Epidemiological principles

Comprehensive content on epidemiology

1. Epidemiological Principles Dr. Navin Adhikari IM Second Year Resident

2. Content Definition Terminology Case Definition Mode of transmission Epidemiological Triad Natural History of Disease Iceberg Phenomenon of Disease Level Of Prevention Control of Disease Measurement in Epidemiology Epidemiological Studies Investigation Of Outbreaks Surveillance

3. Definition Epidemiology is the study of the distribution and determinants of health-related states or events in specified populations , and the application of this study to the control of health problems. - John M. Last in 1988 Components Study - data driven, which relies on a systematic and unbiased methods to the collection, analysis, and interpretation of data. Distribution - deals with the frequency and pattern of disease in specified population.

4. Determinant -any factor, whether event, characteristic, or other definable entity, that brings about a change in a health condition or other defined characteristics. H ealth-related states or events - Anythings that affects the well being of human being. It Includes communicable disease, non-communicable disease chronic diseases, injuries, birth defects, maternal-child health, occupational health, environmental health and human behaviour like amount of exercise and seat belt use.

5. Distribution of Disease It is concerned with the frequency and pattern of health events in a population. Frequency - measurement of frequency of disease, disability or death. T he basic measure of disease frequency is a rate or ratio. Pattern- refers to occurrence of health-related events by time, place, and person . These are activites of descriptive epidemiology . An important outcome of this study is formulation of aetiological hypothesis

6. Determinant of Disease D eterminants are the causes and other factors that influence the occurrence of disease and other health-related events. Uses analytic epidemiological studies to provide the “Why” and “How” of such events. T est aetiological hypotheses and findings provide sufficient evidence to direct prompt and effective public health control and prevention measures

7. TERMINOLOGY I NFECTION : The entry and development or multiplication of an infectious agent in the body of man or animals. S everal levels of infection : colonization (e.g., S. aureus in skin and normal nasopharynx ); subclinical or inapparent infection (e.g., polio); latent infection (e.g., virus of herpes simplex); and manifest or clinical infection . CONTAMINATION : The presence of an infectious agent on a body surface ; also on or in other inanimate articles or substances including water, milk and food. INFESTATION : the lodgement, development and reproduction of arthropods on the surface of the body or in the clothing, e.g., lice, itch mite.

8. EPIDEMICS: The unusual occurrence of disease, specific health-related behaviour or other health related events in a community or region clearly in excess of "expected occurrence ". ENDEMIC :It refers to the constant presence of a disease or infectious agent within a given geographic area or population group, without importation from outside; may also refer to the "usual" or expected frequency of the disease within such area or population group. e .g. tuberculosis

9. SPORADIC are those cases that occur irregularly, haphazardly from time to time, and generally infrequently . e.g., polio, tetanus. A sporadic disease may be the starting point of an epidemic when conditions are favourable for its spread. PANDEMIC : An epidemic usually affecting a large proportion of the population, occurring over a wide geographic area such as a section of a nation, the entire nation, a continent or the world. eg COVID-19 pandemic.

10. SOURCE : Is a person, animal, object or substance from which an infectious agent passes or is disseminated to the host . Source refers to immediate source of infection & may or may not be part of reservoir. RESERVOIR : Is any person, animal, arthropod,plant , soil or substance in which an infectious agent lives & multiplies , on which it primarily depends for survival , & where it reproduces itself in such a manner that it can be transmitted to a susceptible host. HUMAN RESERVOIR : – Cases and Carriers ANIMAL RESERVOIR : E.g. Rabies, Influenza, Yellow Fever, Histoplasmosis RESERVOIR IN NON-LIVING THINGS : E.g. Soil harbour agents for Tetanus, Anthrax, Coccidiomycosis , Mycetoma

11. Examples

12. Case Cases: Persons having particular disease, health disorder or condition under investigation. It can be c linical cases, subclinical cases or Latent Infection. Cases in Epidemiology Primary case : First case of communicable disease introduced into the population unit being studied Secondary cases : Cases that develop from contact with the primary case Index case: First case that comes to the notice of the investigator (first case reported to the health system).

13. Case Definition A case definition is clinical criteria usually with confirmatory laboratory tests, if available, or combinations of symptoms (subjective complaints), signs (objective physical findings), and other findings. C ase definition may have several sets of criteria, depending on how certain the diagnosis is. For example , during an investigation of outbreak of COVID-19, a person with a fever and cough might be classified as having a suspected, probable, or confirmed case, depending on what evidence is present.

14. Carriers Carriers : Infected person or animal that harbours a specific agent in the absence of discernible clinical disease, & serves as a potential source of infection for others Carriers are less infectious than cases but are more dangerous epidemiologically. The " Typhoid Mary " is a classic example of a carrier.

15. MODE OF TRANSMISSION DIRECT TRANSMISSION 1. Direct contact - contact from skin to skin, mucosa to mucosa, or mucosa to skin of the same or another person. eg : STD and AIDS, leprosy, leptospirosis , skin and eye infections 2. Droplet infection - direct projection of a spray of droplets of saliva and nasopharyngeal secretions during coughing, sneezing e.g. common cold, diphtheria, whooping cough, tuberculosis, meningococcal meningitis, 3. Contact with soil – eg hookworm, tetanus, mycosis. 4. Inoculation into skin or mucosa e.g rabies, hepatitis B 5. Transplacental (vertical) : TORCH agents ( Toxoplasma gondii , rubella virus, cytomegalovirus and herpes virus), varicella virus, syphilis, hepatitis B, Coxsackie B and AIDS.

16. INDIRECT TRANSMISSION mechanisms including the traditional 5 F's - "flies, fingers, fomites , food and fluid” Vehicle-borne – through water, food and blood products, , acute diarrhoeas , typhoid fever, cholera, polio, hepatitis A, food poisoning . Vector-borne a. Mechanical -infectious agent is mechanically transported by a crawling or flying arthropod through soiling of its feet or proboscis b. Biological -infectious agent undergoing replication or development or both in vector and requires an incubation period before vector can transmit. three types

17. Propagative : The agent merely multiplies in vector, but no change in form, e.g., plague bacilli in rat fleas. Cyclo-propagative : The agent changes in form and number, e.g., malaria parasites in mosquito. Cyclo-deve / opmental : The disease agent undergoes only development but no multiplicatione.g.microfilaria in mosquito 3. Air-borne a. Droplet nuclei- tuberculosis, influenza, chickenpox b. Dust - streptococcal and staphylococcal infection, pneumonia, tuberculosis, Q fever and psittacosis 4. Fomite -borne e.g diphtheria, typhoid fever, bacillary dysentery, hepatitis A, eye and skin infections 5. Unclean hands and fingers eg staphylococcal and streptococcal infections, typhoid fever, dysentery, hepatitis A and intestinal parasites

18. Chain of Infection

19. Incubation Period: It is the time interval between invasion by an infectious agent and appearance of the first sign or symptom of the disease in question . Median incubation period : It i s the time required for 50% of cases to occur. Generation time : It is the time taken for a person from receipt of infection to develop maximum infectivity. Latent period : It i s the period from disease initiation to disease detection, used in noninfectious diseases as equivalent of incubation period. Serial interval : It is the gap in onset between primary case (first case in the community) and secondary case (case developing through infection from the primary case)

20. Epidemiological Triad The causative factors of disease may be classified as AGENT, HOST and ENVIRONMENT. These three factors are referred to as epidemiological triad . The mere presence of agent, host and favourable environmental factors in the prepathogenesis period is not sufficient to start the disease in man. Interaction of these three factors is required to initiate the disease process in man .

21. Agent Factor Agent factor i s defined as a substance, living or non-living, or a force, tangible or intangible, the excessive presence or relative lack of which may initiate or perpetuate a disease process. Biological agent -viruses, rickettsiae , fungi, bacteria, protozoa and metazoa Nutrient agent- excess or deficiency of carbohydrate, protein, vitamin and minerals Physical agent- cold, heat, pressure, radiation Chemical agent- Endogenous: urea, bilirubin Exogenous:allergens , metals, fumes, dust Mechanical agent - Exposure to chronic friction and other mechanical forces may result in crushing, tearing.

22. Host factor The host factors may be classified as: Demographic characteristics such as age, sex, ethnicity; Biological characteristics such as genetic factors; biochemical levels of the blood (e.g., cholesterol); Social and economic characteristics such as socioeconomic status, education, occupation, stress Lifestyle factors such as personality traits, living habits, nutrition.

23. Environmental factor The environment is defined as "all that which is external to the individual human host, living and non-living, and with which he is in constant interaction“. three components - physical, biological and psychosocial

24. INFECTIVITY : this is the ability of an infectious agent to invade and multiply in a host. Equals to Number infected/ Number exposed . PATHOGENICITY: this is the ability to induce clinically apparent illness. Equals to Number of diseased/ Number infected . VIRULENCE: this is defined as the proportion of clinical cases resulting in severe clinical manifestations (including sequelae ). It is Number of serious condition & mortality/ Number diseased .The case fatality rate is one way of measuring virulence.

25. NATURAL HISTORY OF DISEASE

26. ICEBERG PHENOMENON OF DISEASE Disease in a community may be compared to an iceberg Floating tip: What physician sees in community (Clinical cases) Vast submerged portion: Hidden mass of disease Line of demarcation (water surface): Is between apparent and inapparent infections. Not shown by rabies, tetanus, measles and rubella

27. LEVEL OF PREVENTION

28. PRIMODIAL PREVENTION It is the prevention of the emergence or development of risk factors in countries or population groups in which they have not yet appeared . Modes of Intervention: 1. Individual Education 2. Mass Education Primordial Level is Best level of prevention for Non-communicable diseases.

29. Primary Prevention It is the action taken prior to onset of disease , which removes the possibility that a disease will ever occur Modes of Intervention: 1. Health Promotion : e.g. Health Education, Environmental modifications, Nutritional interventions, Lifestyle and behavioural changes. 2. Specific Protection: immunization (b) use of specific nutrients (c) chemoprophylaxis (d) protection against occupational hazards (e) protection against accidents Primary level of prevention is applied when ‘risk factors are present but disease has not yet taken place .

30. Secondary Prevention It halts the progress of disease at its incipient stage and prevents complications. Modes of Intervention: 1. Early Diagnosis : Detection of disturbances while biochemical, functional and morphological changes are still reversible or prior to occurrence of manifest signs and symptoms 2. Treatment: Shortens period of communicability, reduces mortality and prevents occurrence of further cases (secondary cases) or any long term disability – Secondary level of prevention is applied when disease has possibly set in.

31. Tertiary Prevention Is applied when disease has advanced beyond early stages. It aims to reduce or limit impairments and disabilities, minimize suffering caused by existing departures from good health – Modes of Intervention : 1. Disability Limitation: It prevents the transition of disease from impairment to handicap 2. Rehabilitation : Training and retraining of an individual to the highest possible level of functional ability; It can be medical, vocational, social or psychological.

32. EXAMPLES A patient with fever and cough >3 weeks comes to DOTS Clinic for ‘Sputum for AFB’: Early diagnosis mode of intervention, Secondary Level of Prevention (as disease has possibly set in and sputum for AFB is used to confirm it as a case of tuberculosis) A person sleeps inside a bednet : Specific Protection mode of intervention, Primary Level of prevention (risk factors, i.e , mosquitoes are already present, disease has not yet taken place) A child coming to Immunisation clinic for OPV Vaccine: Specific Protection mode of intervention, Primary Level of prevention (risk factors, i.e , polio infection already present, disease has not yet taken place) . A Urine strip for sugar detection was employed to screen diabetics in a community : Early diagnosis mode of intervention, Secondary Level of prevention (screening is meant for early diagnosis of a disease)

33. STEPS IN DISEASE CONTROL Disease control: Is reducing the transmission of disease agent to such a low level that it ceases to be a public health problem. Control program in Nepal: TB, Leprosy, Dengue Disease elimination : Is complete interruption of transmission of disease in a defined geographical area, but the causative organism may be persisting in environment. Elimination program in Nepal: Poilo , malaria and Kalazar . Disease eradication : Is complete ‘extermination’ of organism– Is ‘tearing out by roots’ of a disease e.g small pox.

34. Measurement in Epidemology M ortality – Crude death rate, Specific death rate, Case fatality rate, Propotional mortality rate, survival rate M orbidity – Incidence and Prevalance , Notification rates, Admission, re-admission and discharge rates , duration of stay in hospital. Disability – DALY, Measurement of medical needs, health care facilities, utilization of health services, other health-related events and demographic events.

35. Measurement Tool Rate : Numerator is a part of denominator and multiplier is 1000 or 10,000 or 100,000 or so on Ratio : Numerator is not a part of denominator and both numerator and denominator are unrelated . Proportion : Numerator is a part of denominator and multiplier is 100.Proportion is always expressed in percentage.

36. EXAMPLES

37. INCIDENCE AND PREVALANCE Both are morbidity measurement Incidence is defined as the ‘ no. of new cases ’ occurring in a defined population during a specified period of time . Incidence = No. of new cases of a disease in a year/ Total population at risk X 1000 It is Rate . Special types of incidence rates: Attack rate : Incidence rate used when population is exposed for a small interval of time, e.g. epidemic Secondary Attack Rate (SAR): Is no. of exposed persons developing the disease within range of incubation period, following exposure to the primary case. Incidence can be determined from: Cohort study

38. Prevalence: Is total current (Old + New) cases in a given population over a point or period of time . Types of prevalence: Point Prevalence - point of time Period Prevalence - period of time Prevalance = No. of total (new + old) cases of a disease in a year / total population X 100 I t is a proportion. It can be determined from : Cross Sectional Study Relationship between Incidence and Prevalence: Given the assumption that population is stable AND incidence & duration are unchanging , Prevalence = Incidence × Mean duration of the disease Example incidence of hepatitis A virus (HAV) infections over a period of 1 year is always greater than its prevalence at a given point because the disease has a very short duration. In contrast, the prevalence of HIV or M. tuberculosis infections is always greater than its incidence because the infection is chronic, and infected persons may live for years after the initial infection.

39. EXAMPLES

40. EPIDEMILOGICAL STUDIES

41. DESCRIPTIVE STUDIES Steps 1.Defining the population to be studied 2. Defining the disease under study 3. Describing the disease by time, place and person 4. Measurement of disease 5. Comparing with known indices 6. Formulation of an aetiological hypothesis

42. ANALYTICAL STUDIES Case control studies : often called retrospective studies . Steps in Case control studies • Statement of the hypothesis • Selection of cases and control • Matching between cases and controls • Measurement of exposure • Analysis and interpretation. Odds ratio : is a measure of estimation of disease risk associated with exposures in case control studies. Also called cross product ratio. OR = ad/ bc for 2x2 contigency table.

43. C ohort is defined as a group of people who share a common characteristic or experience within a defined time period (e.g., age, occupation, exposure to a drug or vaccine, pregnancy, insured persons, etc). Cohort studies are prospective studies. S teps in Cohort studies • Selection of study subject • Selection of comparison group • Obtaining information on exposure • Follow-up • Analysis and interpretation.

44. Relative risk (RR) = Incidence among exposed/ Incidence among non-exposed Attributable risk (AR) = (Incidence among exposed – Incidence among nonexposed )/ Incidence among exposed × 100

45. Bias: Is any ‘systematic error’ in an epidemiological study, occurring during data collection, compilation, analysis and interpretation. Confounding : Any factor associated with both exposure and outcome , and has an independent effect in causation of outcome is a confounder. Example In the study of the role of alcohol in the aetiology of oesophageal cancer, smoking is a confounding factor because it is associated with the consumption of alcohol and it is an independent risk facto r for oesophageal cancer. Matching : Process of selecting controls in a such a way that they are similar to cases. Matching eliminates confounding.

46. Preclinical and clinical trials These are experimental studies, research activities that involve the administration of a treatment or prevention regimen to humans to evaluate its safety and efficacy.

47. Investigation of Outbreak Verification of diagnosis : Is the first step in investigation of an epidemic.It is not necessary to examine all cases, take sample Do not wait for laboratory results for epidemiological investigations Confirmation of existence of an epidemic : Compare with disease frequencies during same period in previous years Defining the population at risk : Obtaining the map of the area. Calculation of appropriate denominator of population at risk. Rapid search for all cases and their characteristics : Medical survey Epidemiological case sheet

48. Searching for more cases :this period is usually taken as twice the incubation period of the disease since the occurrence of last case Data analysis: Time: Construction of an epidemic curve Place: Preparation of a spot map Person: Analysis by age, sex, occupation and other risk factors Formulation of hypothesis Testing of hypothesis Evaluation of ecological factors Further investigation of population at risk Writing report

49. ISOLATION AND QUARANTINE

50. SURVEILLANCE Surveillance: Is the ongoing systematic collection and analysis of data and the provision of information which leads to action being taken to prevent and control a disease, usually one of an infectious nature. Types Passive Surveillance : Data is itself reported to the health system - Example: A patient with fever coming on his own to the PHC Active Surveillance: Health system seeks out ‘actively’ the collection of data, i.e., goes out to community to collect data Sentinel Surveillance: ‘identifying missing cases’ and ‘supplementing notified cases. s done in National AIDS Control Program wherein STD Clinics.

51. Monitoring Versus Surveillance

52. SURVEILLANCE OF COMMUNICABLE DISEASE IN NEPAL A. Early Warning And Reporting System (EWARS)  hospital based sentinel sentinel surveillance system Present  in 118 hospitals covering all 75 districts of Nepal complement the country’s Health Management Information System (HMIS) by providing timely reporting. weekly reporting of detailed line list of cases and deaths (including " zero" reports ) six priority diseases/syndromes— Malaria, Kala- azar , Dengue, Acute Gastroenteritis (AGE), Cholera and Severe Acute Respiratory Infection (SARI), and other epidemic potential diseases/syndromes (like enteric fever, Leptospirosis , Hydrophobia, Chickungunya etc.).

53.   Focuses on immediate reporting (to be reported as soon as possible within 24 hours of diagnosis) of one confirmed case of Cholera, Kala- azar , severe and complicated Malaria and one suspect/clinical case of Dengue as well as 5 or more cases of AGE and SARI from the same geographical locality in one week period . B. Active Surviellience Active surveillance of infectious disease is done if an outbreak occurred or is suspected to keep close track to the number of cases. C. Water Quality Monitoring and Surveillance Regular monitoring of drinking water quality from various sources and distribution sites Regular surveillance of water borne diseases and coordinate with different stakeholders for quality surveillance.

54. REFERANCES Park Textbook of Preventive and Social Medicine 23 rd edition Mandell , Douglas, and Bennett’s Principles and Practice of INFECTIOUS DISEASES Mahajan and Gupta Textbook of Preventive and Social Medicine, 4 th edition Principles of Epidemiology in Public Health Practice by CDC National Annual Report 75/76

55. THANK YOU