Epidemiology

STUDENT HANDOUT:

• Epidemiology Student Handout

INTRODUCTION:

During an 1885 London epidemic, British physician John Snow proved that drinking water was the vehicle for the  spread of cholera.  In the first two months of the epidemic, more than 350 people died from cholera.  Snow suspected that the excrement from cholera patients was highly infectious and  that sewage contamination of the water supply might exist.  In those days, raw sewage was dumped directly into London's Thames River.  Most of the deaths occurred in houses supplied by two water companies.  These two companies obtained their water from a section of the river in  the  heart of London, a heavily populated area.  The third major company, supplying a large number of houses that had very few associated deaths, was found to obtain water from a point distant from the heart of the city. 

Figure 1.  Map of London.  Dots indicate locations of cholera disease.  (Image Source)  (Creative Commons License)

Consequently, water from the heart of London was thought to be the source of contamination and the cause of the epidemic.  By using the map above (Figure 1), the source of the cholera was identified.   Each dot on the map represents a case of cholera.  The epidemic was controlled by stopping the usage of water from the two questionable water companies. Sewage treatment procedures were eventually developed to prevent the occurrence of future waterborne epidemics.  

This was one of the earliest examples of epidemiology, which is the study of the transmission of disease in a population.  Diseases caused by microorganisms are known as infectious diseases.  A communicable disease is one that can be transmitted from one individual to another.  Not all infectious diseases are communicable.  One example of an infectious, non-communicable disease is tetanus.  Tetanus is caused by the bacterium Clostridium tetani.  This bacterium produces a toxin known as tetanospasmin (aka tetanus toxin), which causes muscles to lock.  An individual who has tetanus cannot pass tetanus on to another individual.      

Epidemiology is the study of the transmission of notifiable diseases.  Transmission can be classified into one of four categories:  

• Sporadic = Occasional cases are reported at irregular intervals.  

• Endemic = Disease that has a stable relative frequency in a particular area.

• Epidemic = Disease where the prevalence increases over a short period of time.

• Pandemic = Epidemic across continents.

Epidemiologists also keep track of the morbidity and mortality of a disease. 

OBJECTIVES:

1.  Learn how epidemiologists track the progression of a disease in a population. 

PRECAUTIONS:

1.  Students should wear gloves and goggles while performing this experiment.  

2.  Be sure to follow your teacher’s directions on how to dispose of the test tube contents after class ends. 

MATERIALS:

1. Test tube (contains either water or weak base).

2. Plastic bulb pipet

3. 0.04% thymol blue pH indicator 

PROCEDURE:

1. Each student will be given a test tube and a plastic bulb pipet.  The test tube will contain 5 milliliters of an unknown  solution (either water or a weak base).  The one individual with the weak base will be an asymptomatic carrier of a particular disease.  

2. Find another student with whom to exchange test tube contents.  To exchange:

(a)  Using your bulb pipet, measure 3 milliliters of the solution from your test tube.  The individual you are exchanging with should measure 3 milliliters of solution from their test tube.

(b)  Simultaneously, swap your 3 milliliters of liquid into each other’s test tubes.  

3.  Repeat the process two more times with two other people.  Therefore, you will have completed 3 different exchanges. Take note of who you exchanged with (and in what   

     order).   Record the data in Table 1 of the Laboratory Report Form. 

OBSERVATIONS/CONCLUSIONS:

1. After everyone in the class has completed their exchanges, your teacher will then add a few drops of thymol blue pH indicator. 

(a)  If the pH indicator turns yellow, this means you are not infected.  

(b)  If the pH indicator turns blue, this means you are infected.  

          Record the data in Table 1 of the Laboratory Report Form.

2. Using the information on the board, determine which individual was the asymptomatic carrier. Explain how you arrived at this answer.

QUESTIONS:

1. Look back at the map of London showing the outbreak of cholera (Figure 1).  The dots represent all the cases of cholera.  The X’s represent various water pumps in the city.  

(a)  Circle on the map which water pump was the source of contamination.  

(b)  How did you arrive at this answer?

2.   One method of mitigating the spread of an infectious, contagious disease is by isolation and/or quarantine.  What are these methods?

KEY TERMS:

• asymptomatic carrier

• cholera

• Clostridium tetani

• disease

• epidemic

• morbidity

• mortality

• microorganisms

• notifiable diseases

• sewage contamination

• source of contamination

• vehicle

SUGGESTED READINGS:

Local, state, national (Centers for Disease Control and Prevention (CDC)) and international health organizations (World Health Organization (WHO)) employ epidemiologists to study the progression of diseases.  Each week, the CDC releases a morbidity and mortality report of notifiable diseases (The Morbidity & Mortality Weekly Report (MMWR)).