Epidemiology Exercise

Introduction: Epidemiology is the branch of medicine dealing with the incidence and prevalence of disease in large populations and with detection of the source and cause of epidemics of infectious disease. Many viruses which cause epidemic outbreaks can be transmitted through direct contact, for instance during a simple handshake. Examples include influenza, Norwalk virus, and rhinovirus. In this exercise, we will use Serratiamarcescens to simulate the spread of a viral epidemic through direct contact. Serratiamarcescens has a red pigment, and is easy to visualize on a Petri dish. The results of the class will be pooled, and with some logical detective work the student who initiated the epidemic (the ‘index case’) might be identified.

Materials: (Students will work individually)

  • Piece of candy soaked in sterile media OR media containing Serratiamarcescens (only one person’s candy will be soaked in Serratiamarcescens)
  • TSA plates
  • Sterile swabs

Procedure:

CAUTION: Serratiamarcescens is generally considered non-pathogenic, but has been associated with human infections under certain conditions. Work according to universal precautions, assuming that these bacteria are potentially pathogenic (= harmful). WEAR A GLOVE for handling your piece of candy and for shaking hands. DO NOT CROSS-CONTAMINATE SURFACES with your potentially-contaminated glove. Discard all gloves in the appropriate waste for autoclaving (and not in the regular trash) when finished.

1. Choose a Petri dish containing a piece of candy, and note the number on the dish. If it is an odd number, take two TSA plates and two sterile swabs. If it is an even number, take one TSA plate and one swab.

2. Label the bottom of your TSA plate(s) with your name and ‘Round 1’ (first plate) or ‘Round 2’ (second plate). If you only took one plate, label it ‘Round 2.’

3. Put a glove on the hand with which you do NOT write. Your gloved hand will be your ‘dirty’ hand; your ungloved hand will remain ‘clean.’ Do not contaminate objects or surfaces with your dirty (i.e. gloved) hand!

4. Pick up your piece of candy with your gloved hand, and move it around so that the wet candy gets all over the palm and fingers of the glove. Discard the candy back into the Petri dish when finished. Use the media in the Petri dish to keep your glove wet if it starts to dry out. (A wet glove facilitates transmission of Serratia.)Do not let media drip onto the floor or the lab benches.

5. IN THE ORDER DESIGNATED BY THE INSTRUCTOR, students will shake hands with each other. Do not shake hands out of turn. You can choose to shake hands with any person, preferably not someone right next to you. When shaking hands, make prolonged and thorough contact with the other person’s hand.

NOTE: You must record the entire sequence of handshakes as they happen on the attached report sheet.

6. PEOPLE WITH ODD-NUMBERED CANDIES ONLY: After the entire first round of handshakes is complete, collect as much material as you can from your glove usingasterile swab, and inoculate the TSA plate that is labeled ‘Round 1.’ Cover the entire surface of the agar dish with your swab. Do not let the swab gouge the surface of the agar. Discard the swab in the waste to be autoclaved. Do not re-use swabs.

7. Now, the class will undergo a second round of hand-shaking. Again, go in the order designated by the instructor, and do not shake hands out of turn. Record the sequence of handshakes on the attached report sheet. Try to shake hands with someone you did not shake hands with during Round 1.

8. When the second round of handshaking is complete, take a fresh sterile swab and collect material from your gloved hand. This time, the entire class will sample their gloves. Inoculate the TSA plate labeled ‘Round 2.’ Discard the swab in the waste to be autoclaved. Those with odd-numbered candies should now have two inoculated plates; those with even-numbered candies should only have one inoculated plate.

9. Discard your gloves in the waste to be autoclaved. Incubate your TSA plates in an inverted position (‘upside-down’) at room temperature.

10. During the next lab session, examine the plates for the presence of pink/red colonies. Record the results of the whole class on the attached chart. If pink/red colonies are present, record the results as ‘positive.’ If pink/red colonies are absent, record the results as ‘negative.’

11. Trace the results back to the person who started the epidemic (the ‘index case’). Sometimes it is only possible to narrow it down to two people: the true index case, and that person’s initial contact. The instructor will show you a method using flow charts to help rule out potential suspects. TO RECEIVE CREDIT FOR THIS EXERCISE, YOU MUST ATTACH A PAGE WITH YOUR FLOW CHARTS TO THE REPORT SHEET!

Person Receiving Handshake / Presence of Red Colonies (+ or - )
Person initiating handshake and candy number / Round One / Round Two / Round One
(Only those with odd-numbered candies should have a plate) / Round Two
(All students should have a plate)

Questions:

1. What is the difference between an epidemic and a pandemic?

2. Who was the index case for our epidemic?

3. Why is it necessary to incubate the agar plates at room temperature (and not 37C)?

4. Some of the agar plates grew only white colonies. From where did the bacteria causing these colonies probably come?

5. Cruise ships often place ethanol-based hand sanitizers (62% ethanol) around the ship, ostensibly to reduce the spread of Norwalk virus. Using the Internet, do a small search to determine whether Norwalk virus is susceptible to killing by ethanol. Based on your research, do you believe that the practice of placing hand sanitizer on board cruise ships is effective in reducing the spread of Norwalk virus? Why or why not? To get credit for this exercise, you MUST include the full citation for your reference(s) below.