Experimental epidemiology

& prevention in epidemiology

By Dr Sabah M Abdelkader

Assist Prof of Public Health

Objectives:by the end of this session, students will be able to: • Define experimental epidemiology.• Differentiate its types.• Discuss its aims.• Recognize corner stones of prevention.• Define surveillance.• Discuss its components.• Enumerate its uses.• Explain criteria for diseases under surveillance.• Identify indicators for successful surveillance.

What is experimental epidemiology?• In 1920s, it meant study of epidemics among

experimental animals.• In modern use, it is equated with randomized

controlled trials.• Experimental studies are similar to cohort

studies except that conditions of the study are under direct control of investigator.

• Experimental studies have advantages and disadvantages of prospective cohort studies in addition to: cost, ethics and feasibility.

Types of experimental studies• I-Animal studies: applied in:1-Experimental reproduction of human disease

in animals to study pathogenic phenomena.2-Testing efficacy of preventive and therapeutic

measures e.g. vaccines, treatment.3- Completing natural history of disease.• II-Human experiments:• Essential to investigate diseases that cannot

be reproduced in animals.

Aim of experimental studies• 1-Provide proof of risk factors for a

problem or disease.• 2- Provide a method of measuring

effectiveness and efficiency of heath services for preventing, control and treatment of diseases to improve community health.

• Experiment—investigation designed to untangle cause from effect

– independent variable—imposed treatment or special condition

– dependent variable —specific behavior being studied

– experimental group —participants who are given particular treatment

– comparison (control) group —participants who are not given special treatment but are similar to experimental group in other relevant ways

The Experiment

How to Conduct an Experiment

Prevention in epidemiology• Basic approach of primary prevention is

identifying weak points and break weakest link in chain of transmission:

• 1- Reservoir of infection.• 2- Route of transmission.• 3- Susceptible host.

1- Controlling Reservoir of infection• Elimination of reservoir may be easy

with animal reservoirs e.g. TB, H1N1.• It is not possible with human reservoirs.• Human reservoir control comprises:

early diagnosis, notification, epidemiologic investigation, isolation, treatment, qurantine.

2- Interruption of transmission• Breaking chain of transmission means

changing some components of environment to prevent agent from entering body of susceptible person.

• Examples: water chlorination, food sanitation, hand washing, good cooking, vector control, treatment of patients, personal hygiene.

3- Susceptible hosts• Protection of susceptible host can be

done by one or more of following:• 1- Active immunization:• Universal immunization programs.• WHO EPI schedule. • 2- Passive immunization: with human

immunoglobulins or antitoxins ( short term) e.g. tetanusm Rh incompatibility.

Cont.• 3- Combined passive and active

immunization, immunoglobulins should be administered 3 weeks before active immunization.

• 4- Chemoprophylaxis: causal or clinical e.g. tetracycline for cholera, amantadine for influenza, sulphadiazine for meningitis.

Surveillance • It is collection, analysis, interpretation and

dissemination of information about a selected health event. This information is important to plan, implement and evaluate a health program.

• Examples of health events that can be put under surveillance:

• polio, accident, malnutrition and surveillance of adverse events following immunization.

• Surveillance is a dynamic process and its components are depending on each other.

Components of surveillance

Surveillance

Action to control problem

Collection & reporting of

data

Analysis &interpretation

Dissemination of data

I - Collection and reporting of data• Levels of Collection and reporting of data: • Data about disease under surveillance is

collected:• At the primary health centers or hospital or

clinic ( by the physician).• Then reported to the district level. • From different districts to the provincial level • Then to national level) Ministry of Health). • For certain prescribed diseases, report is

made by the national authority to the WHO.

Methods of data collection: • A) Passive data collection: Data about the

disease is collected by the physician in the primary health care centers routinely in a regular, frequent and timely manner.

• B) Active data collection: The health officer from the districts or other higher level regularly visits the hospitals, primary health care centers or clinics to gather information about the disease under surveillance.

Types of data collected during surveillance of a communicable disease:• 1- Morbidity and mortality data: Number

of cases and /or deaths, time and place of occurrence, personal factors and determinants of the disease.

• 2-Laboratory results: Role of public health lab is to confirm cases, determine bacterial resistance to antibiotics, level of immunity of population, vaccine potency, microbial contamination of environment.

Cont.• 3-Data about the preventive and control

measures: such as the number of vaccinated, the number of houses sprayed by insecticides, the number of contacts received chemo prophylaxis.

• 4- Data related to the environmental conditions: the proportion of people supplied by pure water and coverage of services for garbage and excreta disposal.

Cont.• 5-Data about the vector of disease:

Type, habits and susceptibility to insecticides

• 6-data about the reservoir: Type and distribution.

• 7-Data about the population: Size, distribution, and composition. (demographic data).

II. Data analysis and interpretation:• Data collected from different centers, clinics

and hospitals are manipulated at higher levels to be presented as: tables, graphs and maps or mathematically presented.

III. Dissemination of data :• The results obtained should be

disseminated into two groups:• a) Those who collected the data at the first

level to maintain their interest. • b) Those responsible for taking actions.

The uses of surveillance• I) Identify the disease trend so that

planning of preventive and control programs can be adjusted to meet the new situation.

• 2) Identify, investigate and help the control of outbreaks or epidemics.

• 3) Identify the population at risk for certain disease or death.

• 4) Identify new emerging disease (example HIV infection)

Cont.• 5) It is useful to formulate hypothesis: This is

because the case series (one of the descriptive epidemiological methods) depends mainly on an effective surveillance .The best example is the association between HIV infection and occurrence of pneomocystitis carenii which was formulated by the case series

• 6) Evaluation of preventive and control measures of the disease under study.

Criteria for diseases to be put under surveillance

• 1. It affects large number of population. (High prevalence or incidence)

• 2. It causes substantial mortality or disability.• 3. It is preventable by certain measure such

as immunization• 4. The disease that could cause outbreaks

such as cholera and meningococcal meningitis as it affects few numbers of persons within limited time in certain place.

Cont.• 5. It re-emerges after successful control

measures (TB, cholera, malaria).• 6. A disease, which is the target of national,

regional or international control, program e.g. polio. and malaria

• 7. The measures for the control of this disease should be:

• Effective, Available (resources for implementation), Cheap, Accepted by the public.

Indicators of successful surveillance system

• 1. Completeness: The percentage of reports received in relation to the targeted number of reports .

• Completeness=• actual number of reports received X100 targeted number of reports that should be

received

Cont.• 2. Timely: Effective surveillance system

provides information on time to make the response soon enough to minimize the morbidity and mortality.

• Timely reports = • The number of the reports received on time X 100 The targeted number of reports that should be

received

Cont.• 3. Simple: Surveillance system that collects

too much useless and complex information is not efficient

• 4. Representative: Effective surveillance system should describe the disease in terms of person, place, and time as well it describes the determinants of the disease.

• 5. Flexible: Flexibility is important when a new health problem arises (example when cluster of cases of AIDS were found)

Cont.• 6. Hierarchical: The data must be

disseminated in a hierarchical manner from the most peripheral level to the most central level.

• In this way, health officers at each level should receive data about the area under their legal authority which can be analyzed and used to guide the local control activity.

Thank you