Epidemic investigation

Objectives1- Identify the various patterns of Communicable Diseases (CDs) in the community (Endemic,Epidemic and Pandemic).2- Characterize the major types of epidemic diseases.3- Differentiate between the different types of epidemics (common source versus Propagatedepidemics.4- Describe epidemic curve and its interpretation.5- Illustrate the detailed steps of epidemic investigation.6- Identify the objectives and contents of epidemic investigation reports.

The various patterns of Communicable Diseases (CDs) in the community

Endemic:

This is the occurrence of disease/condition in a given population at frequencies that are constant over a duration of time.

For example, the disease malaria is endemic to tropical areas of the world, such as those in South America or Africa.

Epidemic:

• This is the occurrence of disease cases at a frequency that is higher than the normal for the population in a give period of time.

• Epidemics can occur upon endemic states too.

• For example, 1,000 cases of malaria in a tropical nation may not be an epidemic, but 1,000 cases of malaria in Canada would be an epidemic.

An epidemic may result from:

• A recent increase in amount or virulence of the agent.

• The recent introduction of the agent into a setting where it has not been before.

• An enhanced mode of transmission so that more susceptible persons are exposed.

• A change in the susceptibility of the host response to the agent.

• Factors that increase host exposure or involve introduction through new portals of entry.

Outbreak:An epidemic in small confined group of population.Ex: food poisoning in school, camp,….

Pandemic: is a worldwide epidemic.e.g. Influenza pandemics.

Sporadic:Individual cases of a disease that occur infrequently and irregularly.

Epidemic curve and different types of epidemic

Epidemic curve

• It’s a graphical display of the numbers of incident cases in an outbreak or epidemic, plotted over time.

An epidemic curve can provide information on the following characteristics of an outbreak:• Pattern of spread• Magnitude (the number of cases)

• Time trend ( date of onset of 1st case, date of last case…)

• reveal outliers: are cases at the very beginning and end that may not appear to be related (in time, perhaps in place)

• Exposure and/or disease incubation period:- Average IP: of exposure to an infectious agent until signs and symptoms of the disease

appear.- Minimum IP: minimum time between exposure until appearance of disease.- Maximum IP: maximum time between exposure until appearance of disease.

the major classes of epidemic curve varies according to the type of epidemics.

Types of epidemics

Epidemics can be classified according to their manner of spread through a population:• Common-source – Point– Continuous– Intermittent

• Propagated

Common-source epidemics

• Here the cases of disease arise from a single, shared or 'common' source, such as a batch of bad food, industrial pollution or a contaminated water supply.

• Controlling the source stops the outbreak.

A- point source epidemic • People exposed to the same exposure• over a limited, defined period of time,• all cases occur usually within one incubation

period. The shape of this curve :• rises rapidly and• contains a definite peak at the top• followed by a gradual decline.

Staphylococcus aureus food intoxication from a single meal of food .This produces a single curve that wanes quickly, as long as there is no person-to-person spread.

B-Continuous source epidemic• exposure to the single source that is prolonged over an

extended period of time,

• Continuous exposure may occur over more than one incubation period, cases will rise gradually leading to a plateau rather than a peak.

• The curve ends when the source of the contamination is corrected or when all susceptible people develop immunity.

• The relative flatness of the curve suggests that the infection comes from a common source and there is no person-to-person spread;

exposure to a contaminated water supply, that does not get fixed.

C- Intermittent source epidemic

• People are exposed intermittently to a harmful source

• Period of exposure may be brief or long

• Intermittent exposure often results in an epi curve with irregular peaks that reflect the timing and the extent of exposure

• industrial contaminant being emitted at intervals• common source that is not well controlled, leading to recurrent outbreaks.

Propagated epidemic- a case of disease serves as a source of infection, spread from one

person to another person

- subsequent cases, in turn, serve as sources for later cases (multiple waves)

- The shape of the curve usually contains a series of successively

taller peaks, and one incubation period apart.

-This pattern may continue until the remaining numbers of susceptible individuals declines or until intervention measures take effect.

- Can last longer than common source outbreaks

Steps of epidemic investigation.

Step 1: Prepare for Fieldwork

-Identify the investigation team -Make administrative arrangements-Clarify the role of each team member .-Identify & arrange for resources-Develop communication strategies for team,

hospital employees, and public

Does the observed number of cases exceed the existing number ?• use health department surveillance records.• locally—hospital discharge records, mortality

statistics.• conduct a telephone survey of physicians to

determine whether they have seen more cases of the disease than usual.

• Finally, conduct a survey of the community to establish the background or historical level of disease.

STEP 2. Establish the existence of epidemic

Step 3: Verifying the Diagnosis

• to ensure that the problem has been properly diagnosed

• to rule out laboratory error as the basis for the increase in diagnosed cases.

• summarize the clinical findings with frequency distributions

Step 4a: Establishing 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.

• 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.

Step 4b: Identifying and Counting Cases

• Direct case finding at health care facilities where the diagnosis is likely to be made: physicians , clinics, hospitals, and laboratories.

• In some outbreaks, public health officials may decide to alert the public directly, usually through the local media.

• Identifying contacts to case-patients .

The following items of information should be collected about every case:• identifying information• demographic information• clinical information• risk factor information

Step 5: Descriptive EpidemiologyCharacterize an outbreak by time, place, and person.• Time: epidemic curve.• Place: plotting the affected people over an

area (spot map)• Person: Age, Gender, Race/Ethnicity, Socio-

Economic Status, Behavior related

Step 6: Developing Hypotheses

• All aspects of the investigation should be addressed:

- the source of the agent, - the mode of transmission, (vehicle &vector), - the exposures that caused the disease, - any additional time, person, and place factors.

Step 7: Evaluating Hypotheses

• Either by comparing the hypotheses with the established facts, or

• by using analytic epidemiology to quantify relationships and explore the role of chance.

• Epidemiologic studies ( look for new vehicles or modes of transmission. This is the time to meet with case-patients to look for common links and to visit their homes to look at the products on their shelves).

• Laboratory and environmental studies

Step 8: Refining Hypotheses and carry out Additional Studies

Step 9: Implementing Control and Prevention Measures

As:• Surveillance for hospital employees • Surveillance for cases • Health education • Reduce panic• How to avoid exposures/Contacts• Appropriate quarantine and isolation

measures• reducing host susceptibility • chemoprophylaxis for travelers

Step 10: Communicating the Findings

1- Summarize investigation for requesting authority

2- a written scientific report.

Epidemic investigation reports

An appropriate and timely record should be formulated to the higher authorities. 1. to keep the authorities at the higher level informed so that they can make the appropriate decisions

2. It helps to review the outbreak and response, identify system failures and take corrective measures so that similar events are not repeated.

Contents of the report 1- BACK GROUND : Geographical location ,Demographic status(Population pyramid) Socio-economic situation Organization of health services Surveillance & early warning systems Normal disease prevalence

2- HISTORICAL DATA : About Previous occurrence of epidemics of the same disease locally or elsewhere.

3- METHODOLOGY OF INVESTIGATION : Case definition, Questionnaire used in epidemiological investigation, Survey teams, Retrospective Prospective studies, Collection of Lab. Specimens, Lab techniques.

4- ANALYSIS OF DATA : Natural history of the disease, Different diagnosis , mortality rates, Epidemiological data (time, place, person), testing hypotheses by statistical analysis

5- CONTROL MEASURES : Definitions of strategies and methodology of implementation Constraints, Results.

6- EVALUATION : Significance of results, Cost/Effectiveness Preventive measures.