AN EXPERIMENTAL EPIDEMIC

The science of epidemiology originated and evolved in relation to the great epidemic diseases such as cholera, typhoid fever, small pox and yellow fever. Today its scope embraces all diseases: infectious diseases and those resulting from genetic abnormalities, metabolic dysfunction, malnutrition, neoplasms, psychiatric disorders, and aging, to name but a few. By definition, epidemiology is the science that evaluates the determinants, occurrence, distribution, and control of health and disease in a defined population. A person who practices epidemiology is an epidemiologist. Epidemiologists are, in effect, disease detectives. Their major concerns are the discovery of the factors essential to disease occurrence and the development of methods for disease prevention. What follows are classic examples of the work of two early epidemiologists.¹

The first person to realize that an infectious organism could be transmitted from one person to another was the Hungarian physician Ignaz Phillip Semmelweis. Between 1847 and 1849, Semmelweis observed that women who had their babies at the hospital with the help of medical students and physicians were four times as likely to contract puerperal fever as those who gave birth with the help of midwives. He concluded that the physicians and the students were infecting women with the material remaining on their hands after autopsies and other activities. He suggested that "invisible cadaver particles" might be responsible for the transmission of disease from autopsy to patient. Semmelweis thus required handwashing with a chlorine solution "until the hands were slippery and the odor of the cadaver was gone" before examining patients or delivering babies. This handwashing regimen led to a dramatic decrease in the number of cases of puerperal fever and saved the lives of many women. As a result, Semmelweis is credited with being the pioneer of antisepsis in obstetrics. Unfortunately, in his own time, most of the medical establishment refused to acknowledge his contribution and adopt his procedures. After years of rejection, Semmelweis had a nervous breakdown in 1865. He died a short time later of a wound infection. It is very probable that it was a streptococcal infection, arising from the same pathogen he had struggled against his whole professional life.^1,2

Much of what we know today about the epidemiology of cholera is based on the classic studies conducted by the British physician John Snow in 1853 and 1854. During this period, London was experiencing a series of cholera outbreaks. The water supply for the city came from two sources: the Southwark and Vauxhall Company, and the Lambeth Company.

Snow suspected that water was a possible source of cholera even though the causative organism, Vibrio cholerae, was not identified until 1883 by Robert Koch. In his studies, Snow interviewed cholera patients and found that most of them obtained their drinking water from the Southwark and Vauxhall Company. He also discovered that this company obtained its water from a single pump on Broad Street that received water from the Thames River at the same place where Londoners discharged their sewage. In contrast, Lambeth Company obtained its water from the Thames before it reached the city. Snow concluded that cholera was spread by people's drinking water from the Broad Street pump, water contaminated with raw sewage containing the cholera organism. He arranged for the pump handle to be removed, and the number of cholera cases dropped dramatically.

To commemorate this epidemiological achievement, the John Snow Pub was erected at the site of the old pump. Those who complete the epidemiological intelligence program at the Centers for Disease Control in Atlanta, Georgia receive an emblem bearing a replica of a barrel of Whatney's Ale - the brew dispensed at the John Snow Pub.¹

EXERCISE³

Today we have a classroom epidemic made to order and we will be both the victims and the epidemiologists. In doing this exercise we will use Serratia marcescens a relatively harmless organism - although it can be an opportunist infecting people with debilitating disorders, or treatment with broad-spectrum antibiotics, or subjected to intrumentation such as tracheostomy tubes or indwelling catheters. For the purposes of today's exercise, suppose that this organism is a highly contagious and virulent organism normally spread via food or water or direct contact.

While performing this exercise keep in mind that the liquids you are touching are potentially infectious. Always be mindful of ways to contain and reduce contamination and contagion while at your bench or while walking around looking for someone to shake hands with. Thus, always hold a piece of paper towel under your contaminated hand while walking around and do all of your handshaking over the lab bench top to prevent dripping materials on the floor.

1. You will be given a numbered petri dish. In that dish is a piece of candy. One of the pieces has been dipped in a culture of Serratia marcescens; all of the others have been dipped in distilled water.

2. PUT ON PROTECTIVE GLOVES.

3. Pick up the candy with one of your gloved hands and roll it in that hand. This will get that hand covered with candy. If things start getting a little too sticky dip your fingers back into the liquid bathing the candy to moisten your fingers.

4. Round I Hand Shaking. Student with petri dish #1 will shake hands with someone -- anyone other than the persons on either side or directly across from themself. When student #1 returns to their desk, then student #2 will go and find someone to shake hands with. The handshaking will continue in this manner until everyone has had an opportunity to go off on a handshaking expedition. Feel free to dip your gloved fingers back into the water bathing your candy from time to time - especially before shaking. The instructor will be at the blackboard and will make a record of each handshake.

5. Round I Culturing. Only students #3, 6, 9, 12 , 15, 18, 21, etc. will swab the "candy-handling hand" well and streak it onto a TSA plate. You may need to moisten your fingers before taking the sample.

6. Round II Hand Shaking. Repeat shaking hands as was done in step 4, starting with student #1. However, you don't have to pick the same person as in round I - be promiscuous! Feel free to moisten your fingers from time to time - especially before shaking. The instructor will be at the blackboard and will make a record of each handshake.

7. Round II Culturing. Everyone will swab the "candy-handling hand" and streak it onto a TSA plate. You may need to moisten your fingers before taking the sample. Label your plates with your number as well as your name.

8. Incubate your parafilmed plates at room temp. At the next period the positive plates should show red colonies.

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Record the sequence of the handshakes in a table like the one that follows. In the table, leave room for the culture results which you will see during the next lab period.

Student #	shook hands with:___ during round one.	Culture results after round one	shook hands with:___ during round two.	Culture results after round two
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20

References:

1. The narrative was copied from: L. M. Prescott, J. P. Harley and D. A. Klein, Microbiology; Wm. C. Brown Publishers, Dubuque, 1990

2. Part of the narrative on Semmelweis was also taken from: Sherris Medical Microbiology, An Introduction to Infectious Diseases, 3rd Edition, Kenneth J. Ryan, Editor, Appleton and Lange Publishers, Norwalk, CN, 1994

3. The experiment was adapted from C. S. Mudge and F. R. Smith, A Fundamental Approach to Bacteriology; W. J. Stacey, Inc., San Francisco, 1939