Chapter 10

Chapter 10

Chapter 10 Field Epidemiology What Is Field Epidemiology? Field epidemiology has been defined as the application of epidemiology under a set of general conditions The problem is unexpected A timely response may be demanded Travel to and work in the field is required by

epidemiologists to solve the problem The investigation time is likely to be limited because of the need for a timely intervention Field Investigation Field investigations involving acute problems may differ from conventional epidemiologic studies in three important ways 1. Field investigations often do not start with a clear hypothesis. 2. Acute problems involve an immediate need to protect

the public and resolve the concern. 3. Field epidemiologists must decide when the available information is sufficient to take appropriate action. Steps in Field Investigations 1. 2. 3. 4. 5. 6. 7. 8.

9. 10. 11. 12. 13. 14. Establish the existence of an epidemic (or outbreak) Confirm the diagnosis Establish criteria for case identification Search for missing cases Count cases Orient data according to person, place, and time

Classify the epidemic Determine who is at risk of becoming a case Analyze data Formulate hypotheses Test hypotheses Develop reports and inform those who need to know Execute control and prevention measures Administration and planning activities 1. Establish the existence of an epidemic (or outbreak) Attack rates are appropriate statistics for

investigating disease outbreaks because they describe rapidly occurring new cases of disease in a well-defined population over a limited time period Attack rates are usually calculated by person characteristics (e.g., age, sex, race/ethnicity, and occupation) in order to identify high-risk groups 2. Confirm the Diagnosis Assessment of the clinical findings should be done to assure correctness and reliability of the

findings Clinical diagnosis by appropriately trained individuals Laboratory diagnosis 3. Establish Criteria for Case Identification Standard clinical criteria (what) Loose case definition vs. strict case definition A case may be further characterized by Who When Where

4. Search for Missing Cases Investigation may include Physicians Clinics Health maintenance organizations Hospital emergency rooms Public health clinics Migrant health clinics Related facilities Asymptomatic persons or mild cases and their

contacts should be evaluated Suspected cases vs. probable cases 5. Count Cases Exposure status and disease frequency need to be determined and compared with the appropriate at-risk population 6. Orient data according to person, place, and time Person Inherent characteristics or people (age, race/ethnicity,

sex) Acquired characteristics (immunity or marital status) Activities (occupation, leisure, use of medications) Conditions (socioeconomic state, access to health care) Place Residence,birthplace, place of employment, school district, hospital unit, country, state, county, street address, map coordinates, etc. Census tract

Time Epidemic curve 7. Classify the Epidemic Common source Propagated Mixed 8. Determine who is at risk of becoming a case Clinical, medical, and lab findings need to be confirmed, evaluated, and analyzed for all cases

to substantiate the diagnosis Classify cases by exposure status 9. Analyze Data The epidemiologist gathers, compiles, tabulates, analyzes, and interprets the findings Analysis often involves statistical methods: Frequency tables Bivariate analyses Multiple regression 10. Formulate Hypotheses

For example, in a food-borne outbreak, hypotheses should be developed for the following Infection source, vehicle Suspect foods Transmission mode Pathogen type (based on clinical symptoms, incubation periods) Time factors in the outbreak and course of the disease Place factors in the outbreak Person characteristics and factors in the outbreak

Outside sources of the infection Transmission of the disease outside of the study population Exposed, unexposed, well, and ill cases/individuals 11. Test Hypotheses Statistical tests should be employed to evaluate hypotheses T-test Chi-square test F-test If established facts or information are lacking to

substantiate a hypothesis, more information should be gathered or research hypothesis should be rejected 12. Develop reports and inform those who need to know Narrative of the investigation and review of the course of the epidemic Tables, graphs, charts, or any useful and helpful illustrations are presented, as well as any pertinent epidemiologic data, tests, lab reports, information,

and characteristics Addresses the information presented under hypothesis 13. Execute control and prevention measures Immunization programs Risk factor prevention Behavior change programs 14. Administration and planning activities Organization

Coordination Communication Planning Funding Allocation The Steps Conceptual Provide a logical progression for the investigation Can/should be taken out of order (with caution) Oswego

Oswego An Outbreak of Gastrointestinal Illness Following a Church Supper, Adapted from Centers for Disease Control and Prevention, Epidemiology Program Office, Case Studies in Applied Epidemiology, No. 401-303 http://www.cdc.gov/EIS/casestudies/ xoswego.401-303.student.pdf Gross MB. Oswego county revisited. Public Health Reports 1976;91:160-70. PART I - Background On April 19, 1940, the local health officer in the village of

Lycoming, Oswego County, New York, reported the occurrence of an outbreak of acute gastrointestinal illness to the District Health Officer in Syracuse. Dr. A. M. Rubin, epidemiologist-in-training, was assigned to conduct an investigation. When Dr. Rubin arrived in the field, he learned from the health officer that all persons known to be ill had attended a church supper held on the previous evening, April 18. Family members who did not attend the church supper did not become ill. Accordingly, Dr. Rubin focused the investigation on the supper. He completed interviews with 75 of the 80 persons known to have attended, collecting information about the occurrence and time of onset of symptoms, and foods consumed. Of the 75 persons interviewed,

46 persons reported gastrointestinal illness. Would you call this an epidemic? Would you call it an outbreak? An outbreak, if we consider an outbreak to reflect a more localized situation and an epidemic to refer to more widespread and possibly more prolonged situations. In addition, the term outbreak may be perceived as less alarming than epidemic. Review the steps of an

outbreak investigation The advantage of a list such as this, or an alternative version of this list, is to make sure that important steps are not missed while conducting a field investigation However, the order of these steps is not fixed (e.g., it may be appropriate to implement control measures immediately) In addition, the steps are often dynamic; that is, case definitions, line listings, hypotheses, and so on, can and sometimes should change with further information

Clinical Description The onset of illness in all cases was acute, characterized chiefly by nausea, vomiting, diarrhea, and abdominal pain. None of the ill persons reported having an elevated temperature; all recovered within 24 to 30 hours. Approximately 20% of the ill persons visited physicians. No fecal specimens were obtained for bacteriologic examination. Broad Categories of Diseases

List the broad categories of diseases that must be considered in the differential diagnosis of an outbreak of gastrointestinal illness Pathogen (bacterial, viral, parasitic), toxin (e.g., heavy metals, drugs, insecticides), other (sociogenic, radiation) The investigators suspected that this was a vehicle-borne outbreak, with food as the vehicle.

What is a vehicle? What is a vector? Vehicle a nonliving intermediary such as food, water, biologic product, or fomite (inanimate object) that conveys the infectious agent from its reservoir (habitat where an infectious agent lives, grows, and multiplies) to a susceptible host Vector a living intermediary, most often an insect or arthropod (e.g., mosquito, flea, or tick), that conveys the infectious agent from its reservoir to a susceptible host What are other modes of

transmission? Transmission when an infectious agent leaves its reservoir or host through a portal of exit, is conveyed by some mode of transmission, and enters through an appropriate portal of entry (i.e., oral, respiratory, reproductive, intravenous, urinary, skin, gastrointestinal, conjuctival, transplacental) to infect a susceptible host. Transmission involving a vector may be mechanical (i.e., the agent does not multiply or undergo physiologic changes in the vector) or biological (i.e., the agent undergoes part of its life cycle inside the vector prior to transmission into the new host). Direct transmission direct contact (e.g., mucous membrane to mucous membrane, skin-to-skin, across placenta, fecal-oral),

droplet spread (e.g., sneezes, coughs) Indirect transmission airborne, vehicle-borne, vector-borne If you were to administer a questionnaire to the church supper participants, what information would you collect? Identifying information (e.g., name, address, phone number) Demographic information (e.g., age, sex, occupation) Clinical information (e.g., clinical signs and symptoms, severity or outcome, time of onset, duration, documented medical care, pre-existing medical conditions, medications)

Epidemiology information (e.g., specific exposures and contacts, such as foods eaten at the picnic, how much, when, foods eaten before the picnic and before illness, activities, and contact with ill persons) Abstractor or interviewer information What is a line listing? What is the value of a line listing? Looks like a spreadsheet where each row represents data for a single case and each column represents a variable (such as identifying information, demographic information, clinical

information, epidemiologic information, etc.) Description of the Supper The supper was held in the basement of the village church. Foods were contributed by numerous members of the congregation. The supper began at 6:00 p.m. and continued until 11:00 p.m. Food was spread out on a table and consumed over a period of several hours. Data regarding onset of illness and food eaten or water drunk by each of the 75 persons interviewed are provided in the attached line listing. The approximate time of eating supper

was collected for only about half the persons who had gastrointestinal illness. What is the value of an epidemic curve? A histogram that shows the course of a disease outbreak or epidemic by plotting the number of cases by time of onset. The epidemic curve is a basic tool of field epidemiologists because it is informative in the following ways: Shows the magnitude of the epidemic over time Can distinguish epidemic from endemic disease Potentially correlated events can be noted on the graph

Its shape can provide clues as to whether a point source, intermittent source, or propagated source is responsible Shows where we are in the course of the epidemic (i.e., still on the upswing, down slope, or end) Can indicate how long it took the health department to identify the problem Outlying cases are easily recognized What does the graph tell you? One very early and one very late case Most cases are clustered tightly within a six-hour

interval The tight clustering around a single peak is consistent with a point source outbreak Case Inconsistencies Are there any cases for which the times of onset are inconsistent with the general experience? How might they be explained? Subject #52 was an 8-year-old boy who ate early (11:00 a.m.). The boy may have eaten early and the vehicle already prepared and contaminated by 11:00 a.m.

Subject #16 was a 32-year-old woman. Perhaps she had a longer incubation period or ate later. Other possibilities are that she had an unrelated illness, the information was incorrect, there was a data coding error, or she was a secondary case. How could the data in the line listing be better presented? Data could be separated according to illness status or time of onset Military time could have been used

Determine the mean, median, and range of the incubation period Range: Minimum = 3 hours and maximum = 7 hours. The range is 4 hours The median incubation period is 4 hours Differential Diagnosis How does the information on incubation period, combined with the data on clinical symptoms, help in the differential diagnosis of the illness? Each food-borne illness has a characteristic incubation

period, specific symptoms, and foods with which it is most likely associated. The observed incubation period is too long for heavy metals and too short for viral agents and botulism. Referring to a partial list of incubation periods for major communicable diseases (see Merrill, RM. Introduction to Epidemiology, 5th edition. Sudbury, MA: Jones and Bartlett Publishers; 2010, Table 3-7). Staphylococcal food poisoning has an average incubation period of 2-4 hours. Hypothesis Testing HO: RR =1

HA: RR 1 = 0.05, n = 75 43 /(43 11) Risk Ratio (RR) 5.6; 3 /(3 18) 11/43 18/3 1.95, 16.10

95% CI(5.6) exp ln5.6 1.96 43 11 3 18 Note Three ill people denied eating the vanilla ice cream. They may have not remembered eating the vanilla ice cream, there could be multiple vehicles, or there could have been cross-contamination between foods

by way of dishes, spoons, and servers. Outline further investigations that should be pursued Detailed review of source, ingredients, preparation, and storage of incriminated food Try to explain cases with atypical time of onset Laboratory examination Determine if secondary spread in family members occurred Additional calculations (e.g., age- or genderspecific attack rates)

What control measures would you suggest? Prevent consumption of remaining vanilla ice cream Prevent recurrence of similar events in the future by educating food handlers Ascertain whether a commercial product is involved Eliminate any contaminated sources of food Why was it important to work up this outbreak?

Rule out contamination of a commercial product. If a commercial product is involved, immediate intervention may prevent a substantial number of further cases. Prevent future outbreaks by identifying infected food handler, specific gaps in education or food handling techniques Public health officials need to respond to such problems in a timely manner to maintain a cooperative relationship with local health departments, private physicians, and the community An epidemiologic explanation of the cause of the outbreak may allay community fears and concerns The outbreak investigation may provide opportunities for investigators to answer questions about the agent, host,

environment, incubation period, and so on How does this investigation fit the steps for conducting an outbreak investigation? Prepare for field work Not discussed Establish the existence of an epidemic or outbreak Done Confirm the diagnosis Diagnosis based on signs and symptoms. No fecal specimens were obtained for bacteriologic examination. Establish criteria for case identification No case definition was given in the case study Search for missing cases Not discussed

Count cases Done Orient the data according to person, place, and time Not done, with the exception that time was characterized by the epidemic curve Classify the epidemic Done: point source Fitting Outbreak Investigation? How does this investigation fit the steps for conducting an outbreak investigation? Determine who is at risk of becoming a case Done: those at the church super Analyze the data Done: risk ratios involving the different foods eaten Formulate hypotheses Not formally done

Test hypotheses Not formally done Develop reports and inform those who need to know A report was prepared and disseminated (i.e., see Part IV of the case study) Maintain surveillance to monitor trends and execute control and prevention measures On May 19, all remaining ice cream prepared by the Petrie sisters from raw milk from the Petrie farm was condemned. The method of contamination of ice cream is not clearly understood. Whether positive Staphylococcus nose and throat cultures occurring in the Petrie family was involved is unclear. Carry out administration and planning activities Not specified in the case study

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