Epidemiologic Surveillance An Outbreak Investigation

1
Epidemiologic SurveillanceAn Outbreak
Investigation

• Jennifer Beebe-Dimmer MPH, PhD
• Assistant Research Scientist
• UM Department of Epidemiology

2
Concepts in Infectious Disease Epidemiology
3
Infectious 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 inanimate reservoir
  to a susceptible host either directly or
  indirectly through an intermediate plant or
  animal host, vector or the inanimate environment.
• (Communicable Disease)
• Source Benensen AS, editor. Control of
  Communicable Diseases Manual. Sixteenth Edition,
  1995.

4
Dynamics of Disease Transmission
Epidemiologic Triad

• Human disease results from interaction between
  the host, agent and the environment. A vector
  may be involved in transmission.
• Host susceptibility to the agent is determined by
  a variety of factors, including
• - Genetic background
• - Nutritional status
• - Vaccination
• - Prior exposure

5
Factors associated with increased risk of human
disease

• Host Characteristics Agent
  Environmental Factors
• Age Biologic (Bacteria, viruses)
  Temperature
• Sex Chemical (Poison, smoke) Humidity
• Race Physical (Trauma, radiation) Altitude
• Occupation Nutritional (Lack, excess) Crowding
• Marital Status Housing
• Genetics Neighborhood
• Previous Diseases Water
• Immune Status Food
• Air Pollution
• (In Gordis Epidemiology)

6
Modes of Disease Transmission

• The potential for a given agent to cause an
  outbreak depends on the characteristics of the
  agent, including the mode of transmission of the
  agent
• Two basic modes of transmission
• Direct
• Indirect
• Certain diseases can be transmitted directly or
  indirectly

7
Direct Mode of Disease Transmission

• In an infectious setting, immediate and direct
  transfer of an agent to a host by an infected
  person or animal
• Touching, biting, or sexual intercourse are
  classic examples
• Measles virus airborne by droplet spread or
  direct contact with nasal/throat secretions of
  infected persons
• In a noninfectious setting, the host may have
  direct contact with the agent in the environment
• Children ingesting lead paint from playground
  equipment

8
Indirect Mode of Disease Transmission

• Vehicle-borne
• Transmission through contaminated inanimate
  objects (toys, food, water, surgical utensils, or
  biological products such as blood, tissues or
  organs)
• Vector-borne
• Transmission through simple contamination by
  animal or arthropod vectors or their actual
  penetration of the skin or mucous membranes
• Airborne
• Transmission occurs when microbial, particulate,
  or chemical agents are aerosolized and remain
  suspended in air for long periods of time

9
Incubation Period

• Interval from receipt of infection to the time of
  onset of clinical illness (signs symptoms)
• Different diseases have different incubation
  periods
• No precise incubation period
• A range is characteristic for a disease
• What accounts for this delay?
• Time needed for the pathogen to replicate to the
  critical mass necessary for clinical disease
• Site in the body at which the pathogen replicates
• Dose of the infectious agent received at time of
  infection

10
Outcomes of Exposure to an Agent
The spectrum of severity varies by disease 1.
Exposure, No infection 2. Carrier - Individual
harbors the pathogen but does not show evidence
of clinical illness a potential source of
infection (can transmit the agent) 3.
Subclinical Infection - Disease that is not
clinically apparent leads to immunity,
carrier, or non-immunity 4. Clinical
Infection - Apparent disease characterized by
signs and symptoms results in immunity,
carrier, non-immunity, or severe consequences
such as death
11
Endemic, Epidemic and Pandemic
Endemic - The habitual presence (or usual
occurrence) of a disease within a given
geographic area Epidemic - The occurrence of an
infectious disease clearly in excess of normal
expectancy, and generated from a common or
propagated source Pandemic - A worldwide epidemic
affecting an exceptionally high proportion of
the global population
Number of Cases of Disease
Endemic
Epidemic
Time
12
Disease Outbreaks

• Typically, sudden and rapid increase in the
  number of cases of a disease in a population
• Common Source
• Cases are limited to those who share a common
  exposure
• Foodborne, water
• Propagated
• Disease often passed from one individual to
  another
• Measles, STDs

13
Essential Steps in an Outbreak Investigation
14
Steps of an Outbreak Investigation

• 1) Establish the existence of an outbreak
• 2) Verify the diagnosis
• 3) Define and identify cases
• 4) Describe and orient the data in terms of
  person, place and time
• 5) Develop hypotheses
• 6) Evaluate hypotheses
• 7) Refine hypotheses and carry out additional
  studies
• 8) Implement control and prevention measures
• 9) Communicate findings

15
Step 1 Establish the existence of an outbreak

• Before you decide whether an outbreak exists, you
  must first determine the expected or usual number
  of cases for the given area and time

16
Step 1 Establish the existence of an outbreak

• Data Sources
• 1) Health department surveillance records for a
  notifiable disease
• 2) Sources such as hospital discharge records,
  mortality records and cancer or birth defect
  registries for other diseases and conditions
• 3) If local data is not available, make
  estimates using data from neighboring states or
  national data

17
Step 1 Establish the existence of an outbreak

• Whether or not an outbreak is investigated or
  control measures are implemented is not strictly
  tied to verifying that an epidemic exists
• Other factors may come into play, including
• Severity of the illness
• Potential for spread
• Political considerations
• Public concern and pressure from community
• Availability of resources

18
How do we know when we have an excess over what
is expected?
Public Health Surveillance The ongoing and
systematic collection, analysis, and
interpretation of outcome-specific data for use
in the planning, implementation, and evaluation
of public health practice. (Thacker, Berkleman.
Epidemiologic Reviews 198810164-90)
19
Notifiable Disease

• Disease for which regular, frequent, and timely
  information regarding individual cases is
  considered necessary for the prevention and
  control of disease

20
Summary of Botulism Cases by Year, U.S.,
1979-1999
21
Step 2 Verify the diagnosis

• Two goals in verifying a diagnosis
• 1. Ensure that the problem has been properly
  diagnosed -- the outbreak really is what it has
  been reported to be
• Review clinical findings and laboratory results
  for affected people
• Visit or talk to several of the people who became
  ill
• 2. For outbreaks involving infectious or toxic
  chemical agents, be certain that the increase in
  diagnosed cases is not the result of a mistake in
  the laboratory.

22
Step 3 Define and identify cases

• Establish a case definition - a standard set of
  criteria for deciding whether a person should be
  classified as having the disease under study
• In many outbreaks, a working definition of the
  disease syndrome must be drawn up that will
  permit the identification and reporting of cases
• As the investigation proceeds and the source,
  mode of transmission and/or etiologic agent
  becomes better known, you can modify the working
  definition

23
Step 3 Define and identify cases

• A case definition includes four components
• Clinical information about the disease,
• Characteristics about the people who are affected
  (person),
• Information about the location (place), and
• A specification of time during which the outbreak
  occurred (time).

24
Step 3 Define and identify cases

• To increase sensitivity specificity of
  reporting, we use three classifications of cases
  that reflect the degree of certainty regarding
  diagnosis
• 1) Confirmed
• 2) Probable
• 3) Possible
• The case definition is used to actively search
  for more cases beyond the early cases and the
  ones that presented themselves.

25
Step 3 Define and identify cases

• The following information should be collected
  from every affected person in an outbreak
• 1) Identifying information - name, address,
  phone
• 2) Demographic information - e.g., age, sex,
  race, occupation
• 3) Risk factor information
• 4) Clinical information
• Verify the case definition has been met for every
  case
• Date of onset of clinical symptoms to create an
  epidemic curve

26
Step 3 Define and identify cases

• The first cases to be recognized are usually only
  a small proportion of the total number
• To identify other cases, use as many sources
  possible
• Passive Surveillance - Relies on routine
  notifications by healthcare personnel (recall
  Notifiable Diseases)
• Active Surveillance - Involves regular outreach
  to potential reporters to stimulate reporting of
  specific conditions investigators are sent to
  the afflicted area to collect more information
• Contact physician offices, hospitals, schools to
  find persons with similar symptoms or illnesses
• Send out a letter, telephone or visit the
  facilities to collect information

27
Step 4 Describe and orient the data in terms of
time, place and person

• Characterizing an outbreak by time, place and
  person is called descriptive epidemiology.
• Descriptive epidemiology is important because
• You can learn what information is reliable and
  informative (e.g., similar exposures)
• And what may not be as reliable (e.g., many
  missing responses to a particular question)
• Provides a comprehensive description of an
  outbreak by showing its trend over time, its
  geographic extent (place) and the populations
  (people) affected by the disease

28
Step 4 Describe and orient the data in terms of
time

• The time course of an epidemic is shown by the
  distribution of the times of onset of the
  disease, called the Epidemic Curve.
• Graph of the number of cases of the health event
  by their date of onset
• Provides a simple visual display of the
  magnitude and time trend of the outbreak
• May stratify epidemic curves by place
  (residence, work, school, etc.) or by personal
  traits (age, gender, race, etc.) to assess
  whether time of onset varies in relation to place
  or person characteristics

29
Step 4 Describe and orient the data in terms of
place

• Assessment of the outbreak by place provides
• Information on the geographic extent of the
  problem
• A spot map indicating place of occurrence of
  cases may show clusters or patterns that provide
  clues to the nature and source of the outbreak
• Patterns reflecting water supply, wind currents,
  or proximity to a restaurant, swimming pool,
  school room or workplace
• If the size of overall population varies between
  comparison areas, a spot map of the area may be
  misleading because it only shows number of cases

30
Step 4 Describe and orient the data in terms of
person

• Examine risks in subgroups of the affected
  population according to personal characteristics,
  as well as interaction between characteristics
• - Age, race, sex, occupation, social group,
  medical status
• Characterizing an outbreak by person helps to
  determine which subgroups of the population are
  at risk

31
Examples of Epidemic Curves
32
Step 5 Develop hypotheses

• Though we generate hypotheses from the beginning
  of the outbreak, at this point, the hypotheses
  are sharpened and more accurately focused.
• Use existing knowledge (if any) on the disease,
  or find analogies to diseases of known etiology
• Hypotheses should address
• Source of the agent
• Mode of transmission
• Exposures associated with disease
• and should be proposed in a way that can be tested

33
Step 6 Evaluate hypotheses

• Generally, after a hypothesis is formulated, one
  should be able to show that
• 1) all additional cases, lab data, and
  epidemiologic evidence are consistent with the
  initial hypothesis and
• 2) no other hypothesis fits the data as well
• Observations that add weight to validity
• The greater the degree of exposure (or higher
  dosage of the pathogen), the higher the incidence
  of disease
• Higher incidence of disease in the presence of
  one risk factor relative to another factor

34
Attack Rates

• An attack rate is the proportion of a
  well-defined population that develops illness
  over a limited period of time, such as during an
  epidemic or outbreak
• Useful for comparing the risk of disease in
  groups with different exposures
• Similar to a cumulative incidence
• Often expressed as a percent
• Formula
• Attack Rate Number of new cases occurring in a
  given time period
  Population at risk at the start of the time
  period
• Number of people at risk who
  develop a certain illness
• Total number of people at risk
• (Source Gordis L., Epidemiology)

35
Calculating an Attack Rate in a Foodborne
Outbreak

• In a foodborne outbreak occurring among people
  attending a social function or common
  geographical site
• Calculate an attack rate for people who ate a
  particular item (exposed) and an attack rate for
  those who did not eat the item (unexposed)
• The attack rate is calculated by dividing the
  number who became ill and consumed the item by
  the total number of people who consumed that item

36
Identifying the Source of an Outbreak

• Look for an item with
• A high attack rate among those exposed AND
• A low attack rate among those not exposed (so the
  ratio of attack rates for the two groups is high)
• Ideally, most of the people who became ill should
  have been exposed to the proposed agent so that
  the exposure could explain most, if not all, of
  the cases.

37
Step 7 Refine hypotheses and carry out
additional studies

• Additional epidemiologic studies
• What questions remain unanswered about the
  disease?
• What kind of study used in a particular setting
  would answer these questions?
• When analytic studies do not confirm the
  hypotheses
• reconsider the original hypotheses
• look for new vehicles or modes of transmission

38
Step 7 Refine hypotheses and carry out
additional studies

• Laboratory and environmental studies
• Epidemiologic studies can
• Implicate the source of infection, and
• Guide appropriate public health action
• But sometimes laboratory evidence can clinch
  the findings
• Environmental studies often help explain why an
  outbreak occurred and may be very important in
  certain settings

39
Case-Control Methods Applied to a Foodborne
Outbreak

• The usual approach is to apply the case-control
  methodology to determine what exposures ill
  people had that well people did not have
• List all of the relevant items on the menu
• Determine the proportions of ill and of non-ill
  persons who ate each of the items by
  questionnaire
• Identify the food item with the largest
  difference in attack rates between cases (ill)
  and controls (non-ill)
• Usually one food item stands out as showing the
  greatest difference in proportion between cases
  and controls.

40
Step 8 Implementing control and prevention
measures

• The practical objectives of an epidemic
  investigation are to
• Stop the current epidemic, and
• Establish measures that would prevent similar
  outbreaks in the future.
• Preliminary control measures should be done as
  soon as possible!

41
Elements of Epidemic Control
The elements of epidemic control include 1.
Controlling the source of the pathogen (if
known) Remove or inactivate the pathogen 2.
Interrupting the transmission Sterilize
environmental source of spread vector
control 3. Controlling or modifying the host
response to exposure Immunize the
susceptibles use prophylactic
chemotherapy
42
Step 9 Communicate the findings

• At the end of the investigation, communicate
  findings to others who need to know
• Prepare a final report
• Provide information on the nature, spread, and
  control measures employed
• The report can take several forms
• 1) An oral briefing for local health authorities
• 2) A written report to a journal or the
  Mortality and Morbidity Weekly Report (MMWR)
• 3) Formal presentation of recommendations (a
  blueprint for action)

43
Foodborne Outbreaks
44
Recognizing Foodborne Illnesses

• Patients with foodborne illnesses typically
  present with GI tract symptoms (vomiting,
  diarrhea, abdominal pain)
• But nonspecific symptoms and neurologic symptoms
  may also occur
• Important clues in foodborne etiology
• Incubation period (time between infection and
  clinical illness)
• Duration of the resultant illness
• Predominant clinical symptoms
• Population involved in the outbreak
• Additional clues
• Recent consumption of raw/poorly cooked foods,
  unpasteurized milk or juices, home canned goods
• Foreign travel, travel to coastal areas
• Camping/hiking to areas where untreated water is
  consumed
• Visit to a farm
• Occupation

45
Reporting Foodborne Illness

• Foodborne disease reporting began over 50 years
  ago
• High morbidity and mortality caused by typhoid
  fever and other childhood diarrheal diseases was
  a concern
• Intent of investigating and reporting these cases
  was to obtain information about the role of food,
  milk, and water in outbreaks of GI illness.
• Early reporting efforts led to the enactment of
  important public health measures (e.g.,
  Pasteurized Milk Ordinance)
• Led to a profound decrease in incidence of
  foodborne illnesses

46
A Foodborne OutbreakOswego County, New York
(1940)

• Over a period of a few hours, 46 people became
  ill with gastrointestinal symptoms
• All had attended a church supper the previous
  evening
• A physician in charge of investigating the
  outbreak obtained a detailed history from all 75
  people who attended the supper

From Centers for Disease Control (1976)
47
(No Transcript)
48
Food Specific Attack Rates among Persons
Attending the Supper

• Group A
  Group B
• Persons who ate
  Persons who did not eat
• specified food
  specified food
• Food or
• Beverage Ill Not Ill Total
  AR () Ill Not Ill Total
  AR ()
• Baked Ham 29 17 46
  63.0 17 12 29
  58.6
• Spinach 26 17
  43 60.5 20 12
  32 62.5
• Mashed Potato 23 14 37
  62.2 23 14 37
  62.2
• Cabbage 18 10 28
  64.3 28 19 47
  59.6
• Jello 16 7
  23 69.6 30 22
  52 57.7
• Rolls 21 16
  37 56.8 25 13
  38 65.8
• Ice Cream (Van.) 43 11 54
  79.6 3 18 21
  14.3
• Ice Cream (Choc.) 25 22 47
  53.2 20 7 27
  74.1
• Fruit Salad 4 2
  6 66.7 42 27
  69 60.9

49
Matched-Pairs Analysis

• 
  Ate Chocolate Did not eat
• 
  Ice Cream Chocolate Ice
  Cream Total
• Ate Vanilla Ice Cream
• Ill / Total
  22/28 20/25
  43/54
• Percent Ill
  78.6 80.0
  79.6
• Did not eat Vanilla Ice Cream
• Ill / Total
  3/19 0/2
  3/21
• Percent Ill
  15.8 0.0
  14.3
• Total
• Ill / Total
  25/47 20/27
  46/75
• Percent Ill
  53.2 74.1
  61.3