Introductions to Epidemiology (2) (2).pptx

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1. Epidemiology BY: Wakgari Mosisa (BSc , MPH in Epidemiology)

2. Contents  25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 2 Basic Concepts in Community Health  Definitions History of Epidemiology Use/applications of Epidemiology Scope of epidemiology  Basic assumptions of epidemiology  Theories of disease causation  Levels of disease prevention 

3. Basic Concepts of Health 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 3 HEALTH Definition of health has negative and positive models: Negative model- defines health as the absence of the constraints of health.  That is you are healthy if you are not ill.  The renewal of health in a diseased individual is the removal of the disease.  This model equates health with the absence of disease Positive model is by WHO- " a state of complete physical, mental and social wellbeing and not merely the absence of disease or infirmity".  This definition is also simplified to “ The ability to lead a socially and economically productive life”

4. Basic Concepts Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 4 Disease- is a physiological/psychological dysfunction Illness- a subjective state of the person who feels aware of not being well Sickness- is a state of social dysfunction that is a role that the individual assumes when ill Risk factors - are factors positively associated with the risk of developing disease but are not sufficient. Some risk factors are associated with many diseases while some diseases are associated with many factors .

5. Basic Concepts Cont…  25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 5 Community- refers to a group of people who share something in common.  Examples of common denominators may be neighborhood, workplace, race, religion, or social activity.  Communities are characterized by people’s engagement in activities that demand interrelationship of efforts, they give rise to shared culture, and they are often sited in a particular geographic location.  Community medicine- is a system of delivery of comprehensive health care to the people by a health team to improve the health of a community 

6. Basic Concepts  Cont … 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 6

7. Basic Concepts  Cont … 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 7

8. Basic Concepts  Cont … 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 8 The task of community diagnosis requires statistical knowledge in addition to medical skills Ways that one could make community diagnosis are: Discussion with community leaders and health workers-community felt health problems Attending community gatherings-community felt health problems  Survey of available health records Field survey Analysis of data collected and compilation of report in order to reach at community diagnosis

9. Basic Concepts Cont.… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 9 Community diagnosis provides basis for decision on: The need for intervention Type of intervention needed Target group at whom intervention should be directed Provide baseline for later intervention 

10. Basic Concepts  Cont … 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 10 Public health- Is the science and art of preventing diseases, prolonging life and promoting health through community efforts and government action. Public health is: Preventing disease Prolonging life Improving quality of life Eliminating health inequalities Organizing community to promote active participation

11. Impact of Public Health  25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 11

12. cont …. 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 12 Among the greatest global achievements:  Plague  ==> Quarantine   Cholera ==> Sanitation  Smallpox ==>Immunization For example, over the 20th century, life expectancy for Americans has increased by 30 years.  25 of those years is due to achievements in public health  Less than 4 of those years is due to medical advances.

13. Co n t..  25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 13

14. Co n t..  25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 14

15. Basic Concepts  Cont … 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 15 EPI                                 DEMO                          LOGOS Upon, on, befall        People, population, man             the Study of The study of anything that happens to people “That which befalls man”

16. Definition of Epidemiology 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 16 Many definitions have been proposed over time by different scholars.  Oxford English Dictionary   The branch of medical science which treats of epidemics- old The scientific study of the spread and control of diseases – new Kuller LH: Am J Epid 1991;134:1051 Epidemiology is the study of "epidemics" and their prevention The study of the distribution and determinants of health-related states or events in specified human populations and its application to the control of health problems.                                     - Dictionary of Epidemiology Porta M, ed. Dictionary of Epidemiology, 5th ed. New York: Oxford U. Press, 2008

17. “The Study of…” 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 17

18. Distribution and Determinants... 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 18

19. The Five W’s of Journalism / Epidemiology 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 19  What*            = Diagnosis Who                 = Person  Where              = Place When               = Time Why / How      = Cause, Risk factors, modes of transmission * and How Many   

20. …Distribution…  25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 20 What   = Clinical  Who    = Person         Descriptive Epidemiology(Distribution) Where  = Place     or When    = Time ___________________________________________ Why / How   = Cause,                       Risk factors,         modes of transmission      Analytic Epidemiology (Determinants)  

21. ….Determinants.... 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 21

22. …Health-Related States or Events… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 22

23. Case Definition 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 23 Set of uniformly applied criteria for determining whether a person should be identified as having a particular disease, injury, or other health-related condition  Use simple and objective criteria  Develop a case definition using:  Symptoms or laboratory results  Affected population (person)  Location (place)  Time frame

24. …Populations… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 24 Unit of observation is groups rather than individuals The epidemiologist‘s “patient “is the population Clinician vs. epidemiologist

25. 25 Clinician Vs Epidemiologist (Patient with Diarrhea) 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 25 . Clinician Epidemiologist Main Goal Treatment ( ± Dia g n o s i s) Determine cause, prevent other cases Questions What is wrong with patient? What treatment is appropriate? Exposure/sources? Who else exposed? Potential for spread Action Treatment Intervention (control & prevention) CDC principles of Epidemiology, 3rd ed

26. … Application … 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 26 Not just the study of… It is an applied science with direct practical applications. Practice of epidemiology is both science and art Provides data for decision-makers Tool for public health action

27. History of epidemiology S. N o Year Event 1 400 B.C. Hippocrates suggested that the development of human disease might be related to lifestyle factors and the external environment 2 1662 Graunt analyzed births and deaths in London and quantified disease in a population 3 1839 William Farr( the father of modern vital statistics and surveillance ) Farr set up a system for routine summaries of causes of death 4 1747 Lind used an "experimental" approach to prove the cause of scurvy by showing it could be treated effectively with fresh fruit. 5 1854 John Snow ( considered father of modern epidemiology )formed and tested a hypothesis on the origins of cholera in London: one of the first studies in analytic epidemiology 6 1920 Goldberger published a descriptive field study showing the dietary origins of pellagra 7 1937 Austin Bradford Hill mainly Suggested criteria for establishing causation 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 27

28. 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 28 .

29. Use/applications of Epidemiology (Purposes Of Epidemiology) 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 29 Elucidate the natural history of disease. Describe the health status of the population. Establish causation of disease. Provide an understanding of what causes or sustains disease in populations. Define standards and ranges for normal values of biological and social measures. Guide health and healthcare policy and planning. Assist in the management and care of health and disease in individuals.

30. Scope of epidemiology 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 30

31. Fundamental Assumptions In Epidemiology 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 31 A. “Disease human population does not occur at random ” B. “Human disease has causal and preventive factors That can be identified through systematic investigation of different populations or sub-groups of individuals within a population at different places or times”. Allows for the study of causal and preventive factors

32. Major categories of epidemiology 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 32   Descriptive Epidemiology – Defines the amount and distribution of health problems. It answers the questions: Who, what, and where . Analytic Epidemiology – Analyses the causes or determinants of health and disease. Answers the questions of why and how.

33. Descriptive Epidemiology is an antecedent of Analytic Epidemiology . 33 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

34. Theories/ principles of diseases causation 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 34 Disease causation The cause of a disease is an event, condition, or characteristics that precede the disease event and without which the disease event either would not have occurred at all or would not have occurred at some latter time. Not all associations between exposure and diseases are causal.

35. 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 35 If disease does not develop without the factor being present, then we term the causative factor “ necessary factor ” if the disease always result from the factor then we term the causative factor “sufficient factor “ Example- Tubercle bacilli is a necessary factor for tuberculosis -Rabies virus is sufficient for developing clinical rabies

36. Necessary Causes vs. Sufficient Causes 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 36 If A is necessary for B (necessary cause) that means you will never have B if you don't have A. In other words, if one thing is a  necessary  cause of another, then that means that the outcome  can never happen  without the cause. However, sometimes the cause occurs without the outcome. If A is sufficient for B (sufficient cause), that means that if you have A, you will ALWAYS have B. In other words, if something is a  sufficient  cause, then every time it happens the outcome will follow. The outcome  always follows  the cause. However, the outcome may occur without the cause.

37. What does cause Disease? 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 37 There are different theories : Supernatural Hippocratic Single germ Classic epidemiologic Ecological Multi-factorial causation

38. Supernatural Theory 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 38 In the early past, the disease was thought to be mainly due to either the curse of God or due to the evil force of the demons. Even today superstitions are becoming major obstacles in disease control Most of the literates view that disease is the result of microbes

39. Hippocratic theory 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 39 Hippocrates is credited with being the first physician to reject superstitions and beliefs that credited supernatural or divine forces with causing illness. He separated the discipline of medicine from religion, believing and arguing that disease was not a punishment inflicted by the gods but rather the product of environmental factors, diet and living habits.

40. The Single Germ Theory 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 40 Henle-koch Postulates Sometimes called “pure determinism” Each disease will be caused by a germ Without that germ that disease would not be caused By introducing that germ, that disease can be caused in animals experimentally. And that germ can again be isolated from that sick animal experimented with.

41. Classic Epidemiologic Theory 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 41 Agents Living organisms Exogenous chemicals Genetic traits Psychological factors and stress Nutritive elements Endogenous chemicals Physical forces Host factors: Immunity and immunologic response Host behavior Environmental factors: Physical environment (heat, cold, moisture) Biologic environment (flora, fauna) Social environment (economic, political, culture)

42. The Ecological Theory 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 42 Since disease arises within an ecological system, a basic tenet of epidemiology is that an ecological approach is necessary to explain the occurrence of disease. Interactions among humans, other living creatures, plants, animals, microorganisms, ecosystems, climate, geography, and topography are so complex that despite many studies we are often uncertain about what is happening.

43. Multi-factorial causation theory 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 43 Pettenkoffer contradicted the uni -factorial theory of disease causation and emphasized that disease is Multi-factorial in causation. Several causes or factors acting jointly, cumulatively, by complementing or in an unexplained manner will lead to the disease.

44. Causal Relationships 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 44 A causal pathway may be direct or indirect: Direct causation, A causes B without intermediate effects Indirect causation, A causes B, but with intermediate effects In human biology, intermediate steps are virtually always present in any causal process

45. Epidemiological models in disease causation 45 Models are purposely simplified representations Facilitates the understanding of nature (complex) The following are the widely used ones: Epidemiologic triangle/triad Web of causation model The Wheel model Causal pie model 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

46. 1. The epidemiologic triangle/ triad (balance beam): 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 46 . HOST AGENT ENVIRONMENT

47. Malaria 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 47 . Agent Host Environment Vector

48. The epidemiologic triad Model 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 48 . Agent: Amount, infectivity, pathogenicity, virulence, chemical composition, cell reproduction Environment: Physical, biological, social Host: Intrinsic factors, genetic, physiologic factors, psychological factors, immunity Health or Illness ?

49. Epidemiological models …. 2. The web of causation model especially developed to enhance understanding of chronic diseases de-emphasizes the role of the agent and highlights other factors that encourage the onset of disease 49 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

50. 2. Web of Causation 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 50 . Disease behaviour Unknown factors genes phenotype workplace social organization microbes environment

51. 51 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

52. Example of a Web of Causation 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 52 . Susceptible Host Infection Tuberculosis Vaccination Genetic Overcrowding Malnutrition Tissue Invasion and Reaction Exposure to Mycobacterium

53. Epidemiological models …. 53 3-The Wheel d epict human – environment relations The wheel consists of a hub (the host or human), which has genetic makeup as its core . Surrounding the host is the environment , schematically divided into biological, social, and physical For hereditary diseases, the genetic core would be relatively large. For conditions like measles the genetic core would be of lesser importance; 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

54. 3-The Wheel … 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 54 . The Wheel of Causation Social Environment Genetic Core Physical Environment Biological Environment Host (human)

55. Epidemiological models …. 4. The causal pie model Because the agent-host-environment model does not work well for some noninfectious diseases, several other models have been proposed One of the newer models; is based on the multi-factorial nature of causation in many diseases Factors as pieces of a pie, the whole pie making up the sufficient cause for a disease. Disease may have more than one sufficient cause 55 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

56. Necessary Vs Sufficient Cause 56 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. component causes- The factors represented by the pieces of the pie in this model 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

57. Epidemiological models …. 57 The Rothman’s causal Pie model illustrates the factors that act to cause disease as pieces of a pie, the whole pie making up the sufficient causes for a disease. A cause presented in all pies representing the sufficient causes for a particular disease is considered a necessary cause for the disease. Control of the disease could be achieved by removing one of the components in each "pie" and if there was a factor common to all "pies“ ( necessary cause ), the disease would be eliminated by removing that alone. 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

58. Epidemiological models …. 58 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

59. Exercise 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 59 Some of the risk factors for heart disease are smoking, hypertension, obesity, diabetes, high cholesterol, inactivity, stress, and type A personality . Are these risk factors necessary causes, sufficient causes?

60. Natural History of Diseases 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 60 The natural history of disease refers to the progression of a disease process in an individual over time, in the absence of intervention. The process begins with exposure to the causative agent capable of causing disease & the process ends with recovery, disability, or death. The Course of a disease may be halted at any point in the progression by preventive and therapeutic measures, host factors, and other influences.

61. Stages of Natural History of Disease 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 61 .

62. Stages of Natural . . . 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 62 1. Stage of susceptibility Disease has not yet developed, but the groundwork has been laid by the presence of factors that favor its occurrence. Example: unvaccinated child is susceptible to measles. 2. Stage of Pre-symptomatic (sub-clinical) disease There are no manifestations of the disease but pathologic changes (damages) have started to occur in the body. The disease can only be detected through special tests. The stage may lead to the clinical stage, end in recovery.

63. Stages of Natural . . . 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 63 Examples: Detection of antibodies against HIV in an apparently healthy person. Ova of intestinal parasite in the stool of apparently healthy children. 3. The Clinical stage The person has developed signs and symptoms of the disease. The clinical stage of different diseases differs in duration, severity and outcome. The outcomes of this stage may be recovery, disability or death.

64. Stages of Natural . . . 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 64 Examples: Common cold has a short and mild clinical stage and almost everyone recovers quickly. Polio has a severe clinical stage and many patients develop paralysis becoming disabled for the rest of their lives. Rabies has a relatively short but severe clinical stage and almost always results in death. Diabetes Mellitus has a relatively longer clinical stage and eventually results in death if the patient is not properly treated.

65. Stages of Natural . . . 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 65 4. Stage of disability or death Some diseases run their course and then resolve completely either spontaneously or by treatment. In others the disease may result in a residual defect, leaving the person disabled for a short or longer duration. Still, other diseases will end in death. Disability is limitation of a person's activities including his role as a parent, wage earner, etc Examples: Trachoma may cause blindness Meningitis may result in blindness or deafness. Meningitis may also result in death.

66. Possible outcomes after exposure to an infectious agent 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 66 . Exposure No infection Clinical infection Subclinical infection Death Immunity Carriage Non- immunity Carriage

67. Dynamics of Disease and Infectiousness 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 67 . Latent period Infectious period Non- infectious period Incubation period Clinical disease Recovery Infection Time Onset of symptoms Resolution of symptoms Period Between Exposure to Infectiousness

68. Cont … 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 68 Begins by exposure to a causative agent capable of causing disease. Without intervention, the process ends with recovery, disability or death. The course can be halted at any time in the progression by intervention, host factors and other influences.

69. Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 69 Eventually the host becomes non- infectious by, Clearing the infection, possibly by developing immunity Therapeutic intervention, or Death. The host may become non- clinically ill while still harboring the microbe (also called carrier).

70. Time Course of Disease& Communicability 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 70 . 16

71. Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 71 The natural history of infectiousness includes: Prepatent period : the time interval from infection to becoming infectious (shedding of the agent). Infectious period : the time during which an infected host could infect another host or vector. Incubation period : the time interval from infection to symptomatic disease. Latent period: the time interval from infection to development of infectiousness

72. Cont … 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 72 Non-infectious period: either by recover from the infection or by dea th. The symptomatic period. The probability of developing symptoms or disease after becoming infected is referred as pathogenicity (time of pathogenicity). Asymptomatic period: The host eventually becomes asymptomatic either by recovering from the symptoms or by death . Carrier state develops when the person become asymptomatic but remains infectious

73. Infectious Disease process… 73 Carriers may be Asymptomatic carriers : Transmitting the disease without ever showing manifestations of the disease. Example: Polio, amoebiasis Incubatory carriers: Transmitting the disease during incubation period, i.e. from first shedding of the agent until the clinical onset. Example: Measles, mumps 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

74. Infectious Disease process… 74 Convalescent carriers: Transmitting the disease during convalescence period i.e. from the time of recovery to when shedding stops. Example: Typhoid fever Chronic carriers: Transmitting the disease for a long period / indefinite transmission. E.g.: Hepatitis B 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

75. Infectious disease process 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 75 Infectious diseases result from the interaction of infectious agent, susceptible host/reservoir and environment that brings the host and the agent together. Infection implies that the agent has achieved entry and begun to develop or multiply, whether or not the process leads to disease. A model used to understand the infection process is called the chain of infection

76. Components of infectious Disease Process… 76 The infectious process of a specific disease can be described by the following components (factors). Causative agent Reservoir host Portal of exit Mode of transmission Portal of entry and Susceptible host. 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

77. Components of . … 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 77

78. Chain of Disease Transmission/t he Infectious disease cycle… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 78

79. The Infectious disease cycle… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 79 1 . Infectious agent/etiology Is an organism that is capable of producing infection or infectious disease Necessary cause to produce a disease The biological properties of the agent may play a major role in its epidemiology In order to survive the agent must be able to: Multiply Emerge from the host Reach the new host Infect the new host

80. The Infectious disease cycle… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 80 These can generally be grouped into the following five major taxonomic groups Virus: e.g. HIV; measles; Polio Herpes, etc. Bacteria: e.g. M. tuberculosis; N. meningitidis; N. gonorrhea Protozoa: e.g. Plasmodia species; E. histolytica ; Giardia lamblia , Leishmania species, etc. Fungi: e.g. Candida albicans ; Cryptococcus neoformans ; Histoplasma capsulatum Metazoa : e.g. Helminths

81. Intrinsic properties of Microorganisms 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 81 Infectivity : the ability of an agent to invade and multiply (produce infection) in an exposed host Infectivity is a measure of the progression of an infectious agent from exposure to infection It is the ability to cause infection in an exposed susceptible host and is measured by the infection rate (infectiousness). It is the proportion of exposed persons who become infected  

82. Intrinsic properties of Microorganisms… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 82 Factors influence the infectivity of an agent. Host and environmental factors The dose, The route of entry, source of infection, and the strain of the agent

83. Intrinsic properties of Microorganisms… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 83 Pathogenicity : is the ability of an agent to produce a clinically manifest disease in susceptible host. It is measured by determining the proportion of infections that result in clinically apparent disease  

84. Intrinsic properties of Microorganisms… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 84 Virulence: this is the ability of an agent to produce severe disease. It is measured by determining the proportion of clinical cases resulting in severe clinical manifestations. Some of measures of virulence as it applies to humans are the case fatality rate (CFR)

85. Intrinsic properties of Microorganisms… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 85 Factors that influence the degree of infectivity, pathogenicity, and virulence include: Strain of agent Dose of agent Route of infection Host factors like host age, host nutritional status; host immune response; Treatment, especially on virulence; and Season, through influencing exposure to the agent and other factors that enhance the entry of the agent into the body and its transmission.

86. Infectious Disease process…components 86 2. Reservoirs: an organism or habitat, in which an infectious agent normally lives, transforms, develops and/or multiplies. Reservoirs for infectious agents may be humans , animals, plants or other inanimate objects. Agents with a human reservoir include measles , mumps , and most respiratory pathogens. Animals as a reservoir of infection- brucellosis (cows and pigs), anthrax (sheep), plague (rodents), trichinosis (swine), and rabies (bats, raccoons, dogs, and other mammals). A person who does not have apparent clinical disease, but is a potential source of infection to other people is called a Carrier 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

87. 3. Portal of Exit 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 87 is the way the infectious agent leaves the reservoir Possible portals of exit include all body secretions and discharges: Nose — secretions Mouth — saliva, sputum Skin, intact — pustules, lesions Skin, broken — blood Breast (females) — milk, secretions Vagina (females) — secretions Penis (males) — ejaculate Anus — feces 35

88. Infectious Disease process…components 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 88 This is the site on the reservoir of infection through which the infectious agent escapes from the reservoir. This includes: Gastrointestinal Tract(GIT ) e.g. Typhoid fever, amoebiasis , etc., Respiratory Tract e.g. Tuberculosis, common cold Skin and Mucous membrane e.g. STDS, Scabies, ringworm

89. Infectious Disease process…components 89 4. Mode of Transmission of Infectious Agents It is the mechanism by which an infective agent are conveyed to a susceptible host There are two major modes Direct Transmission In-direct Transmission 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

90. Infectious Disease process…components 90 A. Direct Transmission immediate transfer of the agent from a reservoir to a susceptible host by direct contact or droplet spread. Direct contact: The contact of skin, mucosa, or conjunctiva with infectious agents directly from person or vertebrate animal, Mechanism: via touching, kissing, biting, passage through the birth canal, or during sexual intercourse. Example: HIV, rabies, gonorrhea 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

91. Infectious Disease process…components 91 ii. Direct projection: projection of saliva droplets by coughing, sneezing, singing, spitting or talking. Example: common cold iii. Trans-placental : Transmission from mother to fetus. Example: syphilis 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

92. Vertical Transmission 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 92 Transplacental TORCH Complex Toxoplasma Other (syphilis, VZV) Rubella Cytomegalovirus Herpes Listeria Parvovirus B-19 HIV During Birth Group B Streptococcus Herpes Gonorrhea Breastfeeding HIV HTLV-1, HTLV-2 Others?

93. The Infectious Disease… components B. Indirect Transmission an agent is carried from reservoir to a susceptible host by suspended air particles or animate (vector-mosquitoes, fleas, ticks...) or inanimate (vehicle-food, water, biologic products, fomites) intermediaries. 93 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

94. The Infectious Disease… components 94 Vehicle-borne: Transmission occurs through indirect contact with inanimate objects (fomites): by bedding, toys, or surgical instruments; as well as through contaminated food, water, IV fluids etc. Vehicle-borne food or water – Salmonella, hepatitis A fomites, biologic products – coxsackievirus 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

95. Some Causes of Food-borne, Waterborne Diseases 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 95 Food-borne Bacillus cereus Campylobacter jejuni Clostridium botulinum Clostridium perfringens Cyclospora cayetanensis E. coli O157:H7 Hepatitis A Listeria monocytogenes Norovirus Salmonella species Staphylococcus aureus Yersinia enterocolitica Waterborne Entamoeba histolytica Cryptosporidium parvum Giardia lamblia Hepatitis A Legionella pneumophila Vibrio cholera Parasitic Schistosoma Dracunculus Taenia solium et al.

96. The Infectious Disease… components 96 Vector-borne: The infectious agent is conveyed by an arthropod to a host. Vectors may be biological or mechanical. Biological vector : If the agent multiplies in the vector before transmission. Salivarian E.g. Malaria by the anopheles mosquito Stercorarian E.g. Typhus by ticks or lice Mechanical vector: If the agent is carried by the leg or proboscis. E.g. Trachoma by flies 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

97. Vectors and Vector-borne Diseases 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 97 . Pathogen Vector Disease West Nile virus mosquito WNV E ncephalitis Yersinia pestis flea Plague Borrelia burgdorferi tick Lyme disease Rickettsia prowazekii louse, tick Epidemic typhus Plasmodium falciparum mosquito Malaria Trypanosoma cruzi tsetse fly Chagas‘ disease Onchocerca volvulus Simulium fly River blindness

98. Infectious Disease process…components 98 Non vector intermediate host: Hosts not playing an active role in transporting the agent to humans.  A host in which the agent undergoes an asexual phase of its development. Example: Aquatic snails in the transmission of schistosomiasis. 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

99. 99 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

100. Susceptible Host (via portal of entry ) 5. Portals of Entry 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 100 Eye (conjunctivitis) Mouth – food-borne, water-borne, fecal-oral e.g. Typhoid fever, amoebiasis, etc. Nose/mouth – airborne, droplet (tuberculosis) e.g. Tuberculosis, common cold Skin e.g. Scabies, ringworm Genital tract, anus (STIs) Bloodstream (hepatitis B, HIV) injury site (tetanus), urethral (chlamydial infection), 44

101. Infectious Disease process…components 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 101 6 . Susceptible host This is a person who is highly likely to acquire infection when exposed to the agent Host- a person or animal that harbors an infectious agent under natural conditions Host susceptibility can be seen at the individual level and at the community level. At the individual level: The state of the host at any given time is in the interaction of genetic endowment with the environment over the entire life span. E. g: Genetic factors: sex, blood type, etc. Environmental factors : immunity acquired as a result of past infection

102. Infectious Disease process…components 102 At the community level: Host resistance at the community (population) level is called herd immunity Herd immunity can be defined as the resistance of a community (group) to invasion and spread of an infectious agent, based on the immunity of a high proportion of individuals in the community The high proportion of immunes prevents transmission by highly decreasing the probability of contact between reservoirs and susceptible hosts. 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

103. Infectious Disease process…components 103 Herd immunity best operates when Single reservoir is involved Mode of transmission is direct There is total immunity No shedding of agent by immune host Uniform distribution of immunes No overcrowding 25-May-24 BY Wakgari Mosisa . (BSc, MPHE)

104. Levels of Disease Prevention 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 104 It is important to the levels in details for implementing interventions that prevent or ameliorate infections. It involves the interruption or slowing of disease progression through appropriate intervention. Epidemiology plays a central role in disease prevention by identifying modifiable causes of disease and their risk factors.

105. Cont … 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 105 There are several stages during the course of a disease where we can intervene to control the disease. Three levels, (Primary, Secondary, and Tertiary) I. Primary Prevention The objectives here are to promote health, prevent exposure, and prevent the occurrence of disease.

106. Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 106 A. Health Promotion (Primordial): This consists of general non- specific interventions that enhance health and the body‘s ability to resist disease. The improvement of socio-economic status through the provision of adequate: education, affordable and adequate housing and clothing, etc.

107. Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 107 B. Prevention of exposure: Specific to individual diseases compared to primordial prevention Example Provision of safe and adequate water, proper excreta disposal Vector control; Provision of a safe environment at home Use of bed nets Consistent use of condom

108. Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 108 C. Prevention of Disease: This is when the intervention aims to prevent the initiation of disease, in persons who may already be exposed to the agent An example of intervention, which acts at this stage is immunization. Some times it may be difficult to differentiate interventions in what form of prevention they involved.

109. Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 109 II. Secondary Prevention: Interventions that act after the biological onset of a disease, but before permanent damage sets in. The objective here is to stop or slow the progression of disease and to prevent or limit permanent damage. Strategy at this stage is through early detection and treatment of disease.

110. Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 110 III. Tertiary Prevention Intervention that acts after permanent damage has set in, and the objective of tertiary prevention is to limit the impact of the damage. The impact can be physical, psychological social (social stigma or avoidance by others), and financial. Strategy at this stage in general is rehabilitative.

111. Exercise 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 111 How do we prevent malaria? Primary Secondary Tertiary How do we prevent STIs? Primary Secondary Tertiary

112. Disease control 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 112 Vaccination Shifts individuals from susceptible to recovered state. For some diseases this shift is not permanent. Therapy increases the rate of flow out of the infected states Permanent cure moves the individual from the infected to the recovered state. Temporary cure moves the individual from the infected to the susceptible state . Education through behavioral change, reduces shifting from susceptible to infected.

113. Susceptible Infected Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 113 . Recovered Vaccination Therapy Education Education

114. Estimating Transmission 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 114 Transmission probability is the probability, given that there is contact between an infective source and susceptible host, successful transfer of infective agent will occur so that the susceptible host becomes infected. There are different ways of estimating transmission probability; including secondary attack rate , reproductive number, and the like.

115. Secondary Attack Rate (SAR) 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 115 Measures the spread of disease among contacts of an index case. The index cases are excluded from the numerator and denominator Index case : The case that brings a people (household) to the attention of the public health personnel.

116. Minimum Incubation Period Time Periods for Estimating Household Secondary Attack Rate 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 116 . Onset of Primary Case Definition of Time Interval Maximum Infectious Period Maximum Incubation Period Secondary Cases Onset of Cases in Households 1 Primary Case Co- Primary Case 2 Secondary Case 3 Secondary Case 4 Tertiary Case 5 Time Time

117. Example on SAR 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 117 Consider an outbreak of shigellosis in which 18 persons in 18 different households all became ill. If the population of the community is 1,000, then the overall attack rate is 18 /1,000 x 100% = 1.8%. One incubation period later, 17 persons in the same households as these ―primary cases developed shigellosis. If the 18 households included 86 persons, calculate the secondary attack rate.

118. Example Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 118 .

119. Basic Reproductive Number 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 119 Basic reproductive number, denoted by R , is: the expected number of new infected hosts that one infectious host will produce during his (or her) period of infectiousness in a large, susceptible population. Example: if R = 9 for measles in a population, then one person with measles introduced into that population can be expected to produce 9 (nine) secondary infections before recovering.

120. Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 120 Basic reproductive number, R Does not include the new cases produced by the secondary cases or further down the chain. Does not include secondary cases who do not become infectious.

121. Cont… 25-May-24 BY Wakgari Mosisa . (BSc, MPHE) 121 Basic reproductive number, R for an epidemic to occur, R must be > 1 if R < 1, the disease will not be reproduced, so an epidemic will not occur R is a composite of three important aspects of infectious diseases: the proportion of susceptible contacts, C transmission probability, β duration of infectiousness, d. R = C x β x d