Objectives: During this activity, you will use data from a real investigation of an unexplained outbreak to: • Graph statistical data from an epidemic • Calculate attack rates • Analyze graphical data • Design an experiment to investigate the source of a causative agent of an epidemic • Compre the characteristics of known diseases with an unknown The Situation You are an Epidemic Intelligence Service (EIS) officer at the CDC , Atlanta, GA. A request for epidemic aid comes to the CDC from the Michigan State Health Department. There has been an outbreak of an acute febrile 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 to assess the situation. The department bulilding 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 23 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 15.2O C on June 29 to 27.4OC 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 # 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 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.” 1) Which group of symptoms would you use to determine if a patient represented a “case?” Why would 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 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. 2) Use this data to begin an epidemic curve for the employees. The bar for July 2 has been drawn for you. Date # ill July 1 1 July 2 66 July 3 21 July 4 2 July 5 1 Determining Attack Rate Attack rate is the proportion of the population that is at-risk for infection that actually develops 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% 3) What is the attack rate for the employees at the time of your arrival on Saturday? (show work) 4) What information must you have to determine an attack rate for visitors? Why is it much easier to determine the attack rate for the employees than for the visitors? Continuing the investigation In taking a closer look at the employee and visitor cases you determine that these people vary widely in age, ethnicity, occupation and what they ate or drank just prior to the illness. However, it does seem that the longer someone was in the building, the more likely they were to come down with the illness. In addition, no cases were reported in county employees who worked in the other buildings in the area. 5) What is the one thing that all cases appear to have in common? 6) Formulate a hypothesis as to the source of the illness. Back it up with evidence that you have collected? Q1` You work with your team in the building over the entire weekend and none of you begin to feel any effects of the mysterious illness. You continue your investigation into the building as it reopens on Monday, July 8. Late Tuesday evening, you and your two colleagues become ill. You make the decision to close the building on Friday July 12. By July 17, 95 cases had occurred in the 100 employees exposed to the building. Interestingly, none of their family members had become ill during this period. 7) What does this information tell you about the transmission of the agent causing the outbreak? Contacting the visitors to the health department proves a difficult task. Of the 170 visitors that you were able to locate, 47 were considered to be cases. 8) How would you go about identifying visitors to the health center? 9) Calculate the attack rate for visitors to the health department. (show work) 10) Why do you suppose that the attack rate is considerably lower for visitors than it is for employees? 11) You compile all the data available on July 18 which is expressed in the table below. Add the new information to the epidemic curve that you began above. Place bars for the visitors on top of those for employees, so that it shows the total number of new cases for each day.