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Outbreak Investigation

Outbreak Investigation. Objectives. Determine if an outbreak is occurring Characterise the outbreak Identify additional cases Identify causative agent Identify the source Initiate steps to terminate transmission Learn for the future. Surveillance for disease.

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Outbreak Investigation

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  1. Outbreak Investigation

  2. Objectives • Determine if an outbreak is occurring • Characterise the outbreak • Identify additional cases • Identify causative agent • Identify the source • Initiate steps to terminate transmission • Learn for the future

  3. Surveillance for disease • Certain infectious diseases pose significant threats to the health of the public • It is important that public health know about them • States, federal and international health authorities develop lists of reportable disease.

  4. Surveillance • Physicians, hospitals and clinical laboratories are required to report, usually within a specified time period • Serious diseases are often categorized separately • Basic functions of communicable disease control at the state and local levels is the gathering and analysis of reportable disease data

  5. Examples of Reportable Diseases in Michigan • Enteric diseases • Salmonellosis, shigellosis, Campylbacter, hepatitis A • Sexually Transmitted Diseases • Syphilis, gonorrhea, HIV • CNS diseases • Bacterial and viral meningitis, Arboviral – WNV, EEE • Vaccine Preventable Diseases • Measles, mumps, diphtheria, polio, hepatitis B

  6. Collection and analysis of data • LHD receives reports • LHD logs and reviews data • Number of cases within a specific time period e.g a week • Geographic distribution of cases • Determination if the cases need further investigation • LHD send s reports to MDCH via LHDSurv (soon to be replaced by MDSS) • MDCH sends reports to CDC via NETSS

  7. Data analysis • Need to know background level • Track weekly occurrence of disease over a number of years to establish an average number of cases per week for that particular week • Need to determine when the number of cases is above the background

  8. Establish when an outbreak is occurring • For some diseases, a single case is an outbreak e.g measles, smallpox • In others, use the background as a threshold, but take into account other variables • e.g. is the gender or age frequency similar to previous years, • are the cases clustered geographically

  9. Sometimes the illness is reported before a disease is identified • Many calls from physicians and the public about persons becoming ill with diarrhea, vomiting • Using symptoms try to ascertain the likely disease • Get specimens to send to the laboratory if not already obtained

  10. Investigation • Characterize the data that you have • Try to identify other similar cases occurring • How would you do this? • Fax to ERs • Notices on electronic boards

  11. Case definition • What would you need in a case definition • Time • Place • Symptoms • E.g. All persons who experienced vomiting, diarrhea, nausea or abdominal cramping and who ate at Joe’s Greasy Spoon between Dec 10 and Dec 14.

  12. Data Analysis • Determine which cases fit the case definition • Analyse by variables • Age • Gender • Race • Geography • Risk factor • Food • Drink • Behavior e.g. smoking, outdoor activity, attends day care

  13. Data analysis • Frequencies • Incubation periods • Laboratory tests • Epidemic curve • Attack rates • By person • By place • By risk factor

  14. Hypothesis • Using the data analysis, formulate hypotheses • Select the hypothesis which fits the picture • Hypothesis should address • Source of the disease • Etiologic agent • Method of transmission • Control methods

  15. Hypothesis generation • Hypotheses can be generated at any time during the investigation, and refined as more data becomes available. But beware of making false assumptions • But don’t leave it too late, as hypothesis generation should lead to control measures

  16. Hypothesis testing • Laboratory results can confirm the etiologic agent • Prevent further consumption of implicated food can confirm the transmission/source • Data analysis – • Case – control study using matched or unmatched controls.

  17. Control methods • Initiate control methods based upon hypothesis

  18. Report of findings • So others can learn from our experience

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