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Chapter 10 Field Epidemiology

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© 2010 Jones and Bartlett Publishers, LLC Objectives  Define field epidemiology  Identify the steps of a field investigation  Be familiar with epidemiologic questions that may be helpful in a field investigation.  Define cluster and cluster investigation and discuss the process for investigating clusters  Identify the primary challenges in detecting reported clusters.  Describe methods for assessing reported clusters

© 2010 Jones and Bartlett Publishers, LLC 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

© 2010 Jones and Bartlett Publishers, LLC 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

© 2010 Jones and Bartlett Publishers, LLC Steps in field investigation 1. Establish the existence of an epidemic (or outbreak) 2. Confirm the diagnosis 3. Establish criteria for case identification 4. Search for missing cases 5. Count cases 6. Orient the data according to person, place, and time 7. Classify the epidemic 8. Determine who is at risk of becoming a case 9. Analyze the data 10. Formulate a hypotheses 11. Test hypotheses 12. Develop reports and inform those who need to know 13. Execute control and prevention measures 14. Administration and planning activities

© 2010 Jones and Bartlett Publishers, LLC 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.

© 2010 Jones and Bartlett Publishers, LLC 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

© 2010 Jones and Bartlett Publishers, LLC 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

© 2010 Jones and Bartlett Publishers, LLC 4. Search for missing cases  Investigation may include Physicians Clinics health maintenance organizations hospital emergency rooms public health clinics migrant health clinics and related facilities  Asymptomatic persons or mild cases and their contacts should be evaluated  Suspected cases vs. probable cases

© 2010 Jones and Bartlett Publishers, LLC 5. Count cases  Exposure status and disease frequency need to be determined and compared with the appropriate at-risk population

© 2010 Jones and Bartlett Publishers, LLC 6. Orient the 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 Census tract Street address, Map coordinates, etc  Time Epidemic curve

© 2010 Jones and Bartlett Publishers, LLC 7. Classify the epidemic  Common source  Propagated  Mixed

© 2010 Jones and Bartlett Publishers, LLC 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

© 2010 Jones and Bartlett Publishers, LLC 9. Analyze the data  The epidemiologist gathers, compiles, tabulates, analyzes, and interprets the findings  Analysis often involves statistical methods Frequency tables Bivariate analyses Multiple regression

© 2010 Jones and Bartlett Publishers, LLC 10. Formulate hypotheses  In a food-borne outbreak, for example, hypotheses should be developed for the following: The source of infection The vehicle of infection The suspect foods The mode of transmission The type of pathogen (based on clinical symptoms, incubation periods) The time factors in the outbreak and course of the disease The place factors in the outbreak The person characteristics and factors in the outbreak The outside sources of the infection The transmission of the disease outside of the study population The exposed, unexposed, well and ill cases/individuals

© 2010 Jones and Bartlett Publishers, LLC 11. Test of 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, then more information should be gathered or the research hypothesis should be rejected

© 2010 Jones and Bartlett Publishers, LLC 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

© 2010 Jones and Bartlett Publishers, LLC 13. Execute control and prevention measure  Immunization programs  Risk factor prevention  Behavior change programs

© 2010 Jones and Bartlett Publishers, LLC 14. Administration and planning activities  Organization  Coordination  Communication  Planning  Funding  Allocation

© 2010 Jones and Bartlett Publishers, LLC Disease clusters  An unusual aggregation, real or perceived, of health events that are grouped together in time and space and that are reported to a health agency  It generally occurs in response to the sudden introduction into the human environment of a physical stress, chemical or biological agent, or psychosocial condition.

© 2010 Jones and Bartlett Publishers, LLC Cluster Investigation  Involves reviewing unusual numbers of health-related states or events, real or perceived, grouped together in time and location  Confirm reported disease cases,  Identify whether the number of cases is above what is expected  Identify causal relationships if possible

© 2010 Jones and Bartlett Publishers, LLC Process to cluster investigations (1) Initial response (2) Assessment (3) Major feasibility study (4) Etiologic investigation

© 2010 Jones and Bartlett Publishers, LLC Challenges in cluster investigations  Lack of health tracking data may: Cause long delays in cluster investigations. Prevent public health officials from identifying disease trends. Inhibit the identification of true disease clusters. Reduce the number of cluster investigations carried out by states, meaning that some clusters go uninvestigated.

© 2010 Jones and Bartlett Publishers, LLC Statistical Challenges in Cluster Investigations  Most cluster analyses involve post hoc rather than a priori hypotheses.  Rates have the danger of being overestimated because of “boundary shrinkage” of the population where the cluster is presumed to exist.