1. Chapter 8 Infectious Diseases and Outbreak Investigation

2. Learning Objectives Describe modes of transmission of communicable diseases Name three microbial agents associated with infectious diseases Describe the epidemiology of two infectious diseases State procedures for investigating infectious disease outbreaks

3. Introduction Infectious diseases are important causes of morbidity and mortality in the U.S. and worldwide. – Category of influenza and pneumonia was eighth leading cause of death in U.S. in 2005. – Infectious/parasitic diseases account for almost 20% of mortality worldwide. – Respiratory infections account for 7% of mortality worldwide.

4. Infectious (Communicable) Disease “An illness due to a specific infectious agent or its toxic products that arises through transmission of that agent or its products from an infected person, animal, or reservoir to a susceptible host, either directly or indirectly through an intermediate plant or animal host, vector, or the inanimate environment.”

5. Parasitic Disease An infection caused by a parasite, which “…is an organism that lives on or in a host organism and gets its food from or at the expense of its host.” Example: amebiasis

6. Epidemiologic Triangle One of the long-standing models used to describe the etiology of infectious diseases Includes three major factors: agent, host, and environment

7. Agent “A factor—such as a microorganism, chemical substance, or form of radiation—whose presence, excessive presence, or (in deficiency diseases) relative absence is essential for the occurrence of a disease….”

8. Host “A person or other living animal, including birds and arthropods, that affords subsistence or lodgment to an infectious agent under natural conditions.”

9. Environment The domain in which disease-causing agents may exist, survive, or originate

10. The Epidemiologic Triangle Source: Reprinted from Friis RH, Sellers TA. Epidemiology for Public Health Practice. 4th ed. Sudbury, Massachusetts: Jones and Bartlett Publishers; 2009:439.

11. Infectious Disease Agents With respect to infectious and communicable diseases, agents include specific microbes and vectors involved in the cycle of disease transmission. – Examples of infectious agents: bacteria, rickettsia, viruses, fungi, parasites, and prions

12. Infectivity The capacity of an agent to enter and multiply in a susceptible host and thus produce infection or disease

13. Virulence Refers to the severity of the disease produced, i.e., whether the disease has severe clinical manifestations or is fatal in a large number of cases.

14. Toxin Some infectious disease agents, instead of acting directly, produce a toxin that causes illness. A toxin usually refers to a toxic substance made by living organisms. – Examples of illness: foodborne intoxications such as botulism.

15. Four infectious disease agents Upper left, bacillus anthracis bacteria Upper right, herpes simplex virions Lower left, dermatophyti c fungus (causes ringworm infections of the skin and fungal infections of the nail bed Lower right, Cryptosporidium parvum oocysts Source: Reprinted from Centers for Disease Control and Prevention. Public Health Image Library, ID# 10123 (upper left); ID# 10230 (upper right); ID# 4207 (lower left); ID# 7829 (lower right). Available at: http://phil.cdc.gov/phil/details.asp. Accessed July 3, 2008.

16. Host Characteristics Immunity Other factors related to host responses: – Incubation period – Subclinical illness – Generation time – Carrier status

17. Immunity Refers to the host’s ability to resist infection by the agent. May be either active or passive

18. Active Immunity Immunity that the host has developed as a result of natural infection with a microbial agent Also can be acquired from an injection of a vaccine that contains an antigen – Antigen: A substance that stimulates antibody formation, e.g., a microbial agent Usually of long duration (measured in years)

19. Passive Immunity Immunity acquired from antibodies produced by another person or animal Example 1: newborn infant’s natural immunity conferred transplacentally from its mother Example 2: immunity conferred by injections of antibodies contained in immune serums Usually of short duration (measured in days to months)

20. Herd Immunity The resistance of an entire community to an infectious agent as a result of the immunity of a large proportion of individuals in that community to the agent

21. Incubation Period Time interval between invasion by an infectious agent and the appearance of the first sign or symptom of the disease

22. Subclinical Infection An infection that does not show obvious clinical signs or symptoms Also called an inapparent infection Example: hepatitis A infections among children

23. Generation Time The time interval between lodgment of an infectious agent in a host and the maximal communicability of the host

24. Carrier “A person or animal that harbors a specific infectious agent without discernible clinical disease, and which serves as a potential source of infection.”

25. Index Case Used in an epidemiologic investigation of a disease outbreak to denote the first case of a disease to come to the attention of authorities

26. Environment and Infectious Diseases The external environment comprises these components: – Physical – Climatologic – Biologic – Social – Economic

27. Endemic Refers to a infectious disease agent that is habitually present in an environment (either geographic or population group) Example: Plague is endemic among certain species of rodents in the western U.S.

28. Reservoir A place where infectious agents normally live and multiply Can be human beings, animals, insects, soils, or plants

29. Zoonosis “An infection or infectious agent transmissible under natural conditions from vertebrate animals to humans.”

30. Means of Transmission of Infectious Disease Agents Direct transmission – Person to person Indirect transmission – Vehicle-borne infections – Airborne infections – Vector-borne infections

31. Portal of Exit For an infected person, a portal of exit is the site from which the agent leaves that person’s body. Examples: – Respiratory passages – Alimentary canal – Genitourinary system – Skin lesions

32. Person to Person (Direct Transmission) “Direct and essentially immediate transfer of infectious agents to a receptive portal of entry through which human or animal infection may take place….”

33. Portal of Entry Site where the agent enters the body Example: a skin wound

34. Indirect Transmission Involves intermediary sources of infection such as vehicles, droplet nuclei (particles), and vectors.

35. Vehicle-borne Infections These infections result from contact with vehicles, which are contaminated, nonmoving objects. Examples: – Fomites – Unsanitary food – Impure water

36. Unsafe injection practices and circumstances that likely resulted in transmission of hepatitis C virus (HCV) at clinic A—Nevada, 2007 Source: Reprinted from Centers for Disease Control and Prevention. Acute hepatitis C virus infections attributed to unsafe injection practices at an endoscopy clinic—Nevada, 2007. MMWR. 2008;57:516.

37. Fomite An inanimate object that carries infectious disease agents Examples: – Classroom doorknob – Discarded tissues

38. Airborne Infections Involve the spread of droplet nuclei (particles) that are present in the air. Example: infections caused by stirring up dust that carries fungi or microbes

39. Vector-borne Infections A vector is an animate, living insect or animal that is involved with the transmission of disease agents. Transmission of an infectious disease agent may happen when the vector feeds on a susceptible host.

40. Four vectors of infectious diseases Upper left, a female louse Upper right, a female Aedes aegypti mosquito acquiring a blood meal Lower left, a tick Lower right, a Norway rat. Source: Reprinted from Centers for Disease Control and Prevention. Public Health Image Library, ID# 9250 (upper left); ID# 9255 (upper right); ID# 9959 (lower left); ID# 5445 (lower right). Available at: http://phil.cdc.gov/phil/details.asp. Accessed July 3, 2008.

41. Examples of Significant Infectious Diseases Sexually transmitted diseases Foodborne diseases Waterborne diseases Vector-borne diseases Vaccine-preventable diseases Zoonotic diseases Emerging infections Bioterrorism-related diseases

42. Sexually Transmitted Diseases HIV/AIDS Gonococcal infections Chlamydial genital infections

43. Foodborne Illness Biologic agents of foodborne illness include bacteria, parasites, viruses, and prions (linked to mad cow disease). Examples of bacterial agents include: – Campylobacter – Clostridium botulinum – Salmonella

44. Source: Reprinted from Centers for Disease Control and Prevention. Summary of Notifiable Diseases—United States, 2006. MMWR. 2008;55:42.

45. Vector-borne Diseases Examples of diseases (name of vector in parentheses) from Table 8-6: – Bacterial diseases Lyme disease (tick) – Arthropod-borne (arboviral) diseases Eastern equine encephalitis (mosquito) – Parasitic diseases Malaria (mosquito)

46. Vaccine-Preventable Diseases (VPDs) Conditions that can be prevented by vaccination (immunization) Examples include: – Diptheria – Tetanus – Whooping cough – Hepatitis A and B

47. Zoonotic Diseases Diseases transmitted from vertebrate animals to human beings Examples: – Rabies – Anthrax – Avian influenza – Hantavirus pulmonary syndrome – Toxoplasmosis – Tularemia (rabbit fever)

48. Emerging Infectious Diseases (Emerging Infections) “An infectious disease that has newly appeared in a population or that has been known for some time but is rapidly increasing in incidence or geographic range.” Examples: hepatitis C, E. coli O157:H7.

49. Bioterrorism-Related Diseases Bioterrorism attack: “…the deliberate release of viruses, bacteria, or other germs (agents) used to cause illness or death in people, animals, or plants….” Example: anthrax

50. Methods of Outbreak Investigation Clinical observations Epidemic curve Incubation period Attack rate Case mapping (used by John Snow) Hypothesis formulation and confirmation Draw conclusions

51. Clinical Observations The pattern of symptoms suggests possible infectious agents. Possible symptoms of an outbreak include: – Fever – Nausea – Diarrhea – Vomiting – Headache – Rashes – Stomach pain

52. Epidemic Curve “A graphic plotting of the distribution of cases by time of onset.” May reflect a common-source epidemic or a point-source epidemic.

53. Common-Source Epidemic An “Outbreak due to exposure of a group of persons to a noxious influence that is common to the individuals in the group.”

54. Point-Source Epidemic A type of common-source epidemic that occurs “When the exposure is brief and essentially simultaneous, [and] the resultant cases all develop within one incubation period of the disease…”

55. Incubation Period In conjunction with information about symptoms, provides clues to possible infectious disease agents Example: The incubation period for foodborne illness caused by Salmonella ranges from 6 to 72 hours.

56. Attack Rate Used frequently to describe the occurrence of foodborne illness, infectious diseases, and other acute epidemics Formula: Attack rate = Ill/(Ill + Well) X 100 during a time period

57. Final Steps in Outbreak Investigation Case mapping: Some investigations may use computer software to show the location of cases. Hypotheses: Using the information that has been gathered, the epidemiologist may formulate a hypothesis regarding the causative agent. Draw a conclusion: Plan for the prevention of future outbreaks.

58. Conclusion Infectious diseases remain a threat for the following reasons: – New types of diseases are constantly evolving and imperiling public health. – Outbreaks caused by bioterrorism are a potential threat. – Some infectious disease agents mutate into forms that resist conventional antibiotic treatment.