Chapter 33

1. Chapter 33 Epidemiology of Infectious Disease

2. Epidemiology • science that evaluates occurrence, determinants, distribution, and control of health and disease in a defined human population • health • condition in which organism (and all its parts) performs its vital functions normally • disease • impairment of the normal state of an organism or any of its components that hinders the performance of vital functions • epidemiologist • one who practices epidemiology

3. Figure 33.1

4. Centers for Disease Control and Prevention (CDC) • located in Atlanta, GA • functions as national focus for • developing and applying disease prevention and control • environmental health • health promotion and health education activities designed to improve the health of the people • worldwide counterpart is the World Health Organization (WHO) located in Geneva, Switzerland

5. Epidemiology Terminology • sporadic disease • occurs occasionally and at irregular intervals • endemic disease • maintains a relatively steady low-level frequency at a moderately regular interval • hyperendemic diseases • gradually increase in occurrence frequency above endemic level but not to epidemic level

6. More Terms… • outbreak • sudden, unexpected occurrence of disease • usually focal or in a limited segment of population • epidemic • sudden increase in frequency above expected number • index case – first case in an epidemic • pandemic • increase in disease occurrence within large population over wide region (usually worldwide)

7. 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

8. Prevalence Rate • total number of individuals infected at any one time • depends both on incidence rate and duration of illness

9. 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

10. Correlation with a Single Causative Agent • after recognition of infectious disease in a population, outbreak correlated with specific pathogen • clinical microbiologists help in isolation and identification of pathogen

11. Figure 33.4

12. Human Sources/Reservoirs • carrier • infected hosts who are potential sources of infection for others • types of carriers • active carrier has overt clinical case of disease • convalescent carrier has recovered but continues to harbor large numbers of pathogen • healthy carrier harbors the pathogen but is not ill • incubatory carrier is incubating the pathogen in large numbers but is not yet ill

13. Animal Reservoirs • transmission to human can be direct or indirect • vectors • organisms that spread disease from one host to another

14. More Types of Carriers… • casual, acute, or transient carriers • convalescent, healthy, and incubatory carriers that harbor pathogen for brief time • chronic carriers • convalescent, healthy, and incubatory carriers that harbor pathogen for long time

15. How Was the Pathogen Transmitted? Four main routes: • airborne • contact • vehicle • vector-borne

16. Airborne Transmission • pathogen suspended in air and travels  1 meter • droplet nuclei • small particles (1-4 m 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

17. Figure 33.9

18. 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 m) that travel < 1 meter

19. Figure 33.8

20. 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

21. 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

22. Virulence and the Mode of Transmission Evidence suggests correlation between mode of transmission and degree of virulence • direct contact  less virulent • vector-borne  highly virulent in human host; relatively benign in vector • greater ability to survive outside host  more virulent • e.g., Mycobacterium tuberculosis and Corynebacterium diphtheriae can survive weeks to months outside human hosts

23. Reasons for Increases in Emerging and Reemerging Infectious Diseases • crowding • habitat disruption • shift in distribution of nosocomial pathogens

24. More Reasons… • excessive or inappropriate use of antimicrobial therapy • medical practices that lead to immunosuppression • rapid global transportation systems

25. Control of Epidemics Three types of control measures • reduce or eliminate source or reservoir of infection • break connection between source and susceptible individual • reduce number of susceptible individuals

26. Reduce or Eliminate Source or Reservoir • quarantine and isolation of cases and carriers • destruction of animal reservoir • treatment of sewage • therapy that reduces or eliminates infectivity of cases

27. Break Connection between Source and Susceptible Individuals • chlorination of water supplies • pasteurization of milk • supervision and inspection of food and food handlers • destruction of insect vectors with pesticides

28. Reduce Number of Susceptible Individuals • raise level of herd immunity by • passive immunity following exposure • active immunity for protection

29. Vaccines and Immunization Vaccine • preparation of one or more microbial antigens used to induce protective immunity • may consist of killed, living, weakened (attenuated) microbes or inactivated bacterial toxins (toxoids), purified cell material, recombinant vectors or DNA

30. More on Vaccines and Immunization Immunization • result obtained when vaccine stimulates immunity • vaccines attempt to induce antibodies and activated T cells to protect host from future infection • Vaccinomics is the application of genomics and bioinformatics to vaccine development

31. Table 33.3

32. Recombinant-Vector Vaccines • pathogen genes that encode major antigens inserted into nonvirulent viruses or bacteria which serve as vectors and express the inserted gene • released gene products (antigens) can elicit cellular and humoral immunity

33. DNA Vaccines • DNA directly introduced into host cell via air pressure or gene gun • when injected into muscle cells, DNA taken into nucleus and pathogen’s DNA fragment is expressed • host immune system responds to foreign proteins produced • many DNA vaccine trials are currently being run

34. Table 33.5

35. Bioterrorism Preparedness Bioterrorism • “intentional or threatened use of viruses, bacteria, fungi, or toxins from living organisms to produce death or disease in humans, animals, and plants”

36. Examples of Intentional Uses of Biological Agents for Criminal or Terror Intent • 1984 in The Dalles, OR • Salmonella typhimurium in 10 restaurant salad bars • 1996 in Texas • intentional release of Shigella dysentariae in a hospital lab break room • 2001 in seven eastern U.S. states • use of weaponized Bacillus anthracis spores delivered through U.S. postal systems • all of above caused hospitalizations, the anthrax episode resulted in five deaths

37. Table 33.6

38. Choosing Biological Agents as Weapons • biocrime when chosen as a means for a localized attack vs. bioterrorism when chosen for mass casualties • characteristics that favor their use • invisible, odorless, and tasteless • difficult to detect • take hours or days before awareness that they have been used • fear and panic associated with the anticipation that they were used

39. Nosocomial Infections • result from pathogens that develop within a hospital or other clinical care facility and are acquired by patients while they are in the facility • 5-10% of all hospital patients acquire a nosocomial infection • usually caused by bacteria that are members of normal microbiota

40. Source Endogenous pathogen • brought into hospital by patient or acquired when patient is colonized after admission Exogenous pathogen • microbiota other than the patient’s Autogenous infection • caused by an agent derived from microbiota of patient despite whether it became part of patient’s microbiota following admission

41. Control, Prevention, and Surveillance • proper training of personnel in basic infection control measures • e.g., handling of surgical wounds and hand washing • monitoring of patient for signs and symptoms of nosocomial infection

42. The Hospital Epidemiologist • individual responsible for developing and implementing policies to monitor and control infections and communicable diseases • reports to infection control committee or similar group

43. Bibliography • Lecture PowerPoints Prescott’s Principles of Microbiology-Mc Graw Hill Co. • http://en.wikipedia.org/wiki/Scientific_method • https://files.kennesaw.edu/faculty/jhendrix/bio3340/home.html