Outbreak investigation


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Often diseases will “break out” in certain areas and field epidemiologists are called in to identify the source of the outbreak and institute measures to bring it under control. This is a very challenging job as such research is often hampered in a number of ways.

First, investigators usually arrive on the scene “after the fact” and must reconstruct a picture of what has occurred from various and often inconsistent sources such as personal testimony, records of outpatient visits or hospitalizations, and even school absentee records. There are often a limited number of persons affected, which can make it difficult to formulate statistically valid conclusions.

In addition, some persons directly involved with an outbreak may be reluctant to cooperate with investigators, especially if the results of the study may have financial repercussions, e.g. the owner of a restaurant suspected of food poisoning. Finally, local publicity of an outbreak can hamper a study as media coverage can influence affected persons to form preconceived notions as to the source or cause of an outbreak. Also, media demands on the investigators may require considerable time that could be better spent on the field investigation itself.

In this activity, you will be given some of the details of a real investigation of an outbreak. Using this information you will analyze the situation and form some conclusions as to the cause of the epidemic and make suggestions as to how to remedy the situation.

The Situation

You are an Epidemic Intelligence Service (EIS) officer at the Centers for Disease Control (CDC), Atlanta, Georgia. A request for epidemic aid comes to the CDC from the Michigan State Health Department. There has been an outbreak of acute febrile (feverish) illness in the Oakland County Health Department. You and two other EIS officers are assigned to investigate the outbreak.

You arrive at the health department early on Saturday, July 6, and assess the situation. The department building has two floors, one at ground level and a basement. It is situated on a spacious “campus” of county service buildings and houses the administrative offices for the Oakland County Health Department and several health clinics. During the first 3 weeks of June, the ground adjacent to the building was graded and paved, which raised clouds of dust that enveloped the entire complex at times. Torrential rains occurred during the last week of June, followed by a rapid rise in mean temperature from 15.2°C on June 29 to 27.4°C on June 30.

Beginning on the evening of Monday, July 1, the first employee of the health department came down with an acute febrile illness. By Tuesday evening, over half of the employees had become ill, and on Wednesday, additional cases were reported by some of the visitors to the building. The building was closed on July 4, but then reopened on Friday the 5th.

By the time you arrive, 91 out of the 100 employees who worked in the building are currently ill or have recovered. The director gives you the following list of their symptoms.


Symptom Number with symptom

Malaise (gen. discomfort) 90

Myalgia (muscle aches) 87

Fever 83

Chills 83

Headache 83

Cough 51

Dizziness 48

Nausea 41

Chest pain 40

Joint pain 38

Sore throat 28

Abdominal pain 23

Confusion 18

Bizarre dreams 5

Irritability 2


Defining a Case

Notice that in the list above, not all of the employee patients experienced the exact same symptoms. One of your first jobs is to formally define exactly which persons represent a case of the illness you are studying. If the disease is known, then a clinical definition will suffice. However, in this example, the cause of the illness is unknown, so you will have to formulate a working definition. An example of how this is often expressed is, “If a patient has 3 of these 5 symptoms, then they are classified as a case.”


Which group of symptoms would you use to determine if a patient represented a “case?”

Why might you not want to include a symptom such as “sore throat” in your definition?



Making an Epidemic Curve

In order to determine how the cases are related by time, you will want to make a histogram that plots the number of cases versus time. This will allow you to determine the magnitude of the outbreak, its possible mode of spread, and possibly the incubation period of the agent responsible. In addition to the table of symptoms, the director of the health department provides you with a list of the date that each employee patient first became ill. Use this data to begin an epidemic curve for the employees.


Date Number becoming ill

July 1 1

July 2 66

July 3 21

July 4 2

July 5 1


3. Draw epidemic curve here (a histogram sketch would work):


Determining Attack Rate

Attack rate is the proportion of the population that is at-risk for infection who actually develop the illness over a period of time. You can calculate attack rate by using the following formula:

(Number of cases / Number of people potentially exposed) x 100

4. What is the attack rate for the employees at the time of your arrival on Saturday? (show work).



5. What information must you have to determine an attack rate for the visitors?



6. Why is it much easier to determine the attack rate for the employees than for the visitors at the Health Department?


Continuing the Investigation

Your goal is to identify the agent responsible for the outbreak. What questions would you ask the employees and the visitors? List 3.


Question 1:



Question 2



Question 3:



Employees and visitors were asked for the following information. Results of statistical analyses revealed the following:

Age: No association. (No correlation of age with likelihood of contracting illness).

Ethnicity: No association (All races equally likely to contract illness)

Occupation: No association (All job types within the building equally likely to

contract illness)

What they had eaten that day: No association (Both cases and those who were not

affected had eaten a variety of foods, both from inside and outside the building)

What they had to drink that day: No association (Both cases and unaffected had

had a variety of drinks, including water from the building)

How long they had been in the building: the longer someone was in the building,

the more likely they were to come down with the illness.


In addition, employees in nearby buildings were asked whether they were ill. No other cases were reported.


7. Formulate a hypothesis as to the source of the illness/transmission agent causing the outbreak.

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