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9/10/2010

9/10/2010. Dr. Salwa Tayel. 1. Infectious Disease Epidemiology. By. Dr. Salwa Tayel. Associate Professor Family and Community Medicine Department King Saud University. Dr. Salwa Tayel. 2. Learning Objectives. At the end of this lecture you (will) be able to:

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9/10/2010

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  1. 9/10/2010 Dr. Salwa Tayel Dr. Salwa Tayel 1

  2. Infectious Disease Epidemiology By Dr. Salwa Tayel Associate Professor Family and Community Medicine Department King Saud University Dr. Salwa Tayel Dr. Salwa Tayel 2

  3. Learning Objectives At the end of this lecture you (will) be able to: Explain common definitions and basic concepts used in epidemiology. Describe natural history and spectrum of infectious diseases and their implications for public health. 3 Dr. Salwa Tayel Dr. Salwa Tayel

  4. Infection: • It is the entry, development and multiplication of an infectious agent in the body of man or animal. • Outcome of infection varies. Infectious disease: • A clinically manifest disease of man or animal resulting from infection. In-apparent infection: • The infection does not become manifest at any stage. 4 Dr. Salwa Tayel Dr. Salwa Tayel

  5. 5 Dr. Salwa Tayel Dr. Salwa Tayel

  6. Communicable Disease • An illness due to a specific infectious agent or its toxic products that arises through transmission of that agent from reservoir to susceptible host. • Communicable Disease (typhoid, influenza,.. • Non- Communicable Disease (DM, cancer,.. 6 Dr. Salwa Tayel Dr. Salwa Tayel

  7. Natural history • Naturalhistoryof disease refers to the progress of a disease process in an individual over time, in the absence of intervention. • The natural history of a disease describes the course of the disease in an individual starting from the moment of exposure to the causal agents till one of the possible outcomes occurs. 7 Dr. Salwa Tayel Dr. Salwa Tayel

  8. 8 Dr. Salwa Tayel Dr. Salwa Tayel

  9. Spectrum of disease • The idea that an exposure can lead to varying signs, symptoms and severity of the same disease in the population is the spectrum of disease. • Why do we have varying degrees of severity? Prognosis? • The outcome will depend on the interactions of host, agent and environmental factors. 9 Dr. Salwa Tayel Dr. Salwa Tayel

  10. Classification of diseases according to clinical severity (spectrum) 10 Dr. Salwa Tayel Dr. Salwa Tayel

  11. Class A: Inapparent infection Examples: Tuberculosis, Polio, Hepatitis A, Meningitis, AIDS 11 Dr. Salwa Tayel Dr. Salwa Tayel

  12. Class B: Classic cases Examples: Measles, Chickenpox 12 Dr. Salwa Tayel Dr. Salwa Tayel

  13. Class C: Severe or Fatal infections Examples: Rabies, Hemorrhagicfevers caused by Ebola and Murberg viruses. 13 Dr. Salwa Tayel Dr. Salwa Tayel

  14. Implications for public health 14 Dr. Salwa Tayel Dr. Salwa Tayel

  15. The relation of severity of illness to disease statistics. 15 Dr. Salwa Tayel Dr. Salwa Tayel

  16. 1 Diseased, diagnosed & controlled 2 Diagnosed, uncontrolled 3 Undiagnosed or wrongly diagnosed disease 4 Risk factors for disease 5 Free of risk factors Diagnosed disease Undiagnosed or wrongly diagnosed disease The pyramid and iceberg of disease 16 Dr. Salwa Tayel Dr. Salwa Tayel

  17. Iceberg phenomenon • Cases of illness correctly diagnosed by clinicians in the community often represent only the “tip of the iceberg.” • Many additional cases may be too early to diagnose or may remain asymptomatic. • Examples: Tuberculosis, meningitis, polio, hepatitis A, AIDS. • The risk is that persons with in-apparent or undiagnosed infections may be able to transmit infection to others. 17 Dr. Salwa Tayel Dr. Salwa Tayel

  18. Applications of the concepts of natural history and spectrum of disease • Persons with in-apparent or undiagnosed infections can transmit infections to others. • Control measures must be directed toward all infections capable of being transmitted to others; • both clinically apparent cases and • those with in-apparent or undiagnosed infections. 18 Dr. Salwa Tayel Dr. Salwa Tayel

  19. Chain of infection 23/10/2010 19 Dr. Salwa Tayel Dr. Salwa Tayel

  20. Chain of infection A process that begins when an agent leaves its reservoir or host through a portal of exit, and is conveyed by some modeoftransmission, then enters through an appropriate portal of entry to infect a susceptible host. 23/10/2010 20 Dr. Salwa Tayel Dr. Salwa Tayel

  21. Cycle of infection Agent Susceptible Host Reservoir IP PC Portal of Inlet Portal of Exit Mode of transmission 23/10/2010 21 Dr. Salwa Tayel Dr. Salwa Tayel

  22. The requisites (essentials) for the perpetuation of communicable diseases: • The elements of the cycle of infection: • Presence of microbiological agent. • Presence of reservoir. • Portal of exit. • Mode of transmission. • Portal of entry (inlet). • Presence of susceptible host. 23/10/2010 22 Dr. Salwa Tayel Dr. Salwa Tayel

  23. 1. Agent: Microorganisms are responsible for disease production (viruses, bacteria, protozoa, parasites, fungi,.. 23/10/2010 23 Dr. Salwa Tayel Dr. Salwa Tayel

  24. Mechanisms of disease production (pathogenesis): • Invasiveness: (Pneumococcosis, measles). • Toxicity: (Tetanus, Botulism). • Hypersensitivity: (Tuberculosis). • Others: (Immune suppression; AIDS). 23/10/2010 24 Dr. Salwa Tayel Dr. Salwa Tayel

  25. Agent factors that affect disease transmission: Infectivity, Pathogenicity, Virulence, Antigenicity,… 23/10/2010 25 Dr. Salwa Tayel Dr. Salwa Tayel

  26. Infectivity The ability of an agent to invade and multiply (produce infection) in a susceptible host. How to measure (Infectivity);ease & spread of infection? Secondary Attack Rate The proportion of exposed susceptible persons who become infected. Examples: High infectivity: Measles, Chickenpox Low infectivity: Leprosy 23/10/2010 26 Dr. Salwa Tayel Dr. Salwa Tayel

  27. Pathogenicity Is the ability of the organisms to produce specific clinical reaction after infection Itrefers to the proportion of infected persons who develop clinical disease. Examples: High pathogenicity: Measles, Chickenpox (Class B) Low pathogenicity: Polio, Tuberculosis, Hepatitis A, Meningitis, AIDS (Class A) It can be measured by: Ratio of clinical to sub-clinical case= 23/10/2010 27 Dr. Salwa Tayel Dr. Salwa Tayel

  28. Virulence It refers the ability of organisms to produce severe pathological reaction. It is the proportion of persons with clinical disease who become severely ill or die. Examples: Rabies, Hemorrhagic fevers caused by Ebola and Murberg viruses. (Class C) It can be measured by: Case fatality rate 23/10/2010 28 Dr. Salwa Tayel Dr. Salwa Tayel

  29. Antigenicity (Immunogenicity) The ability of the organism to produce specific immunity (antibodies or antitoxin). It can be measured by: Second attack frequency: Second attacks are rare in measles, mumps and chickenpox. Re-infection occurs as in case of common cold,syphilis and gonorrhea. 23/10/2010 29 Dr. Salwa Tayel Dr. Salwa Tayel

  30. 2. Reservoir of infection The reservoir of an agent is the habitat in which an infectious agent normally lives, grows, and multiplies. Types of reservoirs: Humans, animals, and the environment. Human reservoirs Two types of human reservoir exist: • Cases: persons with symptomatic illness • Carriers 23/10/2010 30 Dr. Salwa Tayel Dr. Salwa Tayel

  31. Pathogen Reservoirs Humans are the most important reservoir of human infectious disease. 23/10/2010 31 Dr. Salwa Tayel Dr. Salwa Tayel

  32. Carrier A person or animalwithoutapparent disease who harbors a specific infectious agent and is capable of transmitting the agent to others. 23/10/2010 32 Dr. Salwa Tayel Dr. Salwa Tayel

  33. Carriers are dangerous because: • They do not show any clinical manifestation so they carry normal life. • The carrier and his contacts are not aware of their condition so, they take no precautions. • It is difficult to discover them. • It is not always possible to deal with them. • Chronic carriers can remain infectious for a long time leading to repeated introduction of the disease to contacts. 23/10/2010 33 Dr. Salwa Tayel Dr. Salwa Tayel

  34. Types of Carriers: • Asymptomatic (In-apparent) carrier: • The carrier state that may occur in an individual with an infection that is in-apparent throughout its course • Examples: Polio virus, meningococcus, hepatitis A virus • Incubatory, Convalescent, Post-Convalescent carriers: • The carrier state may occur during the incubation period, convalescence, and post convalescence of an individual with a clinically recognizable disease. • Examples of Incubatory carrier: Measles, chickenpox 23/10/2010 34 Dr. Salwa Tayel Dr. Salwa Tayel

  35. Convalescent carriers: Examples:Diphtheria, hepatitis B viruses and Salmonella species **According to duration of carriage: The carrier state may be (transient carrier or chronic carrier). Chronic carriers: They continue to harbour an agent for anextended time (months or years) following the initial infection. Examples: Hepatitis B virus and Salmonella typhi 23/10/2010 35 Dr. Salwa Tayel Dr. Salwa Tayel

  36. Animal reservoirs Zoonoses:Infectious diseases that are transmissible under normal conditions from vertebrate animals to humans. (with humans as incidental hosts) • Zoonoticdiseases include: • brucellosis (cows and pigs), • anthrax (sheep), • plague (rodents), • rabies (bats, dogs, and other mammals). 23/10/2010 36 Dr. Salwa Tayel Dr. Salwa Tayel

  37. Zoonoses Zoonoses are Human Diseases with Animal Reservoirs. 23/10/2010 37 Dr. Salwa Tayel Dr. Salwa Tayel

  38. Zoonoses Toxoplasmosis 23/10/2010 38 Dr. Salwa Tayel Dr. Salwa Tayel

  39. Environmental reservoirs: • Soil, and water • Soil: Agents live and multiply in the soil. • Examples: • -Tetanus spores and • - Fungal agents; (those causing histoplasmosis) • Pools of water are the primary reservoir of • Legionnaires’ bacillus. 23/10/2010 39 Dr. Salwa Tayel Dr. Salwa Tayel

  40. BibliothecaAlexandrina Thank you Website http://faculty.ksu.edu.sa/73234/default.aspx salwatayel@hotmail.com 23/10/2010 40 Dr. Salwa Tayel Dr. Salwa Tayel

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