EPI 5240: Introduction to Epidemiology Overview of Communicable Disease Epidemiology October 26, 2009

1. EPI 5240:Introduction to EpidemiologyOverview of Communicable Disease EpidemiologyOctober 26, 2009 Dr. N. Birkett, Department of Epidemiology & Community Medicine, University of Ottawa

2. Session Overview • Basic concepts of infectious disease epidemiology • Spectrum of disease • Transmission methods • Epidemics and outbreaks • Key concepts • Methods of investigation/control • Approaches to disease surveillance • Sample outbreak investigation exercise

3. Agents of Infectious Diseases • Bacteria • Viruses • Rickettsia • Fungi (mycoses) • Protozoa • Helminths • Prions

4. Infections: Sources and agents (1) Foodborne • Botulism • Clostridium Perfringens • Salmonellosis • Shigellosis • Staphylococcal disease • Traveler’s disease • Trichinosis Person-to-person spread • Aseptic Meningitis • Viral hepatitis • Respiratory Infections (influenza) • Herpes Simplex • Streptococcal disease (rheumatic fever) • Tuberculosis • Leprosy Water & Foodborne • Amebiasis • Cholera • Giardiasis • Legionellosis • E Coli

5. Infections: Sources and agents (1) Vaccine preventable • Chickenpox • Diphtheria • Measles • Mumps • Poliomyelitis • Tetanus Arthropod Borne • Encephalitis (West Nile) • Lyme Disease • Malaria • Plague • Rocky Mountain Spotted Fever Sexually Transmitted • HIV/AIDS • Gonorrhea • Syphilis • Chlamydia Trachomatis Zoonotic • Psittacosis • Q fever • Rabies • Hantavirus Prions • Kuru • vCJD Opportunistic fungal/fungal • Coccidioidomycosis • Candidiasis

7. Variation in incubation period

10. Where would Lassa Fever go? Ebola virus?

12. Some Definitions INFECTION • An infectious agent has entered and established itself in (or on) a host. COLONIZATION • Organism is present on the surface of the body and grows at a rate sufficient to maintain its numbers • There is no host reaction to the organism. CONTAMINATION • The presence of an organism on the surface of a body or inanimate object of an infectious agent which can serve as a source of infection. The organism need not be actively growing.

13. Terminology (1) • Infectivity • The ability of an agent to invade and multiply in a host (an infection). • Dose of organism required to establish infection in 50% of animals. • Pathogenicity • The ability of an agent to produce clinically apparent illness.

14. Terminology (2) • Virulence • The proportion of clinical cases which produce severe disease and/or permanent sequelae. • Immunogenicity • The ability of an agent to produce specific immunity against the agent • Can be produced in general body or within specific sites such as the GI tract. • Determines the ability of an agent to re-infect a host • Measles vs. gonorrhea • Seroepidemiology • The use of the immune status of people to study disease severity, distribution, etc. • Lassa fever

15. Terminology (3) • Reservoir • Living organisms or inanimate matter in which infectious agent normally lives and multiplies • Fomites (Vehicle) • Inanimate objects contaminated with infectious agent (not the reservoir). Example would be toys in a daycare centre. • Vector • An animate source of an infectious agent. The vector may be infected with the organism (e.g. mosquitoes and malaria) or just be a mechanical carrier (e.g. flies). There is disagreement about whether vectors are restricted to insects or can also include small mammals.

16. Terminology (4) • Zoonoses • Diseases transmitted to humans from animals (e.g. anthrax) • Carriers • An infected person without apparent clinical disease who remains infectious (e.g. Typhoid Mary) • Index Case • The person (case) who brings the infection to the attention of the medical community or the public agency. Sometimes used to refer to the person who brings the infection into a community. This will often (but not always) be the same person.

17. Terminology (5) • Attack Rate • The probability that people will get ill from the disease. Usually applied in an outbreak situation. It is a cumulative incidence (incidence proportion) type measure. # new cases in group = ------------------------------------ # people in group

18. Terminology (6) • Secondary Attack Rate • Probability of infection in a closed group who are at risk but excluding the index case(s). Formula is: # new cases in group – index case(s) = ----------------------------------------------------- # people in group – index case(s) • Risk of H. Influenza infection in family members within 30 days of disease in an index case (compared to general public) ≈ 600

19. Terminology (7) • Case Fatality Rate (CFR) • The probability of death in people with an infection.

20. Pathogenic Mechanisms (1) • Direct Tissue Invasion • Production of Toxins • Allergic Host Reaction • Resistant/latent infection (carriers) • Enhancement of host susceptibility to drugs (e.g. Reye’s syndrome and ASA). • Immune Suppression

21. Reservoirs (examples of transmission patterns) • Human Human Human • Animal Animal Animal Human Animal Vector Vector Human

23. Mechanisms of Spread (1) • kissing • sexual intercourse • hand shaking • droplets • spores in soil Indirect transmission Direct transmission • Vehicle borne • fomites (e.g. toys) • food • IV fluid • Vector borne • mechanical (e.g. soiled feet of insect) • biological (e.g. malaria) • Airborne • dust • droplet nuclei

24. Factors Influencing Spread of Disease in a Population (1) • Period of infectivity in relation to symptoms • Mumps • Carrier state • Herd immunity • Attack Rate • Secondary attack rate • Type of spread • Person to person • common vehicle • vector-borne • zoonoses • Transmission mechanisms • sexually transmitted vs. droplet spread

25. Develop disease Immune

26. Epidemics (1) • Epidemic (now often called ‘outbreak’) • the occurrence in a community or region of cases of a disease/condition/behaviour clearly in excess of normal expectancy • Endemic • the occurrence of a disease/condition at a relatively constant level in a given setting (often required to be at a high rate) • Pandemic • an epidemic covering a very wide area and affecting a large proportion of the population • Pathogen • Infectious and non-infectious substance capable of producing tissue damage or initiating a process which can lead to a disease.

27. Epidemics (2) • Common conditions increasing likelihood of an epidemic • The introduction of a new pathogen or an increased amount of, or a change in the virulence (infectivity) of, a pathogen. • An adequate number of exposed and susceptible persons. • An effective means of transmission between the source of the pathogen and the susceptible person.

29. Epidemics (4) • Types of epidemics • Common source • Point source • Ongoing exposure • Need not be geographically localized • Propagated/progressive • Mixed • Note that epidemics can arise from behaviour as well as from traditional infectious sources. • Intravenous drug users and HIV/HPC • Mass hysteria • Epidemic curve • Spot maps

30. Epidemic Curves: point source (1)

31. Epidemic Curves: propagated (2) 10 days

34. Epidemics (3) • Incubation period and causal agent

38. Epidemic of hepatitis in institution

39. Epidemic of hepatitis in institution

41. SARS

44. Epidemic Control (1) Twin goals: • Understand the cause, etc. • Minimize the impact to the affected community Goals can conflict: • need to collect full information base • need to take action in absence of full information Effective and clear communication with general public is essential • SARS outbreak • designate one spokesperson • regular press briefings

45. Epidemic Control (2) Step 1: Identify potential investigation team members and resources/prepare for fieldwork. • Identify a lead investigator Step 2: Establish the existence of an outbreak Step 3: Verify the diagnosis • contact patients, verify history, symptoms, lab tests, etc.

46. Epidemic Control (3) Step 4: Construct a working case definition • Uses a few simple clinical criteria • May be restricted by person, place and/or time • do not include an exposure or risk factor you want to test. • Is ‘loose’ or ‘sensitive’ if used for case-finding • Is ‘specific’ or ‘tight’ if used for hypothesis testing. • Your case definition may (should) change as you acquire more information. Step 5: Find cases systematically and develop a case listing.

47. Epidemic Control (4) Step 6: Perform descriptive epidemiology • time: epidemic curve • place: spot map • person: look for commonalities. Tends to follow case series methods at this point but may involve comparison to ‘known’ community information. • Can use cohort or case-control methods is appropriate Step 7: Develop hypotheses. Step 8: Evaluate hypotheses.

48. Epidemic Control (5) Step 9: As necessary, reconsider and refine hypotheses and conduct additional studies Step 10: Implement control and prevention measures. • your PRIMARY goal. Implement as soon as feasible even if information base is incomplete. Step 11: Communicate your findings. Step 12: Maintain surveillance to monitor trends and evaluate control/prevention measures.

49. Actions to take in first 24 hours • Determine extent of outbreak • Establish etiological agent (if possible). • Identify all persons at risk • Identify key clinical & epidemiological features • age, sex, race • candidate exposures of interest • data of onset, place of onset, etc. • Obtain relevant environmental samples • when in doubt, take a sample! Think CSI. • Designate a public spokesperson and set-up media communications system • Organize investigative team • Call for help (if needed)

50. Actions for ‘acute’ outbreak control Isolation • separation of infected persons or animals from others during the period of communicability • usually isolate for at least two incubation periods. Quarantine • restrictions on the activities of well people who (may) have been exposed to a communicable disease during its period of communicability. • active surveillance is an alternative • usually quarantine for at least two incubation periods. • More controversial than isolation since it affects people who are not currently ill (and may never get ill). Immunization • passive or active. Passive (IGG) is more useful for acute outbreaks. Chemoprophylaxis