Epidemiological Investigations

1. Epidemiological Investigations

2. The objective of this lecture is to know how to investigate epidemics when the cause is known or unknown

3. عرفت الهوى مذ عرفت هواكا وأغلقت قلبي عمن عداكاوقمت أناجيك يا من ترى خفايا القلوب ولسنا نراكاأحبك حبَين: حب الهوى وحبا لأنك أهل لذاكافأما الذي هو حب الهوى فشغلي بذكرك عمن سواكاوأما الذي أنت أهل له فكشفك للحجب حتى أراكاوما الحمد في ذا ولا ذاك لي ولكن لك الحمد في ذا وذاكا

4. Obtain some information: • 1- Make preparations (scientific, administrative, consulting,..) • 2. The probable Dx. including symptoms and lab data • 3- Confirm that there is an epidemic actually caused by • -The number of cases • -The distribution of the cases with regard to person, place and time • -Preliminary estimates of rates calculated after obtaining the denominator • -The trend of the discussed area : including the median endemic index • 4. Determine the local facilities available to aid in the Dx.

5. II Go to the scene of epidemic : A- identify your self to the health authorities

6. B- Establish a base for the confirmation of DX. by going to the place and examine the cases (if you are a physician) or ask for assistance of a specialist according to the type of the disease, this is done by: • complete clinical characterization of the cases )criteria for case definition) • establish clear procedures for collection and processing of Dx. Specimens • A lab. confirmation of Dx. Is important if possible to eliminate miss-Dx.

7. C- Know what diseases are occurring in the area at the same time to avoid overlapping of symptoms. • D- Determine if cases of the same disease are occurring concurrently in adjacent or geographically related area. • E- ask your self (( Is this an epidemic )), compare number of cases with the expected No. • Certain factors make the determination difficult: • Marked recent or in the population size or in the number of cases • Over-estimation due to miss-Dx. or duplicate reporting • Normal seasonal variation • making numerical comparison of the number of cases rather than rates

8. III- Collect data by a case Hx. form

9. IV- Establishing a case definition Your next task as an investigator is to establish a case definition. A case definition is a standard set of criteria for deciding whether an individual should be classified as having the health condition of interest. A case definition includes clinical criteria and--particularly in the setting of an outbreak investigation--restrictions by time, place, and person. You should base the clinical criteria on simple and objective measures such as elevated antibody titers, fever 38°C, three or more loose bowel movements per day, or myalgias severe enough to limit the patient’s usual activities.

10. To be classified as: confirmed, a case usually must have laboratory verification. A case classified as probable usually has typical clinical features of the disease without laboratory confirmation. A case classified as possible usually has fewer of the typical clinical features. For example, in an outbreak of bloody diarrhea and hemolytic-uremic syndrome caused by infection with E. coli O157:H7, investigators defined cases in the following three classes:

11. •Definite case: E. coli O157:H7 isolated from a stool culture or development of hemolytic-uremic syndrome in a school-age child resident of the county with gastrointestinal symptoms beginning between November 3 - 8, 2009 • Probable case: Bloody diarrhea, with the same person, place, and time restrictions • Possible case: Abdominal cramps and diarrhea (at least three stools in a 24-hour period) in a school-age child with onset during the same period

12. V- Identifying and counting cases •identifying information • demographic information • clinical information • risk factor information • reporter information

13. VI- Performing Descriptive Epidemiology Once we have collected some data, we can begin to characterize an outbreak by time, place, and person. In fact, we may wind up performing this step several times during the course of an outbreak. Characterizing an outbreak by these variables is called descriptive epidemiology, because we describe what has occurred in the population under study. This step is critical for several reasons:

14. First, by looking at the data carefully, we become familiar with them. we can learn what information is reliable and informative (such as if many cases report the same unusual exposure), and learn what may not be as reliable (for example, many missing or “don’t know” responses to a particular question). Second, we provide a comprehensive description of an outbreak by portraying its trend over time, its geographic extent (place), and the populations (persons) affected by the disease.

15. A – Analyze host characteristics • To identify factors that are common to the cases & differentiate them from the unaffected pop. , factors of similarity within the cases series are identified , there will be a bias for formulation of hypotheses to explain the sequence of events that caused the epidemic.

16. B- Analyze the distribution of cases geographically : • to identify exposure of cases to a common location or locations different from that of general or non-ill population , this may provide a clue as to the source of infection and the most common procedure is to spot the cases on maps by : • Place of residence • - Areas of recreation • Place of occupation • Place of onset • Distinguishing between primary and secondary case by • place and time

17. By this we will be able to recognize clusters, however, it is essential to know the distribution of the general pop. as cases may be distributed proportionally to the population distribution Also we may study the remarks and surroundings of the area like rivers , factories , sewage drainage , animal population , … which may sign us an idea about the cause, source or mode of transmission of the disease.

18. C- Analyze the distribution of cases on a time scale: obtain a picture of the sequential development of the epidemic ( epidemiological curve ) Exposure ----- Incubations----- onset---- Dx.----- death or recovery.

19. 1. Dates when describing the occurrence of an infection in time, several dates may be of interest: - The date of exposure : very important but difficult to know - The date of onset : the commonly used date , however , the disease may not be recognized in it’s early stages ---onset dates will vary. - The date of Dx. : may be the day a physician first sees the case , however great variation exists in this date depending on the need of the pat. to see a doctor early or late in the disease , and the ease of diagnosing the disease , also depend on: the patient’s economic status, the severity of the illness, the availability of a doctor or merely when the doctor makes his mind or changes it.

20. The date of reporting : may vary widely as does the DX. • The date of hospitalization : may be the only one available for some diseases or cases , it is of limited use because only few cases may be severe enough to require hospital care. • The date of death , recovery , or development of a chronic state : have the disadvantages of the others, moreover , they may be so far separated from the date of exposure that they have little epidemiological value.

21. 2- Interpretation of the epidemic curve: -one abrupt rise usually means one exposure but due to the element of damage and difficult diffuse mechanisms of individuals, the incubation period of cases will vary. A series of waves usually indicates - that the force causing the epidemic has operated more than once - that a second source of infection has evolved, - that carriers are at large - or that the original cases may be spreading the disease. Its possible to have concurrent epidemics with different causes

22. Missed , unrecognized and unreported cases may distort the true picture of date onset • Over-reporting and secondary cases distort the true picture of the decline of the epidemic , causing a tailing out of the curve , one skewed to the right • The period of time between the onset and peak of the epidemic may vary because of the numbers of the contacts, the vehicles of infection and the number of cases in the epidemic, the severity of cases , …etc.

23. The decline may indicate the removal of the source of infection , the reduction of susceptible in the population , the introduction of control measures , …etc. • Prolongation of the epidemic may indicate prolonged exposure to the initial source or to carriers and contacts in the population. • It is fallacious to presume at the onset of a study that there can only be one disease for the case series , unusual symptom complexes or peculiar epidemic curves, bimodal or double peaked curves that may suggest the involvement of a second disease agent.

24. VII – Obtain a comparable group: To see the difference between the cases and the population by choosing a random probability sample preferably among persons sharing the same event (non cases) after matching the same characteristics. • neighbors of cases • patients from the same physician practice or hospital who do not have the disease in question • friends of cases

25. VIII- Analysis of cases & their contacts with the environment A- Consideration of remote and recent exposure, this depend on the incontinence of the disease, the mobility of the population, the origin of food and water supplies , … B- Be alert to the possibility that the source may not have been ill, it may be necessary to use some techniques, to uncover unapparent infection and missed cases such as : serological tests, isolation of the virus (or bacteria), X-rays, questioning people or re-examining medical and hospital records

26. IX- Classify the severity: usually by CFR : CFR may be unusually high, this have several epidemiological explanations: - The organism may be unusually virulent - The dosage of the agent may have been unusually high - There may be many mild cases not reported or diagnosed - cases of another and more severe disease may have been misdiagnosed as the disease under study - A super-imposed secondary infection may cause mortality - There was a lack of proper RX - The pathogen has developed resistance to a previously effective drug.

27. 2. An unusually low CFR suggests the reveres of any of the above factors , or the introduction of a new effective RX.

28. X- Developing & evaluating hypotheses We usually begin to generate hypotheses with the first phone call. But at this point in an investigation, after talking with some case-patients and with local public health officials, and having characterized the outbreak by time, place, and person, our hypotheses will be sharpened and more accurately focused. The hypotheses should address the source of the agent, the mode of transmission, and the exposures that caused the disease. Also, the hypotheses should be testable, since evaluating hypotheses is one of the goals of the next step in an investigation.

29. We can generate hypotheses in a variety of ways. First, consider what we know about the disease itself: What is the agent’s usual reservoir? How is it usually transmitted? What vehicles are commonly implicated? What are the known risk factors? In other words, simply by becoming familiar with the disease, we can, at the very least, “round up the usual suspects

30. XI- Refining Hypotheses Even when the analytic study identifies an association between an exposure and disease, we often will need to refine the hypotheses. Sometimes need to obtain more specific exposure histories. Sometimes we need a more specific control group to test a more specific hypothesis. For example, in many hospital outbreaks, investigators use an initial study to narrow their focus. They then conduct a second study, with more closely matched controls, to identify a more specific exposure or vehicle

31. XII- Communicating the findings (Report) • 1. Oral briefing for local authorities: The briefing should be attended by the local health authorities and persons responsible for implementing control and preventive measures. Usually these persons are not epidemiologists, so we must present our findings in clear and convincing fashion with appropriate and justifiable recommendations for action. This presentation is an opportunity for us to describe what we did, what we found, and what we think should be done about it. We should present our findings in scientifically objective fashion, and should be able to defend our conclusions and recommendations.

32. 2. written report that follows the usual scientific format of introduction, background, methods, results, discussion, and recommendations. By formally presenting recommendations, the report provides a blueprint for action. It also serves as a record of performance and a document for potential legal issues. It serves as a reference if the health department encounters a similar situation in the future. Finally, a report that finds its way into the public health literature serves the broader purpose of contributing to the knowledge base of epidemiology and public health.