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I NVESTIGATION OF EPIDEMICS By Dr Sabah M.A.Abdelkader Assist.Prof of public health.

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Presentation on theme: "I NVESTIGATION OF EPIDEMICS By Dr Sabah M.A.Abdelkader Assist.Prof of public health."— Presentation transcript:

1 I NVESTIGATION OF EPIDEMICS By Dr Sabah M.A.Abdelkader Assist.Prof of public health

2 O BJECTIVES After completing this unit a student will be able to: Define the terms "epidemic" and "outbreak" Differentiate between point- source and propagated epidemics Explain why health agencies investigate reported epidemics Discuss the steps of an epidemic investigation


4 INTRODUCTION Occurrence of an epidemic always signals shift in existing balance between agent, host and environment. This calls for a thorough investigation of cases to uncover factors responsible and to guide in control measures. Epidemiology has an important role in investigation of epidemics.

5 O BJECTIVES OF INVESTIGATION A- Define magnitude of outbreak in terms of time, place, person. B- Determine particular conditions and factors responsible for epidemic. C- Identify causes, sources of infection, and modes of transmission to determine control measures. D- Make recommendations to prevent recurrence.

6 D EFINITIONS Epidemic: Is the occurrence of more cases of disease than would normally be expected in a specific place or group of people over a given period of time. One case of small pox today anywhere in the world (except due to accidental exposure in the labs which still store the virus) will be considered an epidemic because none is expected. Outbreak: Refers to a localized health problem. Often used to denote local epidemic.

7 T YPES OF OUTBREAKS : Point source: In which a simultaneous exposure of many susceptible individuals to a source of disease agent results in an explosive increase in the number of cases of the disease over a short period of time, e.g. outbreak of salmonella food poisoning.

8 CONT.: Propagated: In which an infectious agent is propagated in a community by passage from person to person, e.g. outbreak of measles. Transmission continues until the number of susceptible is depleted or susceptible individuals are no longer exposed to infected persons.

9 C ONT. Point sourcePropagated The epidemic curve rises and falls rapidly. Explosive, there is clustering of cases within a narrow interval of time. All cases develop within one incubation period of disease. Epidemic curve rises and falls gradually. Cases occur over a much longer period. Cases occur within more than one incubation period of the diseases.

10 S TEPS OF AN O UTBREAK I NVESTIGATION 1. Confirm diagnosis. 2. Confirm existence of outbreak. 3. Define a case and count cases. 4. Orient data in person, time, place. 5. Determine who is at risk. 6. Develop a hypothesis and test it. 7. Determine control measures. 8. Plan a more systematic study. 9. Implement control measures. 10.Prepare a written report.

11 C ONT. N.B: The steps are presented here in a logical order. In practice, however, several may be done at the same time, or they may be done in a different order. For example, control measures should be implemented as soon as the source and modes of transmission are known, which may be early or late in any particular outbreak investigation.

12 C ONT. Each of these steps is important to complete before moving to next steps. It is a process where each step is dependent on successful completion in previous steps.

13 S TEP 1: V ERIFY THE D IAGNOSIS First, You must ensure that the problem has been properly diagnosed- that it really is what it has been reported to be. Second, You must to be certain that the increase number of diagnosed cases is not the result of a mistake in the laboratory. Verifying the diagnosis requires that you review the clinical findings (the symptoms and features of illness) and the laboratory results for the people who are affected. Finally, you should visit several of the people who became ill. You should see and talk to some of them to gain a better understanding of the diseases and in addition, you may be able to gather critical information by asking such questions as:

14 C ONT. What was their exposure before becoming ill? What do they think caused their illness? Do they know anyone else with the disease? Do they have anything in common with others who have the disease? Conversations with patients are very helpful in generating hypotheses about the cause, source, and spread of disease.

15 S TEP 2: E STABLISH THE E XISTENCE OF AN O UTBREAK : Compare disease frequency during same period of previous years. For a notifiable disease, or you can use health department surveillance records. For other disease, you can find data from local sources such as hospital discharge records, death (mortality) records, and cancer or other health registries.

16 C ONT. If local data are not available, you can make estimates using data from: A-National data. B-You might consider conducting a telephone survey of physicians to determine whether they have seen more cases of the disease than usual. C- You can also conduct a survey of people in the community to establish the background level of disease. D- Epidemic threshold: any amount of disease greater than the upper limit.

17 S TEP 3: D EFINE AND I DENTIFY C ASES A case definition includes four components: Clinical information about the disease. The presence of fever of at least 38 o C. Characteristics about the person who is affected, You might restrict the definition to those who attended a wedding, or ate at a specific restaurant. Information about the location or place, e.g. living in a certain area or working at a particular plant. Specification of time during which the outbreak occurred, the criterion might be onset of illness within the past 2 months.

18 T YPES OF INFORMATION TO BE COLLECTED : Identifying information : name, address, and telephone number. Addresses also allow you to map the geographic extent of the problem. Demographic information: age, sex, race, and occupation. This provides the details that you need to characterize the population at risk. Clinical information: to verify that the case definition has been met. Date of onset allows you to create a graph of the outbreak. Risk factor information: to tailor your investigation to the specific disease in question; e.g. in hepatitis A, you would look at exposure to food and water sources.

19 A LINE LISTING FOR AN OUTBREAK OF HEPATITIS A Diagnostic Signs and Symptoms Lab Case #Initials Report Onset Physician Diagnosis NVAFDUJHAIgMOtherAgeSex 1JG10/1212/6Hep A SGOT37M 2BC10/1210/5Hep A Alt62F 3HP10/1310/4Hep A+-+++S*S* +SGOT30F 4MC10/1510/4Hep A+-++?-+ HbS/ Ag- 17F 5NG10/1510/9NA F 6RD10/1510/8Hep A M 7KR10/1610/13Hep A SGOT =240 43M

20 S TEP 4: D ESCRIBE AND O RIENT THE D ATA ( DESCRIPTIVE EPIDEMIOLOGY ) A- Time: An epidemic curve provides a great deal of information: 1. outbreak’s magnitude. where you are in the course of the epidemic, and possibly to project its future course. 2. Identifying disease and its usual incubation period, you may be able to estimate a probable time period of exposure and develop a questionnaire focusing on that time period. 3. Finally, you may be able to draw inferences about the epidemic pattern, for example, whether it is an outbreak resulting from a common source exposure, from person-to-person spread, or both.

21 H OW TO DRAW AN EPIDEMIC CURVE ? To draw an epidemic curve, you first must know the time of onset (or date of onset) of illness for each person. The number of cases is plotted on the Y - axis; the unit of time on the X -axis. Show the pre- and post-epidemic period on your graph to illustrate activity of disease during those periods.

22 B – P LACE : Assessment of an outbreak by place provides information on the geographic extent of a problem and may also show clusters or patterns that provides clues to the identity and origins of the problem. A simple and useful technique for looking at geographic patterns is to plot, on a “spot map” of the area where the affected people live, work, or may have been exposed. Based on map, calculate attack rate & secondary attack rate ?????

23 C – P ERSON : You determine what populations are at risk for the disease. Personal characteristics (e.g. age, race, sex, or medical status) or exposures (e.g., occupation, leisure activities, use of medications, tobacco, and drugs) are important factors because they may be related to susceptibility to the disease and to opportunities for exposure. For example, if you were investigating an outbreak of hepatitis B, you should consider the usual high-risk exposures for that infection, such as intravenous drug use, sexual contacts, and health care employment.

24 S TEP 5: D ETERMINE WHO IS AT RISK. Where did cases come from? Identify population with same criteria of cases: Geographic location. Time period. Population characteristics. Look for any cases in population at risk. Population at risk is important because: They will be used as controls to test hypothesis. This population will be target for prevention measures.

25 S TEP 6: D EVELOP H YPOTHESIS AND TEST IT You can develop hypotheses in a variety of ways: First, consider what you 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? Test hypothesis using a suitable study design: Retrospective study to compare cases to controls Possible prospective study if exposure is still present.

26 C ONT. Case-control studies compare people with a disease (case-patients) with a group of people without the disease (controls). You then can calculate measure of association – called an odds ratio - to quantify the relationship between exposure and disease. Cohort studies compare groups of people who have been exposed to suspect risk factors with groups who have not been exposed. Calculate incidence rate, relative risk of exposure to a certain suspected item can finally lead to incrimination of the exposure responsible for the epidemic.

27 S TEP 7: D ETERMINE CONTROL MEASURES In common source epidemics, cause may no longer exist. E.g ????? For an outbreak in which causative agent is still a risk, it is important to consider and institute control measures to stop current outbreak: destroying implicated food, shutting down contaminated water source. In propagated epidemic: treating carriers, vaccinating population at risk. Control measures are dependent on identified source of outbreak.

28 S TEP 8 : P LAN A MORE SYSTEMATIC STUDY AND REFINE HYPOTHESIS Comparison of the hypotheses with the established facts you would use this method when your evidence is so strong that hypothesis does not need to be tested. Analysis epidemiology used when the cause is less clear. With this method, you test your hypotheses by using a comparison group to quantify relationship between various exposures and the disease ( cohort studies and case- control studies). When analytic epidemiological studies do not confirm your hypotheses, you need to reconsider your hypotheses and look for new vehicles or modes of transmission. This is the time to meet with case- patients to look for common links.

29 C ONT. Goal in step 6 was to quickly find source of outbreak to be able to control. In outbreak, the quickest study design is retrospective because there is access to a group of cases from outbreak. However, investigator can perform prospective study beginning with healthy individuals with their exposure initially identified. Prospective study allows testing control measures working or not, to make additional prevention interventions.

30 S TEP 9: I MPLEMENTING C ONTROL AND P REVENTION M EASURES : Control measures, which can be implemented early if you know the source of an outbreak, should be aimed at specific links in the chain of infection, the agent, the source, or the reservoir. For example, an outbreak might be controlled by destroying contaminated foods or exclusion of food handler (diseased or carrier) from work until he or she is well after treatment. In some outbreaks, you would direct control measures at reducing susceptibility, example immunization against rubella.

31 C ONT. Same activities as step 7 but these activities should be ongoing and constant. Conduct training programs. Keep surveillance for future disease. Communicate with population and health officials to look for further symptoms of disease.

32 STEP 10: P REPARE A WRITTEN REPORT An oral briefing for local health authorities and people responsible for implementing control and prevention. A written report : It serves as: A- A record of performance. b- A document for potential legal issues. c- A reference if the health department encounters a similar situation in the future. d- Finally, it contributes to the scientific knowledge of epidemiology and public health.

33 C ONTENT OF REPORT Description of settings Documenting methods Presenting results Documenting specific causative agent and source Listing recommendations for prevention


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