not exceed 10% of the total paper (since the focus of these assignments is critical thinking). Use your own words and build on the ideas of others. When material is copied verbatim from external sources, it MUST be enclosed in quotes. The references should be cited within the text and also listed at the end of the assignment in the References section (APA format recommended).

Organization: Subheadings should be used to organize your paper according to question

Format: APA format is recommended for this assignment. See Syllabus page for more information on APA format.

Grammar and Spelling: While no points are deducted, assignments are expected to adhere to standards guidelines of grammar, spelling, punctuation, and sentence syntax. Points may be deducted if grammar and spelling impact clarity.

The following items will be assessed in particular:

• Achievement of learning objectives for SLP assignment

• Relevance (e.g. all content is connected to the question)

• Precision (e.g. specific question is addressed. Statements, facts, and statistics are specific and accurate).

• Depth of discussion (e.g. present and integrate points that lead to deeper issues)

• Breadth (e.g. multiple perspectives and references, multiple issues/factors considered)

• Evidence (e.g. points are well-supported with facts, statistics and references)

• Logic (e.g. presented discussion makes sense, conclusions are logically supported by premises, statements, or factual information)

• Clarity (e.g. writing is concise, understandable, and contains sufficient detail or examples)

• Objectivity (e.g. avoid use of first person and subjective bias)