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
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. tetanusmRh 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.
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 pneomocystitiscarenii 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.