1. Epidemiology of Infectious Diseases
Chapter 12
Epidemiology of Infectious Diseases

2. Learning Objectives State modes of infectious disease transmission.
Define categories of infectious disease agents.
Identify the characteristics of agents.

3. Infectious Diseases (Importance)
They are significant cause of morbidity and mortality worldwide.
Infectious agents are associated with some types of cancer.
They cause disease outbreaks in institutions.

4. Epidemiologic Triangle
A model used to explain the etiology of infectious diseases.
Recognizes three major factors in the pathogenesis of disease: agent, host, and environment. 2

5. Diagram of Epidemiologic Triangle

6. Microbial Agents of Infectious Disease
Protozoa
Helminths
Arthropods
Bacteria
Viruses and rickettsia
Mycoses (fungal diseases) 3

7. Bacteria
Once were the leading killers, but now are controlled by antibiotics.
Remain significant causes of human illness.
Tuberculosis and salmonellosis are common diseases caused by bacteria.
Emergence of antibiotic-resistant strains a growing concern.

8. Viruses and Rickettsia
Viral hepatitis A, herpes, and influenza are caused by viruses.
Rickettsial agents produce Q fever and Rocky Mountain spotted fever.

9. Mycoses (Fungal Diseases)
Valley fever (coccidioidomycosis), ringworm and athlete’s foot.
Opportunistic mycoses infect immunocompromised patients.
Candidiasis, cryptococcosis, and aspergillosis. 4

10. Protozoa
Cause malaria, amebiasis, babesiosis, cryptosporidiosis, and giardiasis.

11. Helminths Found in tropical areas.
Include intestinal parasites such as roundworms, pinworms, and tapeworms.
Are responsible for trichinellosis and schistosomiasis.

12. Arthropods Act as insect vectors.
Examples: mosquitos, ticks, flies, mites.
Transmit diseases such as malaria and encephalitis. 5

13. Characteristics of Infectious Disease Agents
Infectivity
The capacity of an agent to produce infection or disease.
Measured by the secondary attack rate.
Pathogenicity
The capacity of the agent to cause disease in the infected host.
Measured by the proportion of individuals with clinically apparent disease. 6

14. Characteristics of Infectious Disease Agents (cont’d)
Virulence
Refers to the severity of the disease.
Measured by the proportion of severe or fatal cases. If fatal, use case fatality rate.
Toxigenicity
The capacity of the agent to produce a toxin or poison. 7

15. Characteristics of Infectious Disease Agents (cont’d)
Resistance
The ability of the agent to survive adverse environmental conditions.
Antigenicity
The ability of the agent to induce antibody production in the host. Related to immunogenicity. 8

16. Host: Definition (Refer to Glossary)
A person (or animal) who permits lodgment of an infectious disease agent under natural conditions.

17. Host
Once an agent infects the host, the degree and severity of the infection will depend on the host’s ability to fight off the infectious agent.
Two types of defense mechanisms are present in the host: nonspecific and disease-specific. 9

18. Nonspecific Defense Mechanisms
Examples include skin, mucosal surfaces, tears, saliva, gastric juices, and the immune system.
Nonspecific defense mechanisms such as immunity may decrease as we age. 10

19. Disease-Specific Defense Mechanisms
Immunity (resistance) against a particular agent.
Types of immunity:
Active: administration of a microorganism to invoke an immunologic response that mimics the natural infection.
Passive: short-term immunity provided by a preformed antibody. 11

20. Active Immunity
Natural, active--results from an infection by the agent.
Artificial, active--results from an injection with a vaccine that stimulates antibody production in the host. 12

21. Passive Immunity
Natural, passive--preformed antibodies are passed to the fetus during pregnancy and provide short-term immunity in the newborn.
Artificial, passive--preformed antibodies are given to exposed individuals to prevent disease.

22. Environment
The domain external to the host in which the agent may exist, survive, or originate.
The environment consists of physical, climatologic, biologic, social, and economic components that affect the survival of the agents and serve to bring the agent and host into contact. 13

23. Reservoirs of Infectious Diseases
The environment can act as a reservoir that fosters the survival of infectious agents.
Examples: contaminated water supplies or food; soils; vertebrate animals. 14

24. Animal Reservoirs Animals can be reservoirs of infectious agents.
Zoonoses--infectious diseases that are potentially transmittable to humans by vertebrate animals. Examples: rabies and the plague.

25. Direct Transmission from Reservoir
Spread of infection through person-to-person contact.
Portal of exit--site where infectious agents leave the body, e.g., respiratory system, skin lesions. 15

26. Direct Transmission (cont’d)
Portal of entry--locus of access to the human body, e.g., mouth and digestive system.
Agent must exit in large enough quantities to survive in the environment and overcome the defenses at the portal of entry into the host.

27. Inapparent Infection No symptoms of infection present.
Important because disease can be transmitted to unsuspecting hosts.
In asymptomatic individuals, clinicians can look for serologic evidence of infection.
Example: Increase in antibodies and enzymes in patients with hepatitis A virus. 16

28. Incubation Period
The time interval between exposure to an infectious agent and the appearance of the first signs and symptoms of disease.
Provides a clue to the time and circumstance of exposure to the agent.
Useful for determining the etiologic agent. 17

29. Herd Immunity
Immunity of a population, group, or community against an infectious disease when a large proportion of individuals are immune either through vaccinations or prior infection. 18

30. Generation Time
Time interval between lodgment of an infectious agent in a host and the maximal communicability of the host.
Can precede the development of active symptoms.
Useful for describing the spread of infectious agents that have large proportions of subclinical cases.
Applies to both inapparent and apparent cases of disease. 19

31. Colonization and Infestation
Colonization--agents multiply on the surface of the body without invoking tissue or immune response.
Infestation--the presence of a living infectious agent on the body’s exterior surface, upon which a local reaction may be invoked. 20

32. Iceberg Concept of Infection
Active clinical disease accounts for only a small proportion of host’s infections and exposures to disease agents. (Refer to next slide.) 21

33. Iceberg Concept (cont’d)

34. Indirect Transmission
The spread of infection through an intermediary source:
Vehicles--e.g., contaminated water, infected blood, food.
Fomites--inanimate objects laden with disease-causing agents.
Vectors--living insects or animals involved with transmission of the disease agent. 22

35. Measures of Disease Outbreaks
Attack rate
Secondary attack rate
Case fatality rate 23

36. Attack Rate Similar to an incidence rate.
Used when the nature of the disease or condition is such that a population is observed for a short period of time.
Formula:
___Ill __ X 100 during a time period
Ill + Well 24

37. Secondary Attack Rate
An index of the spread of disease in a family, household, dwelling unit, dormitory or similar circumscribed group.
A measure of contagiousness.
Useful in evaluating control measures. 25

38. Secondary Attack Rate: Definition
The number of cases of infection that occur among contacts within the incubation period following exposure to a primary case in relation to the total number of exposed contacts.

39. Secondary Attack Rate (%) (Multiply fraction by 100.)
Number of new cases in group - initial case(s) Number of susceptible persons in the group - initial case(s)
Initial case(s) = Index case(s) + coprimaries
Index case(s) = Case that first comes to the attention of public health authorities.
Coprimaries = Cases related to index case so closely in time that they are considered to belong to the same generation of cases. 26

40. Case Fatality Rate (CFR)
The number of deaths caused by a disease among those who have the disease.
Examples of diseases with a high CFR are rabies and AIDS. 27

41. Formula for CFR
Number of deaths due to disease “X” x Number of cases of disease “X”
Sample calculation: Assume that an outbreak of plague occurs in an Asian country. Health authorities record 98 case of the disease, all of whom are untreated. Among these, 60 deaths are reported.
CFR = (60/98) x 100 = 61.2% 28

42. Investigation of Infectious Disease Outbreaks
Define the problem.
Appraise existing data.
Case identification
Clinical observations
Tabulation and spot maps
Identification of responsible agent 29

43. Investigation (cont’d)
Formulate a hypothesis.
Test the hypothesis.
Draw conclusions and formulate practical applications.

44. Epidemiologically Significant Categories of Infectious Diseases
Foodborne illness
Water- and foodborne diseases
Sexually transmitted diseases
Vaccine-preventable diseases
Diseases spread by person-to-person contact
Zoonotic diseases
Mycoses (fungal diseases)
Arthropod-borne diseases 30

45. Foodborne Illness
One of the most common infectious disease problems in the community.
Examples include:
Staphylococcus aureus--present in contaminated food. Produces a toxin.
Trichinosis--associated with inadequately cooked pork products. 31

46. Foodborne Agents
Source: Data are from Heymann DL, Control of Communicable Diseases
Manual, 18th ed. Washington, DC: American Public Health Association; 2004.

47. Water and Foodborne Diseases
Examples include:
Amebiasis--intestinal disease.
Cholera--acute enteric disease.
Giardiasis
Legionellosis
Schistosomiasis--infection caused by adult worms in the bloodstream. The cycle involves alternate human and snail hosts. 32

48. Schistosoma mansoni Life Cycle
Source: Reprinted from Centers for Disease Control and Prevention. Schistosomiasis in US Peace Corps Volunteers—Malawi, 1992.
MMWR, vol 42, p 568, July 30, 1993.

49. Sexually Transmitted Diseases: AIDS
Risk factors in U.S.
Male-to-male sex
Injection drug use
In 2005, the case rate was 19.8 per 100,000 for 33 U.S. states.
The human immunodeficiency virus (HIV) is an acute problem worldwide.
For example, approximately 24.7 million cases were living with HIV in Sub-Saharan Africa in 2006. 33

50. Vaccine-Preventable Diseases
Vaccines are routinely given to children for the prevention of several diseases, including:
Chickenpox, Diphtheria, Haemophilus influenzae type b infections, hepatitis A, hepatitis B, influenza, measles, meningococcal meningitis, mumps, pertussis, paralytic poliomyelitis, pneumococcal disease, rotaviral enteritis, rubella, and tetanus. 34

51. Diseases Spread by Person-to-Person Contact
One example is tuberculosis.
Resurgence of TB (from late 1980s until mid-1990s) due to:
Increase in persons infected with HIV.
Increase in homeless population.
Importation of cases from endemic areas. 35

52. U.S. TB Cases,
Source: Reprinted from Centers for Disease Control and Prevention. Tuberculosis morbidity—United States, 1992.
MMWR, vol 42, p 696, September 17, 1993.

53. U.S. TB Cases,
TB incidence on the decline for most racial/ethnic groups.
Extensively drug-resistant tuberculosis (XDR TB) was the focus of media attention in 2007.

54. Zoonotic Diseases
Zoonosis--a disease that under natural conditions can be spread from vertebrate animals to humans. Example: Q fever.
Zoonotic diseases may be either:
Enzootic--similar to endemic in human diseases.
Epizootic--similar to epidemic in human diseases. 36

55. Mycoses
Coccidioidomycosis (San Joaquin Valley fever )--caused by the fungus Coccidioides immitis.
Usually attacks the lungs.
Cases of infection usually have had contact with contaminated soil.
Other examples: blastomycosis, ringworm, athlete’s foot, candidiasis, cryptococcosis, and aspergillosis.

56. Arthropod-Borne Diseases
Include arboviral diseases.
Blood-feeding arthropod vectors transmit disease agents to vertebrate hosts.
Examples of vectors: sand flies, ticks, mosquitoes.
Examples of diseases: encephalitis, Lyme disease, chikungunya fever

57. Emerging Infections
Suddenly increase in incidence or geographic scope.
Many infections appear when an existing pathogen gains access to new host populations.
Examples: U.S. hantaviral pulmonary syndrome, Lyme disease, AIDS, hepatitis C, hemorrhagic fever.
Environmental changes may contribute to their emergence.

58. Emerging Infectious Diseases

59. Emerging Infectious Diseases (cont’d)