INVESTIGATION of EPIDEMIC LEARNING OBJECTIVES Recognize trends of disease occurrence. Recognize trends of disease occurrence. Define epidemic and its characteristics Define epidemic and its characteristics Know why and why to investigate an epidemic Know why and why to investigate an epidemic Know the steps in conducting an investigation Know the steps in conducting an investigation Understand the importance of a case definition Understand the importance of a case definition Understand how to interpret an epidemic curve Understand how to interpret an epidemic curve Know what elements should be included in a Know what elements should be included in a report report Patterns of disease occurrence 1) endemic level; 2) Hyperendemic disease; 3) Sporadic occurrence 4) Epidemics or outbreak, differences?? 5) Pandemic. Declaration of epidemic: An epidemic a situation when the observed number of cases unaccountably exceeds the expected number in specific place. Comparison . Even one case of small pox today anywhere in the world, will be considered an emergency condition. Why? Why? Seasonality and time period. Does epidemics restricted only to infectious diseases can occur as epidemics? NO NO The incubation period of an outbreak of CHD has been computed to be 10 years. What does the disease needs to start ? How the epidemic can be precipitated: a recent increase in amount or virulence of the a recent increase in amount or virulence of the agent. ( genetic change) agent. ( genetic change) the recent introduction of the agent or factor into the recent introduction of the agent or factor into a novel setting a novel setting an enhanced mode of transmission (more an enhanced mode of transmission (more susceptible are exposed) susceptible are exposed) factors involve introduction through new portals factors involve introduction through new portals of entry of entry some change in the susceptibility of the host some change in the susceptibility of the host response to the agent (more susceptible ) response to the agent (more susceptible ) Epidemic Classification 1- Common Source Epidemics Point Source Epidemic Point Source Epidemic Continuous or intermittent. Continuous or intermittent. 2- Propagated Epidemics the affected individuals had an exposure to a common agent. 1- A Common source outbreak the affected individuals had an exposure to a common agent. point source outbreak:, - single exposure, - one incubation period. - one incubation period. - peak epidemic curve. - peak epidemic curve. Contineous or intermittent source outbreak: - longer, irregular pattern with the exposure being either intermittent or continuous. - longer, irregular pattern with the exposure being either intermittent or continuous. - (irregular pattern curve). - (irregular pattern curve). In a point-source outbreak, there is a sudden peak in the incidence and quick fall. No fresh cases occur after some time. Cases Time ( days ) Fig 1. point-source outbreak 2. Propagated (serial) outbreak: - a successive series of peaks - increasing no. of cases with each generation How to anticipate epidemics? 1.Surveillance systems: The systematic collection, analysis, interpretation, and dissemination of health data, to gain knowledge of the pattern of disease occurrence. Aims: control and prevention. 2. Hospital data: Large number of cases being admitted in hospitals can point to outbreaks [e.g. dengue outbreak] How to anticipate epidemics? 3. Data from Laboratories: Microbiological or pathological data from labs can also point to outbreaks [e.g. V. cholera 0139, specific cancer] 4. Media: In countries where no surveillance systems exist, lay press is a source, Goals of an outbreak investigation: To identify the source of illness To guide public health intervention Objectives of an epidemic investigation 1- to define the magnitude of the epidemic outbreak in term of time, place and person. 2- to identify the cause, source of infection and modes of transmission 3- to determine the conditions responsible for the occurrence of the disease. 4- to determine measures to control epidemics and recommendations to prevent its recurrence. EPIDEMIC CURVE. A histogram that shows the Does all outbreaks should be investigated?? : No; Diseases requiring investigations 1.Endemic diseases with epidemic potential. 2.Even a single case of diseases for which eradication/elimination goals have been set. 3.Rare but internationally important diseases with high case fatality rates. 4.Outbreaks of unknown etiology. 5.Public concern. During the previous year, nine residents of a community died from the same type of cancer. List some reasons that might justify an investigation. No. of cases Pop. Size Common ca? Treatable? Under Screening ? How to Investigate an Epidemic: 1.Confirmation of the existence of an epidemic 2. Verification of diagnosis: 3. Define cases and conduct case finding. 4. count cases Determine risky groups 5. Orient data 6. Generate hypotheses 7. Compare the hypothesis with established facts 8. Testing the hypotheses 9. Propose and implement control measures. 10. Write the report. 1. Ensure the existence of an epidemic: Real epidemic pseudo-epidemic (excluding artifact). (either due to better surveillance or greater awareness about the disease). Steps in an outbreak investigation Artifact: is a misleading observed differences due to a human factor. Types: Numerator errors: Denominator errors: Numerator errors: 1. misinterpret case definition: e.g: DM 2. Change in the awareness of the condition (case recognition) 3. Enhance case finding (diagnostic test) 4. Lab. Errors (technicians) Denominator errors: 1. Wrong denominator ( pop. At risk) 2. Calculation at country level and extrapolation to a local level. 3. The Pop. Is under-enumerated 2. Verify diagnosis Clinical findings Laboratory findings If unknown disease? 3. Define a case and conduct case finding Develop a specific case definition using: What is case definition? Initially should be general. Components of Case Definition Person Type of illness (e.g., a person with...) Place Location of suspected exposure Time Based on incubation (if known) Define a case and conduct case finding Conduct surveillance using case definition Existing surveillance Active surveillance (e.g. review medical records) Interview case-patients 4. Count cases and determine risky group. To get an idea of the magnitude of the problem. Health institutes Physician, lab, community, press reports. Questionnaire/Survey: - among the population you believe to be at risk. - A questionnaire is solicited to those ill and well, associated with the incident, - Patient may be asked if he knew of other cases in the home,family,neigbourhood,school,workplace, - It assists in developing better hypotheses about the etiologic agents identity, source, including the means and time of transmission. 5. Orient and tabulate data Create line listing Create line listing Person Person - tables - tables Place Place Where were they infected? Where were they infected? Epi map Epi map Time Time When were they infected? When were they infected? Create an epidemic curve Create an epidemic curve Type of outbreak. Type of outbreak. 6. Formulate hypothesis Develop hypotheses based on: Develop hypotheses based on: - available information - available information - interviews of several case-patients - interviews of several case-patients Compare it with literature reviews of previous outbreaks Compare it with literature reviews of previous outbreaks 8. Test hypothesis Conduct an analytic study to test hypotheses Conduct an analytic study to test hypotheses Retrospective cohort study Retrospective cohort study Case-control study Case-control study Measure of association Measure of association 9. implement control measures If an obvious source of the contamination is identifiedinstitute control measures immediately! If an obvious source of the contamination is identifiedinstitute control measures immediately! treating the source, and cases, implement essential prevention to inhibit further exposure and future outbreaks. treating the source, and cases, implement essential prevention to inhibit further exposure and future outbreaks. Create mechanism to evaluate both short- and long-term success Create mechanism to evaluate both short- and long-term success 10. WRITE THE REPORT CONTENTS OF REPORT : CONTENTS OF REPORT : Back ground Historical data, Back ground Historical data, Methodology of investigation, Methodology of investigation, Analysis of data (clinical data, Epideiological data, modes of transmission, Lab data, Interpretation of data) Analysis of data (clinical data, Epideiological data, modes of transmission, Lab data, Interpretation of data) Control measures Control measures Evaluation Preventive measures Evaluation Preventive measures During August, a county health department received reports of 12 new cases of tuberculosis and 12 new cases of aseptic meningitis. Tuberculosis does not have a striking seasonal distribution; however, aseptic meningitis, which is caused primarily by a viral infection, is highly seasonal, and peaks from August to October. What additional information is needed to determine whether either of these groups of cases is an outbreak?