The Epidemiology of Infectious Disease

1. The Epidemiology of Infectious Disease Chapter 37 Todd Kitten, Ph.D.

2. Epidemiology • The science that evaluates occurrence, determinants, distribution, and control of health and disease in a defined human population • Concerned primarily with the occurrence of disease as categorized by time, place, persons, and causes.

3. Epidemiology • Diseases investigated can be infectious or non-infectious, and can concern injury and fatality not caused by diseases.

4. http://www.phppo.cdc.gov/phtn/catalog/pdf-file/LESSON1.pdf

5. Epidemiology • Diseases investigated can be infectious or non-infectious, and can concern injury and fatality not caused by diseases. • Often concerns associations

6. Types of Associations • Not statistically associated • Statistically associated • Non-causal • Causal • Indirectly causal • Directly causal

7. Epidemiology • Diseases investigated can be infectious or non-infectious, and can concern injury and fatality not caused by diseases. • Often concerns associations • Can concern associations that are beneficial rather than harmful • Alcohol and CHD • Fluoride

8. Fluoride and dental caries Lilienfeld, D.E. and Stolley, P.D. 1994. Foundations of Epidemiology, 3rd Ed., Oxford Press.

9. Epidemiology • Diseases investigated can be infectious or non-infectious, and can concern injury and fatality not caused by diseases. • Often concerns associations • Can concern associations that are beneficial rather than harmful • Alcohol and CHD • Fluoride • Can concern experiments rather than just observation

10. Fluoride and dental caries Lilienfeld, D.E. and Stolley, P.D. 1994. Foundations of Epidemiology, 3rd Ed., Oxford Press.

11. Epidemiology Terminology • sporadic disease • occurs occasionally and at irregular intervals (typhoid fever) • endemic disease • maintains a relatively steady low-level frequency at a moderately regular interval (common cold) • hyperendemic diseases • gradually increase in occurrence frequency above endemic level but not to epidemic level (common cold during the winter) • epidemic • sudden increase in frequency above expected number • index case – first case in an epidemic

12. Figure 37.1

13. More terms… • outbreak • sudden, unexpected occurrence of disease • usually focal or in a limited segment of population • often synonymous with “epidemic” • pandemic • increase in disease occurrence within large population over wide region (usually worldwide)

14. Animal diseases • epizootiology • deals with factors that influence frequency of a disease in an animal population • enzootic • moderate prevalence of disease • epizootic • sudden outbreak of disease • panzootic • wide dissemination • zoonoses • diseases of animals that can be transmitted to humans

15. Measuring Frequency • three important statistical measures of disease frequency • morbidity rate • prevalence rate • mortality rate

16. Morbidity rate • an incidence rate • number of new cases in a specific time period per unit of population # new cases during a specific time # individuals in population

17. Prevalence rate • total number of individuals infected at any moment in time • depends both on incidence rate and duration of illness

18. Mortality rate • number of deaths from a disease per number of cases of the disease # deaths due to given disease size of total population with disease If 500 people in a town of 100,000 become infected with HIV and 100 die, the mortality rate is…

19. Infectious Disease Epidemiology • tries to determine: • causative agent • source and/or reservoir of disease agent • mechanism of transmission • host and environmental factors that facilitate development of disease within a defined population • best control measures

20. Recognition of an Infectious Disease in a Population • involves use of surveillance methods • cases of a disease recognized by its characteristic disease syndrome • set of signs and symptoms characteristic of a disease • signs • objective changes in body that can be directly observed • symptoms • subjective changes experienced by patient

21. Some surveillance methods • review of death certificates • field investigation of epidemics • investigation of actual cases

22. Investigation of a GI illness outbreak http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

23. Investigation of a GI illness outbreak http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

24. Investigation of a GI illness outbreak If you were to administer a questionnaire to the church supper participants, what information would you collect? • What did you eat? • How much did you eat? • How long after you ate did you begin to feel sick? • How long did it last? • Did anyone at the supper show illness prior to the supper? • What did you drink? • How was the food prepared/ stored? http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

25. Investigation of a GI illness outbreak http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

26. Investigation of a GI illness outbreak http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

27. Investigation of a GI illness outbreak Without having clinical isolates from the infected individuals for examination, how might you tentatively identify the causative agent? • Temperature of storage vs organism? • Food vs organism? • Homemade vs commercial? • Symptoms • Culture ice cream. It the outbreak has already ended, what is the value of working up the case? http://www.cdc.gov/eis/casestudies/xoswego.401-303.student.pdf

28. Course of infectious disease • incubation period • period after pathogen entry but before signs and symptoms appear • prodromal stage • onset of signs and symptoms • not clear enough for diagnosis • period of illness • disease is most severe and has characteristic signs and symptoms • convalescence • signs and symptoms begin to disappear

29. Recognition of an Epidemic • two types of epidemics • common source epidemic • propagated epidemic

30. Figure 37.2

31. Figure 37.3

32. Herd immunity • resistance of a population to infection and to spread of an infectious organism because of the immunity of a large percentage of the population

33. Figure 37.4

34. Figure 37.4

35. Herd immunity • resistance of a population to infection and to spread of an infectious organism because of the immunity of a large percentage of the population • level can be altered by introduction of new susceptible individuals into population • level can be altered by changes in pathogen • antigenic shift – major change in antigenic character of pathogen • antigenic drift – smaller antigenic changes

36. The Infectious Disease Cycle: Story of a Disease Figure 37.5

37. What Pathogen Caused the Disease? • Koch’s postulate (or modifications of them) are used if possible • clinical microbiology lab • plays important role in isolation and identification of pathogen • communicable disease • can be transmitted from one host to another

38. What was the Source and/or Reservoir of the Pathogen? • source • location from which pathogen is transmitted to host • period of infectivity • time during which source is infectious or is disseminating the organism • reservoir • site or natural environmental location in which pathogen is normally found • sometimes functions as source of pathogen

39. Human sources/reservoirs • carrier • infected host • can be casual (acute or transient) carrier – short time • can be chronic carrier – long time

40. Types of carriers • incubatory carrier • harbors pathogen but is not yet ill • active carrier • has overt clinical case of disease • convalescent carrier • has recovered from disease but continues to harbor large numbers of pathogen • healthy carrier • harbors pathogen but is not ill

41. Animal reservoirs • numerous diseases are zoonoses • transmission to human can be direct or indirect • vectors • organisms that spread disease from one host to another

42. The Infectious Disease Cycle: Story of a Disease Figure 37.5

43. How Was the Pathogen Transmitted? • Vertical transmission • Horizontal transmission: • airborne • contact • vehicle • vector-borne

44. Airborne Transmission • pathogen suspended in air and travels  1 meter • droplet nuclei • small particles (1-4 mm diameter) • can remain airborne for long time • can travel long distances • usually propelled from respiratory tract of source organisms by sneezing, coughing, or vocalization • dust particles also important route of airborne transmission

45. Contact Transmission • coming together or touching of source/reservoir and host • direct contact (person-to-person) • physical interaction between source/reservoir and host • e.g., kissing, touching, and sexual contact • indirect contact • involves an intermediate (usually inanimate) • e.g., eating utensils, bedding • droplet spread • large particles (>5 mm) that travel < 1 meter

46. Vehicle Transmission • vehicles • inanimate materials or objects involved in pathogen transmission • common vehicle transmission • single vehicle spreads pathogen to multiple hosts • e.g., water and food • fomites • common vehicles such as surgical instruments, bedding and eating utensils

47. Vector-Borne Transmission • external (mechanical) transmission • passive carriage of pathogen on body of vector • no growth of pathogen during transmission • internal transmission • carried within vector • harborage transmission – pathogen does not undergo changes within vector • biologic transmission – pathogen undergoes changes within vector

48. The Infectious Disease Cycle: Story of a Disease Figure 37.5

49. Why Was the Host Susceptible to the Pathogen? • two main factors • defense mechanisms of host • Can be related to age, sex, genetic factors, immune status, nutrition, environment • pathogenicity of pathogen

50. The Infectious Disease Cycle: Story of a Disease Figure 37.5