Outbreak Investigation

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