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Epidemiology for enviromental Health.pptx

  1. . Debre Markos University College of Medicine and Health Science Department of Public Health Epidemiology for BSc Environmental Health 2nd year students By Mengistie K (BSc, MPH Field Epidemiology) March, 2023 Debre Markos, Ethiopia
  2. Course syllabus  Year: II  Semester: II  Course Tile: Epidemiology  Module category: Core  Module code: Enph-M2091  Module number:9  Course Code: PuHe2093  Course Credit: 5ECTS(3 cr hr)=6o hrs  Course Delivery Duration: 14/07/2015 -01/08/2015
  3. Course objectives • At the end of the course the student will be able to:  Define epidemiology and discuss its importance to health science student s in general and to environmental health students in particular   Understand and make use of the principles of Epidemiology   Describe concepts of disease causation   Calculate the measures of disease and death   Differentiate types of study design   Investigate and control outbreaks and epidemics   Describe the purpose and types of surveillance   Identify the factors that affect validity of studies
  4. Introduction to Epidemiology Learning Objectives: At the end of this unit the student is expected to: • Define Epidemiology • Identify the main issues in the definition • Discuss the uses of Epidemiology
  5. Definition • Epidemiology is the study of the frequency, distribution and determinants of diseases and other health related conditions in human populations, and the application of this study to the promotion of health, and to the prevention and control of health problems.
  6. Major components of the definition 1. Population. The main focus of epidemiology is on the effect of disease on the population rather than individuals. For example malaria affects many people in Ethiopia but lung cancer is rare. If an individual develops lung cancer, it is more likely that he/she will die. Even though lung cancer is more killer, epidemiology gives more emphasis to malaria since it affects many people. 2. Frequency. This shows that epidemiology is mainly a quantitative science. Epidemiology is concerned with the frequency (occurrence) of diseases and other health related conditions. Frequency of diseases is measured by morbidity and mortality rates.
  7. Major components of the definition… 3. Health related conditions. Epidemiology is concerned not only with disease but also with other health related conditions because every thing around us and what we do also affects our health. Health related conditions are conditions which directly or indirectly affect or influence health. These may be injuries, births, health related behaviors like smoking, unemployment, poverty etc. 4. Distribution. refers to the geographical distribution of diseases, the distribution in time, and distribution by type of persons affected
  8. Major components of the definition… 5. Determinants. are factors which determine whether or not a person will get a disease. 6. Application of the studies to the promotion of health and to the prevention and control of health problems. The whole aim in studying the frequency, distribution, and determinants of disease is to identify effective disease prevention and control strategies
  9. History of Epidemiology Although epidemiological thinking has been traced to the time of Hippocrates, the discipline did not flourish as an independent discipline until the 20th century. Some key dates and contributions to the development of epidemiologic thinking and methods include: 460 B.C – Hippocrates, the father of modern medicine. For the first time in the fifth century B.C. he suggested that the development of human disease might be related to the external as well as personal environmental of an individual 9 introduction to Epidemiology
  10. 1854 - John Snow demonstrated that the risk of mortality due to cholera was related to the drinking water provided by a particular supplier in London.  He used a "natural experiment" to test his hypothesis. In another study conducted by Snow in 1854, he linked an epidemic of cholera to a specific pump, the "Broad Street Pump". History of Epidemiology 10 introduction to Epidemiology
  11. Basic Epidemiologic Assumptions Human disease does not occur at random: there are patterns of occurrence in which some behavioral and environmental factors (exposures) increase the risk of acquiring/developing a particular disease among group of individuals. Human disease has causal and preventive factors that can be identified through systematic investigation of populations or group of individuals within a population in different places or at different times. Thus, identifying these factors creates opportunity for prevention and control of diseases in human population either by eliminating the cause or introducing appropriate treatment. 11 introduction to Epidemiology
  12. Scope of epidemiology Its scope at the beginning was limited to understanding epidemics.  Now it is the basis of advancing our understanding of all kinds of diseases whether they belong to communicable, non communicable or injury category. It is used in laboratory sciences, clinical medicine and public health. Its scope in public health ranges from routine surveillance to research strategies for the testing of hypotheses about causes, measurement of health and disease risks and evaluations of preventive, diagnostic and therapeutic programme and technologies 12 introduction to Epidemiology
  13. Scope of epidemiology… Epidemiology is also a collection of applied disciplines, i.e., every disease entity has its own epidemiology (infectious, cardio-vascular, cancer, etc.). Other studies focus on health risks (occupation, smoking, diet, social conditions, etc.). More recently, epidemiologic methods have been applied to chronic diseases, injuries, birth defects, maternal and child health, occupational health, and environmental health. Now, even health behaviors, such as care-seeking, safety practices, violence, and hygienic practices are valid subjects for epidemiologic investigation. 13 introduction to Epidemiology
  14. Uses of Epidemiology  to determine, describe, and report on the natural course of disease, disability, injury, and death  to aid in the planning and development of health services and programs  to provide administrative and planning data  to study the cause (or etiology) of disease(s), or conditions, disorders, disabilities, etc.
  15. Uses of Epidemiology…  to determine the primary agent responsible or ascertain causative factors  to determine the characteristics of the agent or causative factors to determine the mode of transmission  to determine contributing factors  to identify and determine geographic patterns
  16. Two Broad Types of Epidemiology: 1. Descriptive Epidemiology - Defines the amount and distribution of health problems in relation to person, place and time. It answers the questions who, where and when. 2. Analytic Epidemiology – involves explicit comparison of groups of individuals to identify determinants of health and diseases.  It answers the questions why and how Descriptive Epidemiology is antecedent to analytical epidemiology
  17. Con… Three essentials characteristics (Variables) of disease that we look for in descriptive studies are  Person Place Time
  18. Person age, gender, ethnic group genetic predisposition concurrent disease diet, physical activity, smoking risk taking behavior SES, education, occupation
  19. Geographic Place  presence of agents or vectors Climate Geology population density economic development  nutritional practices medical practices
  20. Time calendar time  time since an event physiologic cycles age (time since birth) Seasonality temporal trends
  21. Communicable disease epidemiology Definition It is the study of circumstances under which infection disease occur in a population; including factors that influence their frequency, spread and distribution. 21
  22. Cont… • Occur in epidemic forms, and can be sudden public health problem (Plague, SARS, bird flue, etc) • The problem is aggravate by: • Poor socio-economic status • Poor personal and environmental hygiene • Inadequate health service coverage, etc. • Epidemiological transition • Change in demographic characteristics of people • Emergence of antibiotic resistant strains of microbes 22
  23. Natural history of disease definition : is the course of a disease in an individual over time without intervention. • The process begins with exposure to the causative agent capable of causing disease. Without medical intervention, the process ends with recovery, disability, or death
  24. Stage natural history of a disease There are four stages in the natural history of a disease. These are 1. Stage of susceptibility-period of exposure: • Disease has not yet developed ,but there are factors that favor occurrence. Examples: • A person practicing casual and unprotected sex has a high risk of getting HIV infection. • An unvaccinated child is susceptible to measles. • High cholesterol level increases the risk of coronary heart disease. 2. Stage of sub clinical disease (pre-symptomatic stage) : • the disease process has already begun but , the disease is not manifested . • The disease can only be detected through special tests.
  25. 3. Stage of clinical disease : • signs and symptoms of the disease are manifested in this stage . 4.Stage of disability or death : • The disease has occurred and left over damage to the body that limits the activity of the victim(disability) or has ended with the death of the victim. Examples: • Trachoma may cause blindness. • Meningitis may also result in death. N.B. recovery can takes place at any stage in the course of the disease.
  26. The stages in the natural history of disease cont..
  27. Course of an infectious disease over time The different periods that are encountered in the course of infectious disease : • Prepatent period • Incubation period • Communicable period • Generation time • Latent period
  28. Course of an infectious disease cont… Prepatent period • This is the time interval between infection (or biological onset), and • The point at which the infection can first be detected • It measured by the time of first shedding of the agent by the host • In some conditions, like the AIDS, it is the so called "window period"
  29. Course of an infectious disease cont.. Incubation period • This is the time interval between infection and the first clinical manifestations of the disease • Between the biological onset and clinical onset Communicable period • The period during which an infected host can transmit the infection to others which can be measured by the time interval during which the agent is shed by the host.
  30. Generation time/period • The period b/n the onset of infection in a host and the maximal communicability of that host • The maximal communicability may be during or after the incubation period Latent period • The time interval between recovery and the occurrence of a relapse in clinical disease • E.g. in cases of malaria and epidemic typhus.
  31. Time Course of a Disease in Relation to Its Clinical Expression and Communicability 31 Generation period
  32. Components of Infectious Disease Process • Infectious diseases result from the interaction between the infectious agent, host/reservoir and environment. Fig 2. Components of infectious diseases Host Environment Agent 32
  33. Models in infectious disease 1. Epidemiologic triangle and triad (balance beam). Traditional model of infectious disease causation Agent Host Environment Epidemiologic triangle Host Agent Environment Balance beam 33 N
  34. Sufficient and component causes model • It is also called multiple causality of diseases • In this traditional model, each component cause is seen as necessary and sufficient cause in itself to produce the effect. • Necessary cause: A causal factor whose presence is required for the occurrence of the disease. • Sufficient cause. A causal factor or collection of factors whose presence is always followed by the occurrence of the disease. 34
  35. Levels of Disease Occurrence Diseases in a community occur 1. At difference levels of existence and 2. Excess of predictable levels (expected) 1. Level of occurrence of disease Endemic: the usual presence of disease from low to moderate level Hypo/Hyper-endemic: a persistently lower or high level of disease Sporadic: Normally does not occur, but occasional cases occur at irregular intervals 35
  36. Cont…. Excess of expected levels Epidemic/ Outbreak : An excess occurrence of disease over expected level at certain time. Pandemic: An epidemic that affects several countries or continents. (eg HIV/AIDS, Swan flue, etc) 36
  37. Chain of disease transmission • IT is a series of events , which must occur in order for disease causing organisms to cause infection . • There are six successive events implicated in the chain of disease transmission. 1. The agent 2. Its reservoir(s) 3. Its portal(s) of exit 4. Its mode(s) of transmission 5. Its portal(s) of entry 6. The human host
  38. 1.Infection agent This is an organism capable causing infection or infectious disease . metazoan ,protozoa , bacteria , fungus ,Virus. • A. Infectivity :- ability of an agent to cause infection to susceptible host . • Can be measured by infection rate • The proportion of exposed persons who become infected. infection rate = total no of infected person x100 total no susceptible people exposed
  39. B. Pathogenicity • Ability of micro organism to induce disease • The proportion of infected persons who develop clinical disease. Pathogenicity =total no of clinical cases total no of subclinical cases
  40. C. From disease to disease outcome  Virulence: the proportion of persons with clinical disease who become severely ill or die It is measured by Case-fatality-rate or hospitalization rate Case-fatality-rate = No of death of a specific disease Total No of cases of that specific disease X 100 Hospitalization rate = No of hospitalized persons of a specific disease Total No of cases of that specific disease X 100 40
  41. D. Resistance • ability of the agent to survive adverse environmental conditions during transmission from one host to another
  42. Outcomes at Each Stage of Infection Exposure Infection Disease Disease Outcome Infectiousness (Infection rate) Pathogenesis (Clinical to sub-clinical ratio) Virulence (Case-fatality rate, Hospitalization rate) 42
  43. 2. Reservoir of infection • It is an organism or habitat, in which an infectious agent normally lives, growth and multiplies. Types of reservoirs: 1. human reservoir • Agents with a human reservoir include • measles • mumps, and • most respiratory pathogens. • Human reservoirs may be persons with symptomatic illness, or carriers.
  44. Reservoir of infection cont… • A carrier is a person without apparent disease who is nevertheless capable of transmitting the agent to others. Carriers may be: • Asymptomatic carriers (transmitting infection without ever showing signs of the disease), • Examples: - measles, chicken pox, mumps, viral hepatitis, AIDS, rabies. • Incubatory carriers (transmitting infection by shedding the agent before the onset of clinical manifestations), • Examples: - typhoid fever, Diphtheria, HBV • Convalescent carriers (transmitting infection after the time of recovery from the disease). • Examples:- polio , amebiasis , VHA, meningococcal • Chronic carriers shed the agent for a long period of time, or even indefinitely. • Examples: HBV and typhoid fever.
  45. Reservoir of infection cont… 2. Animals reservoir :-cause zoonotic diseases -bovine TB-cow to man - brucellosis –cows, pigs and goats to man - anthrax -cattle, sheep, goats, horses to man - rabies -dogs, foxes and other wild animals to man 3. Non-living things:- soil , food , water . E.g. clostridium tetani ,clostridium botulinum etc
  46. 3. Portal of exit This is the site through which the agent escapes from the reservoir. • All body secretions and discharges. • GIT: TF, dysentery, cholera, ascariasis • Respiratory: TB, Common cold, • Skin and mucus membranes: syphilis • Blood and tissues (placenta).
  47. 4.Mode of transmission • This is the mechanism by which an infectious agent is transferred from a reservoir of infection to anew host . • There are two major modes: 1. Direct Transmission- immediate transfer of the agent from a reservoir to a susceptible host by direct contact or droplet spread. • Example: ⎯ Touching ⎯ Kissing -direct projection ⎯ Sexual intercourse ⎯ Blood transfusion ⎯ Trans-placental
  48. Cont… 2. Indirect Transmission- an agent is carried from reservoir to a susceptible host by suspended air particles or by animate (vector-mosquitoes, fleas, ticks...) or inanimate (vehicle-food, water, biologic products, fomites) intermediaries. • Example: • ⎯ Vehicle-born: food, water, towels, ... • ⎯ Vector-borne: insect animals, ... • ⎯ Airborne: dust, droplets • ⎯ Parenteral injections
  49. 5.Rout of entry /portal of entry • This is the site on the susceptible host through which an infectious agent gets in to the susceptible host. GIT:-TF Respiratory:- TB skin :- STD, scabies The manner of entry is one of the factors which determine whether or not the agent will establish infection .
  50. 6. Susceptible host Definition:- A person lacking sufficient resistance to a particular pathogenic agent to prevent disease if exposed. Level of susceptibility depends up on • Age : extreme of age • Nutritional status • Stress • pre- existing medical conditions • Immune status
  51. Principles of communicable disease control • Definition :- reduction of incidence and prevalence of communicable diseases to a level where it can not be a major public health problem. There are three main methods of controlling communicable diseases: 1. Elimination of the Reservoir a. Man as reservoir: When man is the reservoir, eradication of an infected host is not a viable option. Instead, the following options are considered:
  52. Cont… Detection and adequate treatment of cases: arrests the communicability of the disease Isolation: separation of infected persons for a period of communicability of the disease. Quarantine: limitation of the movement of apparently well person or animal who has been exposed to the infectious disease for a duration of the maximum incubation period of the disease.
  53. Cont… b. Animals as reservoir: Action will be determined by the usefulness of the animals, how intimately they are associated to man and the feasibility of protecting susceptible animals. For example: • Plague: The rat is regarded as a pest and the objective would be to destroy the rat and exclude it from human habitation. • Rabies: Pet dogs can be protected by vaccination but stray dogs are destroyed.
  54. Cont… • c. Reservoir in non-living things: Possible to limit man’s exposure to the affected area (e.g. Soil, water, forest, etc.).
  55. 2. Interruption of transmission • This involves the control of the modes of transmission from the reservoir to the potential new host through: • Improvement of environmental sanitation and personal hygiene • Control of vectors • Disinfections and sterilization
  56. 3. Protection of susceptible host: • The chain of infection may be interrupted if the agent does not find a susceptible host. This can be achieved through: • Immunization: Active or Passive • Chemo-prophylaxis- (e.g. Malaria, meningococcal meningitis, etc.) • Better nutrition • Personal protection. (e.g. wearing of shoes, use of mosquito bed net, insect repellents, etc.)
  57. Levels of Disease Prevention • Disease prevention means to interrupt or slow the progression of disease • Epidemiology plays a central role in disease prevention by identifying those modifiable causes. • There are three levels of prevention
  58. 1. Primary prevention • The main objectives of primary prevention are promoting health, preventing exposure and preventing disease • keeps the disease process from becoming established by eliminating causes of disease or increasing resistance to disease • Has 3 components. A. Health promotion:- consists of general non-specific interventions that enhance health and the body's ability to resist disease • Improvement of socioeconomic status, provision of adequate food, housing, clothing, and education are examples of health promotion
  59. Primary prevention… B. Prevention of exposure:- is the avoidance of factors which may cause disease if an individual is exposed to them. • Examples can be provision of safe and adequate water, proper excreta disposal, and vector control
  60. Primary prevention… C. Prevention of disease:- is the prevention of disease development after the individual has become exposed to the disease causing factors • Immunization is an example of prevention of disease. • Immunization acts after exposure has taken place • Immunization does not prevent an infectious organism from invading the immunized host, but does prevent it from establishing an infection. • If we take measles vaccine, it will not prevent the virus from entering to the body but it prevents the development of infection/disease
  61. 2. Secondary prevention • its objective is to stop or slow the progression of disease so as to prevent or limit permanent damage. • It can be achieved through detecting people who already have the disease as early as possible and treat them. • It is carried out before the person is permanently damaged. • Examples: • Prevention of blindness from Trachoma • Early detection and treatment of breast cancer to prevent its progression to the invasive stage, which is the severe form of the disease
  62. 3. Tertiary prevention • is targeted towards people with permanent damage or disability. • It is needed in some diseases because primary and secondary preventions have failed, and in others because primary and secondary prevention are not effective. • It has two objectives: 1. Treatment to prevent further disability or death 2. To limit the physical, psychological, social, and financial impact of disability It can be done through rehabilitation Retraining of the remaining functions for maximal effectiveness.
  63. Tertiary prevention… • Example: When a person becomes blind due to vitamin A deficiency, tertiary prevention (rehabilitation) can help the blind or partly blind person learn to do gainful work and be economically self supporting.
  64. . Basic measurement in epidemiology
  65. Epidemiologic Questions (Measurement) Epidemiology ≈ Measuring ≈ Quantitative Method 3/26/2023 65 Frequency Association/Effect Impact What proportion of the population is affected by the health related event? (who, when, where?) How fast is the exposure or outcome spreading? Are there differences in the outcome between exposed and not exposed Are there differences in exposure between people with the outcome and those without the outcome of interest? Is the health event a problem any more? What is the impact of introducing a preventive exposure? What is the impact of removing a harmful exposure?
  66. Epidemiologic Measures Epidemiology ≈ Measuring ≈ Quantitative Method 3/26/2023 66 Frequency Association/Effect Impact Prevalence ( Point, Period) Incidence (Cumulative, Density) Relative Risk Odds Ratio Crude Vs specific Crude Vs Adjusted Attributable Risk among exposed Crude Vs Adjusted Crude Vs Adjusted Relative Risk Reduction Attributable Risk among the population Prevented Fraction
  67. Measures of Frequency • Counts • Ratios • Proportions • Rates 3/26/2023 67
  68. Count Common descriptive measure First step in calculating rates  Essential for service delivery, planning 3/26/2023 68
  69. Targets and Number of < 5 years children who have been measured for their weight in Oromia Zone, Amhara Region, Ethiopia from 2013 to 2016 Years Targets for weight measurement Number of weights measured 2013 39905 41449 2014 70402 98216 2015 71650 157865 2016 74256 181619 3/26/2023 69
  70. Ratio  One number divided by another number  No specific relationship necessary between the numerator and denominator(numerator NOT necessarily included in the denominator  Common descriptive measure  Numerator and denominator can be unrelated  Either numerator or denominator usually set to 1 3/26/2023 70
  71. Ratio… • Numerator not included in Denominator 3/26/2023 71
  72. Ratio…. • A city of 4 million people has 400 clinics. • Calculate the ratio of clinics per person. • Ratio=400/4000,000 =1 clinic per 10,000 3/26/2023 72
  73. Proportion • Definition: comparison of a part to a whole • Numerator must be included in the denominator • Ranges between 0 and 1 (0–100%) • Percentage = proportion x 100 3/26/2023 73
  74. Proportion… • Numerator included in Denominator 3/26/2023 74
  75. Rate • Numerator – number of EVENTS observed for a given time Denominator – population in which the events occur – includes time interval Example: rate of new acute malnutrition cases in City A • 200 cases/100,000 population/year 3/26/2023 75
  76. Rate • Numerator – number of EVENTS observed for a given time Denominator – population in which the events occur – includes time interval Example: rate of new acute malnutrition cases in City A • 200 cases/100,000 population/year 3/26/2023 76
  77. Risk Definition: proportion of an initially disease-free population that develops disease during a specified (usually limited) period of time new cases during period Size of population at start of period Synonyms: - "Attack rate" - Probability of getting disease - Cumulative incidence - Incidence proportion 3/26/2023 77
  78. Summary Ratio Division of two numbers, related or unrelated Proportion Division of two related numbers; numerator is a subset of denominator Rate Division of two related numbers; denominator is the population; Denominator includes time
  79. Summary… • All rates are proportions • All rates are ratios too • All proportions are ratios • But all proportions are not rates • All ratios are not proportions 3/26/2023 79
  80. Common measures of Disease Frequency (Morbidity) • Incidence (risk): Measures new cases of a disease that develop over a period of time • Prevalence (Burden): Measures existing cases of a disease at a particular point in time or over a period of time 3/26/2023 80
  81. Incidence Incidence 3/26/2023 81 Cumulative Incidence (CI) or Risk of Disease Incidence Rate (IR)
  82. Incidence… Risk (Cumulative Incidence) • Measures the risk (the likelihood, probability) that an individual will contract the disease during a certain time period • Cumulative incidence relates occurrences of new cases to the population at the beginning of the study period 3/26/2023 82
  83. Incidence… • Incidence rate must take into account • Number of individuals who become ill in a population • Time periods experienced by member of the population during which the events occur 3/26/2023 83
  84. Attack rate A type of incidence used during disease outbreak in a narrowly defined population over a short period of time Measured during an outbreak The denominator of the attack rate is the number of people who are at risk of becoming ill Example: x = 30 people got sick, out of y = 100 people who attended dinner party Attack Rate = 30/100 = 0.30 = 30% 3/26/2023 84
  85. Prevalence The size of cases present in a population (estimates disease burden)  Commonly used with chronic disease or those with long duration Two types of Prevalence •Point prevalence •Period prevalence 3/26/2023 85
  86. 3/26/2023 86
  87. Comparing Incidence and Prevalence Incidence New cases or events over period of time Useful to study factors causing disease, Useful in “risk” estimation Prevalence All cases at point/period of time Useful for measuring size of problem For planning 3/26/2023 87
  88. Increased By By longer duration of the disease Prolongation of life of patients without cure Increase in new cases (increase in incidence) In-migration of cases Out-migration of healthy people In-migration of susceptible people Improved diagnostic facilities (better reporting) Decreased By Shorter duration of the disease High case fatality Decrease in new cases (decrease in incidence) In-migration of health people Out-migration of cases Out-migration of susceptible people Improved cure rate of cases) Factors influencing Prevalence 3/26/2023 88
  89. Measures of disease frequency Example Suppose we followed a population of 150 persons for one year, and 25 had a disease of interest at the start of follow-up and another 15 new cases developed during the year What is the point prevalence at the start of the period? 25/150 = 0.17 = 17% What is the period prevalence for the year? (25 + 15) / 150 = 0.27 or 27% What is the cumulative incidence for the one year period? 15/125 = 0.12 = 12% 3/26/2023 89
  90. Measurements of Mortality Mortality rates and ratios measure the occurrence of deaths in a population using different ways. Rates whose denominators are the total population are commonly calculated using either the mid - interval population or the average population. This is done because population size fluctuates over time due to births, deaths and migration. 3/26/2023 90
  91. Crude Death rate (CDR) = Total no. of deaths reported during a given time interval X 1000 Estimated mid interval population The Crude Death Rate measures the proportion of the population dying every year, or the number of deaths in the community, per 1000 population It reflects the risk of death in that community or country 3/26/2023 91
  92. Age- specific mortality rate = No. of deaths in a specific age group during a given time X 1000 Estimated mid interval population of specific age group • One example of age specific mortality rate is Infant Mortality Rate. Sex- specific mortality rate = No. of deaths in a specific sex during a given time X 1000 Estimated mid interval population of same sex 3/26/2023 92
  93. Proportionate mortality ratio = No. of deaths from a specific cause during a given time x 100 Total no. of deaths from all causes in the same time The proportionate mortality ratio asks the question: What proportion of deaths are due to a certain cause? For example when we say the proportionate mortality ratio for HIV/AIDS is 30 %, this means out of 100 total (of all) deaths 30 of them died from HIV/AIDS. 3/26/2023 93
  94. Case Fatality Rate (CFR) • = No. of deaths from a specific disease during a given time x 100 No. of cases of that disease during the same time • Case fatality rate represents the probability of death among diagnosed cases or the killing power of a disease. • Example: In 1996 there were 1000 tuberculosis patients in one region. Out of the 1000 patients 100 died in the same year. • Calculate the case fatality rate of tuberculosis. • CFR = 100 x 100 = 10 % 1000 That means 10% of tuberculosis patients will die once they develop the disease 3/26/2023 94
  95. Neonatal Mortality Rate = No. of deaths < 28 days of age reported during a given time X 1000 No. of live births reported during the same time Example: In 1996 there were a total of 5000 live births in “Zone B”. Two hundred of them died before 28 days after birth. Calculate the Neonatal Mortality Rate (NMR). NMR = 200 X 1000 = 40 per 1000 live births 5000 That means out of 1000 live births in 1996, 40 of them died before 28 days after birth. 3/26/2023 95
  96. Infant Mortality Rate (IMR) = No. of deaths under 1 year of age during a given time X 1000 No. of live births reported during the same time interval Infant mortality rate reflects the health of the community in which the child is being brought up. • Thus, it is high among people who have little health care, chiefly because infections, such as pneumonia, diarrhea and malaria, are common among their infants. • Malnutrition is also one of the killer of infants in developing countries. • The infant mortality rate in Ethiopia is one of the highest in the world (96.8 per 1000 live births). • That means out of 1000 live births about 97 die before they celebrate their first birth day. 3/26/2023 96
  97. . Public Health Surveillance
  98. Learning Objectives • Define Surveillance • Describe the types of surveillance • Discuss the activities of surveillance • Identify public health important diseases that are under surveillance in Ethiopia • Identify Features of a good surveillance system
  99. What is Public Health Surveillance? • “Ongoing systematic collection, analysis, interpretation, and dissemination of data regarding a health related event for use in public health action to reduce morbidity and mortality and to improve health.” • “Watch over careful” observation and timely intervention.
  100. Problem Response Surveillance: What is the problem? Risk Factor Identification: What is the cause? Intervention Evaluation: What works? Implementation: How do you do it? Public Health Approach
  101. Types of Surveillance 1. Indicator based surveillance • Structured data collection through routine Surveillance • It includes: • Integrated diseases surveillance • Laboratory surveillance • Nutritional surveillance 2. Event based surveillances • Media scanning (website and news paper} • Rumor (patient report, community concern, clinician concern) • Sectoral information,
  102. Types of Surveillance… Population-based Active Surveillance Passive Sentinel Active Passive
  103. Types of Surveillance… Population based Surveillance Targets the entire population Representative Based on existing public health structure Increase potential for detection of rare diseases Relatively poor data Sentinel Surveillance Target the selected population Lack of representativeness Selected physicians or facilities involved Specified diseases reported High quality data collected
  104. Types of Surveillance… Passive Surveillance Provider initiated reporting May not be representative Less work for health department, simple Limited consistency of reporter, reporting May not be timely May fail to identify outbreaks Voluntary Active Surveillance public health system initiated reporting Validate representativeness Requires more health department resources Assure more complete reporting Can be used with specific investigations Can be used for brief periods
  105. Purposes of Public Health Surveillance Assess public health status Trigger public health action  Define public health priorities Evaluate programs
  106. Uses of Public Health Surveillance ♦ Estimate magnitude of a health condition ♦ Determine geographic distribution of illness ♦ Portray temporal trends (seasonality) of a disease ♦ Detect outbreaks/define a problem ♦ Generate hypotheses, stimulate research ♦ Evaluate prevention and control measures ♦ Monitor changes in infectious agents ♦ Detect changes in health practices ♦ Facilitate planning
  107. Health Related Events Under Surveillance Diseases, conditions, injuries, deaths  Risk factors Host - behavioral risk factors (e.g. smoking, lack of exercise, poor diet, risky sexual behavior)  Agent - Antibiotic resistance, antigenic patterns, virulence Environment - pollution, insect density Health care practices  Blood products, adverse drug effects
  108. Disease under Surveillance in Ethiopia… Immediately Reportable 1. Acute Flaccid Paralysis 2. Anthrax 3. Avian Human Influenza 4. Cholera 5. Dracunculiasis/Guinea warm 6. Measles 7. Neonatal tetanus 8. Pandemic Influenza A(H1N1) 9. Rabies 10. Small pox 11. SARS 12. Viral Hemorrhagic Fever(VHF) 13. Yellow Fever 14. Maternal death 15. Perinatal death Weekly Reportable 1. Dysentery 2. Malaria 3. Meningitis 4. Relapsing 5. Typhoid Fever 6. Typhus 7. Severe Acute Malnutrition 8. New HIV infection 9. ARI 10. Viral load
  109. What are the criteria 1. Diseases which have high epidemic potential 2. Diseases of international concern 3. Diseases targeted for eradication or elimination 4. Disease of public health importance 5. Diseases which have effective prevention and control measures
  110. Activities in Surveillance • The different activities carried out under surveillance are: 1. Data collection and recording 2. Data compilation, analysis and interpretation 3. Reporting and notification 4. Dissemination of information
  111. Features of a good surveillance system • Using a combination of both active and passive surveillance techniques • Timely notification • Timely and comprehensive action taken in response to notification • Availability of a strong laboratory service for accurate diagnoses of cases
  112. Epidemiological study Design 112
  113. Learning Objectives • To overview different types of study designs • To identify descriptive Epidemiological study designs • To identify Analytical Epidemiological study designs • To describe the advantage of observational and experimental study designs 113
  114. Epidemiological designs Descriptive studies: • Case Reports/Case series • Correlational studies • Cross sectional studies Analytic study designs • Case control, Cohort studies • Experimental /Interventional studies 114
  115. Descriptive study design Descriptive epidemiology is a way of organizing data related to health and health related events by person (Who), place (Where) and time (When) in a population. Information organized as such is easy to communicate and provides information about: 1) the magnitude of the problem, 2) the populations at greatest risk of acquiring a particular disease, and 3) the possible cause(s) of the disease. 115
  116. Cross-sectional study design ♦Observation of a cross-section of a population at a single point in time – Unit of observation and analysis: The individual ♦Usually conducted to collect information about prevalence – Also known as “prevalence studies” ♦No independent reference groups ♦Non-directional in time = ‘snapshot 116
  117. Uses of Cross-sectional ♦Estimate prevalence of disease or their risk factors ♦Estimate burden ♦Measure health status in a defined population ♦Plan health care services delivery ♦Set priorities for disease control ♦Generate hypotheses ♦Examine evolving trends – Before / after surveys – Iterative/repeated cross-sectional surveys 117
  118. Data collected in cross-sectional study ♦ Disease ♦ Exposure to potential risk factors ♦ Practices – Dietary intake – Costs and utilization of health care services – Healthy / unhealthy behaviors – Physiologic measurements 118
  119. Cross-sectional study design ♦Descriptive – Estimate prevalence ♦Analytic – Compare the prevalence of a disease in various subgroups, exposed and unexposed – Compare the prevalence of an exposure in various subgroups, affected and unaffected 119
  120. Limitations of Cross-sectional study design • Limited capacity to document causality • Exposure/outcome measured at the same time • Prevalence-incidence bias • Neyman • Not suitable for the study of rare / short diseases 120
  121. Analytic Epidemiological study designs The subject of interest is the individual within the population. The objective is not to formulate but to test the hypothesis. To evaluate an association between exposure and disease. Focuses on the magnitude of the association between the exposure and the health problem under the study. 121
  122. Case control studies Definition: A case-control study is one in which persons with a condition ("cases") and suitable comparison subjects ("controls") are identified, and then the two groups are compared with respect to prior exposure.  – subjects are sampled by their outcome status. 122
  123. Case-Control Studies – Timing Exposure ? ? Exposed Unexposed Disease Yes (case) No (control) Investigator
  124. Case-Control Studies – Flow Chart Source Population Cases Controls Exposed Unexposed Exposed Unexposed
  125. Con… Information is obtained by simple observation of the event. Cases (subjects having a specific disease) and controls (subjects not having the disease) are compared for their exposure status. Assess retrospectively on exposure status Relatively cheaper, (Time and Cost) Measure of association is using Odds ratio 125
  126. Case-Control Studies – Steps 1. Identify cases of disease of concern 2. Identify appropriate non-diseased comparison group (“controls”) 3. Document exposures among cases and controls 4. Calculate odds ratios 5. Perform statistical tests or calculate confidence intervals
  127. Sources of Cases • Hospitals. • General population: 127
  128. Where to Find Controls • Population-based • Hospital- or clinic-based • Neighbors • Friends • Other, such as • Co-workers • Classmates 128
  129. Why Have Controls? • Provide estimate of prevalence of exposure in population • “Expected” prevalence of exposure among cases if no association 129
  130. Advantages of case control study oUseful for studying several potential exposures. oEfficient for rare diseases. oEfficient in resources and time oRelatively quick, disease and exposure measurements can be made at the same point in time. Disadvantages of case control study oProne to selection bias and recall bias. oNot suitable for rare exposures. oMay be difficult to establish that "cause ”preceded "effect” oCannot yield a population-level measure of disease incidence or prevalence. 130
  131. COHORT STUDIES Definition: An epidemiologic design that identifies comparison groups according to their exposure status. Or It is the analytical method of epidemiologic study that compares exposed and non-exposed groups. 131
  132. Definition • Cohort study is a type of analytical study which is undertaken to obtain additional evidence to refute or support existence of association between suspected cause and diseases. • Other names of cohort study are Longitudinal study, incidence study and forward looking study 132
  133. Design of a Cohort Study 133 Individuals “choose” their exposure status
  134. Types of Cohort study 1. Prospective - The outcome has not occurred at the beginning of the study 2. Retrospective - Both exposure and outcome status have occurred at the beginning of the study 134
  135. Measure of Association for Cohort study • Risk ratio if cumulative incidence study • Rate ratio if person-time follow-up study • Incidence of disease in exposed divided by incidence of disease in unexposed 135
  136. Advantage valuable when the exposure is rare  can examine multiple effects of a single exposure  can reveal temporal relationship  allows direct measurement of risk minimize bias in ascertainment of exposure 136
  137. Limitations inefficient in evaluation of rare diseases Expensive time consuming loss to follow-up create problem 137
  138. Experimental/Intervention studies Key Features of Experimental Design 1) Investigator manipulates the condition under study 2) Always prospective 138
  139. Experimental Studies • Investigator can “control” the exposure • Similar to laboratory experiments except that living populations are the subjects • Generally involves random assignment to groups • Clinical trials are the most well known experimental design • The ultimate step in testing causal hypotheses 139
  140. Cont… • In an experiment, we are interested in the consequences of some treatment on some outcome. • The subjects in the study who actually receive the treatment of interest are called the treatment group. • The subjects in the study who receive no treatment or a different treatment are called the comparison group. 140
  141. Classification of Intervention Studies: Based on population • Clinical trial - usually performed in clinical setting and the subjects are patients. • Field trial- used in testing medicine for preventive purpose and the subjects are healthy people. • Community trial - a field trial in which the unit of the study is group of people/ community. 141
  142. Classification of Intervention Studies: Based on design • Uncontrolled trial - no control group. control will be past experience (history). • Non-randomized controlled - there is control group but allocation into either group is not randomized. • Randomized controlled - there is control group and allocation into either group is randomized. 142
  143. Classification of Intervention Studies: Based on Trial Objective • Phase I - trial on small subjects to test a new drug with small dosage to determine the toxic effect. – Scale: 20-80 healthy individuals • Phase II - trial on small group to determine the therapeutic effect. – Scale: 100-200 patients • Phase III - study on large population to test effectiveness. – Randomized Controlled Trial (RCT) • Phase IV – Post marketing surveillance – Long term prospective assessment of effects & side- effects 143
  144. Experimental studies: Advantages • The major advantage of experimental studies lie in the strength of causal inference that can be made. • it is very difficult to make causal inferences based on observational studies. • Experimental studies offer the best design for controlling confounding variables. • Randomized Controlled Trials (RCTs) • Gold standard for epidemiologic research 144
  145. . Outbreak/Epidemic Investigation
  146. Learning Objectives • Define common terms in outbreak investigation • Identify Types of Epidemic • Describe steps of Epidemic Investigation • Discuss the purpose of Epidemic management • Identify different epidemic management methods 146
  147. Levels of disease occurrence • Diseases occur in a community at different levels at a particular point in time. • Some diseases are usually present at a predictable level. • This is called the expected level/endemic. • The examples of expected level are endemic and hyper endemic. • But sometimes they occur in excess of what is expected. • Outbreak • Epidemic, and • Pandemic 147
  148. Definition of terms related to the level of disease occurrence 1. Endemic: Presence of a disease at more or less stable level. Malaria is endemic in the lowland areas of Ethiopia. 2. Hyper endemic: Persistently high level of disease occurrence. 3. Sporadic: Occasional or irregular occurrence of a disease. When diseases occur sporadically they may occur as epidemic. 4. Cluster: Aggregation of cases in a given area over a particular period without regard to whether the number of cases is more than expected 148
  149. 5. Epidemic: The occurrence of disease or other health related condition in excess of the usual frequency in a given area or among a specific group of people over a particular period of time 6. Outbreak: Epidemics of shorter duration covering a more limited area. 7. Pandemic: An epidemic involving several countries or continents affecting a large number of people. • For example the worldwide occurrence of HIV/AIDS is a pandemic 149
  150. Epidemics Three points to be kept in mind 1. Epidemic refers to  Acute and chronic infection  Non infectious diseases  Other related conditions 2. No minimum number 3. Knowledge of the usual frequency 150
  151. Types of epidemics • Epidemics (outbreaks) can be classified according to • the method of spread or propagation, • nature and length of exposure to the infectious agent, and duration. 151
  152. 1. Common Source Epidemics • Disease occurs as a result of • exposure of a group of susceptible persons to a common source of a pathogen, • often at the same time or within a brief time period. • When the exposure is simultaneous, the resulting cases develop within one incubation period of the disease and this is called a point source epidemic. • E.g. Food borne epidemic 152
  153. 153 Number of cases Time Usual rate Epidemic Fig. The epidemic curve for point source epidemic Characteristics: •Sharp rise and fall •Unimodal peak •Short duration
  154. • If the exposure to a common source continues over time, it will result in a continuous common source epidemic. • E.g. A waterborne outbreak that spreads through a contaminated community water supply • The epidemic curve may have a wide peak because of the range of exposures and the range of incubation periods. 154
  155. 155 Usual rate Number of cases Time Epidemic Pattern of a continuous common source epidemic Flat top
  156. 2. Propagated/ Progressive Epidemics • The infectious agent is transferred from one host to another. • It can occur through direct person to person transmission or it can involve more complex cycles in which the agent must pass through a vector as in malaria. • Propagated spread usually results in an epidemic curve with a relatively gentle upslope and somewhat steeper tail. • An outbreak of malaria is a good example of propagated epidemic. • When it is difficult to differentiate the two types of epidemics by the epidemic curve, spot map (studying the geographic distribution) can help 156
  157. 157 Time Pattern of a propagated type epidemic Usual rate Number of cases Epidemic Characteristics: •Slow increase •Several peaks •Sharp fall
  158. 3. Mixed Epidemics • The epidemic begins with a single, common source of an infectious agent with subsequent propagated spread. • Many food borne pathogens result in mixed epidemics. 158
  159. Investigation of an Epidemic • The purpose is • to determine the specific cause or causes of the outbreak at the earliest time and • to take appropriate measure directed at controlling the epidemic and preventing future occurrence. 159
  160. Steps in epidemic investigation 1. Verify (confirm) the existence of an epidemic • This initial determination is often made on the basis of available data. • Compare the number of cases with the past levels to identify whether the present occurrence is in excess of its usual frequency. • Instead of comparing absolute numbers it is advisable to compare rates like incidence rate
  161. Steps… 2. Prepare for fieldwork  Before leaving for the field you should be well prepared to under take the investigation.  Preparations can include: a. Investigation- – appropriate scientific knowledge, • supplies, and equipment to carry out the investigation b. administration, and c. consultation. 161
  162. Steps in epidemic investigation…. 3. Verify (confirm the diagnosis) • Always consider whether initial reports are correct • Carry out clinical and laboratory investigations on the reported cases. • For example the already collected blood film slides can be seen by laboratory experts to check whether the initial report was correct. • It is important to investigate the index case (the first case that comes to the attention of health authorities) and other early cases. • The sooner the index case and other early cases are investigated, the greater the opportunity to arrest the outbreak at earliest stage possible. 162
  163. Steps in epidemic investigation…. 4. Identify and count cases Remember excess may be due to • Changes in local reporting producers • Changes in case definition • Improvement of diagnostic • A standard case definition is required to differentiate cases and non cases • Confirmed / definite – a case with clinical features and laboratory investigation • Probable – a case with typical clinical features without laboratory confirmation • Possible a case with fewer of typical clinical features 163
  164. Steps in epidemic investigation…. 5. Describe the epidemic with respect to person, place and time • Each case must be defined according to standard epidemiologic parameters: • the date of onset of the illness, • the place where the person lives or became ill, • and the socio-demographic characteristics (age, sex, education level, occupation). • The tools to be used when characterizing the epidemic are • Epidemic curve, • spot map and • attack rates. 164
  165. Steps in epidemic investigation…. • Epidemic curve is an important tool for the investigation of disease outbreaks. • In epidemic curve the distribution of cases is plotted over time, usually in the form of histogram, with the date of onset of cases on the horizontal axis, and the number of cases corresponding to each date of onset on the vertical axis. 165
  166. • Spot map is a map of locality where the outbreak has occurred, on which the location of cases is plotted. • The spot map is often helpful in detecting the source of an outbreak • Mapping disease can be done at kebele, woreda, regional, and national level • One limitation of spot map is that it does not take into account underlying geographic differences in population density • Thus the spot map needs to be supplemented by calculation of place specific attack rates. 166
  167. 6. Formulate hypothesis • The hypothesis should addressed • Source of the agent • Mode of transmission • Exposure that cause the disease • All factors that can contribute to the occurrence of the epidemic should be assessed. • The epidemic investigating team should try to answer questions like: • Why did this epidemic occur? • Are there many susceptible individuals? • Is the temperature favorable for the transmission of the diseases? • Are there breeding sites for the breeding of vectors? Etc 167
  168. Steps in epidemic investigation…. 7. Search for additional cases • Using active and passive case detection • Investigation of in apparent asymptomatic person 8. Analyze the data Interpret findings 9. Make a decision on the hypothesis tested 10. Intervention and follow up • Intervention must be as soon as possible • Control • Mode of transmission • Destroying contaminated food • Sterilizing 168
  169. Steps in epidemic investigation…. 11.Reporting • Comprehensive report to concerned agencies • Factor leading to epidemics • Evaluation of measures • Recommendation for prevention of similar episodes 169
  170. Managing Outbreak/Epidemic • Management of epidemics requires an urgent and intelligent use of appropriate measures against the spread of the disease. • Action to be taken is dependent on the type of the disease as well as the source of the outbreak. However, the actions can be generally categorized as presented below to facilitate easy understanding of the strategies.
  171. A. Measures Directed Against the Reservoir Domestic animals as reservoir:  Immunization. Example – giving anti-rabies vaccine for dogs Destruction of infected animals e.g anthrax Wild animals as reservoir: post-exposure prophylaxis for human beings- Example: rabies Humans as reservoir Isolation of infected persons. is not suitable in the control of diseases in which a large proportion are unapparent infection(without sign and symptom)
  172. Humans as reservoir… Treatment to make them noninfectious- e.g., tuberculosis. Quarantine- is the limitation of freedom of movement of apparently healthy persons or animals who have been exposed to a case of infectious disease.
  173. B. Measures that interrupt the transmission of organisms i. Purification of water ii. Pasteurization of milk iii. Inspection procedures to ensure safe food supply. iv. Improve housing conditions Actions to reduce transmission of respiratory infections include ventilation of rooms.
  174. C. Measures that reduce host susceptibility immunization (vaccination). Example vaccination for meningitis Chemoprophylaxis: for example, use of chloroquine to persons traveling to malaria endemic areas. After the epidemic is controlled, strict follow up mechanisms should be designed so as to prevent similar epidemics in the future.
  175. Thank you!!