Unit 1: Food InspectionTopic: Investigation of FBD outbreak

Lecture 5

ObjectivesAt the end of the session, students should be able to:

• Accurately define terms related to FBD investigation• Explain correctly the epidemiological triad, using a diagram• Differentiate between direct and indirect disease transmission using

examples• Explain clearly the value of understanding incubation periods in

respect of investigating diseases• Differentiate correctly among point source, propagated source and

continuous exposure using epi-curves• Outline clearly the rationale for investigating outbreaks• List the steps involved in the investigation of FBDs, based on CDC

protocol• Explain clearly the steps involved in the investigation of FBD

outbreaks

Outline• Concepts in Infectious Disease Epidemiology– Infectious disease– Epidemiological Triad– Disease risk factors– Disease transmission

• Direct & Indirect• Incubation period• Endemic, epidemic, pandemic• Point source vs propagated spread

– Outbreak Investigation Steps– FB outbreak

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: Heymann David L. Control of Communicable Diseases Manual. Ninetenth Edition, 2008.

Dynamics of Disease Transmission

• 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- Immune system

AGENT

HOST

VECTOR

ENVIRONMENT

EpidemiologicTriad

Agent Environment

Host

vector

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

• Diseases can be transmitted directly or indirectly

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

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 by animal

• Mechanical transmission• Biological transmission

Airborne– Transmission occurs when microbial, particulate,

or chemical agents are aerosolized and remain suspended in air for long periods of time

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

Outcomes of Exposure to an Agent

The spectrum of severity varies by disease:1. Exposure, No infection2. 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-immunity4. Clinical Infection - Apparent disease

characterized by signs and symptoms; results in immunity, carrier, non-immunity, or severe consequences such as death

Endemic & Epidemic Endemic • The habitual presence (or usual occurrence) of a disease within a given geographic areaEpidemic • The occurrence of an infectious disease clearly in excess of normal expectancy, and generated from a common or propagated source

“Endemic”

“Epidemic”

Time

Number of Casesof Disease

Pandemic

• A worldwide epidemic affecting an exceptionally high proportion of the global population

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

Point Source

Point source vs Propagated

• Propagated– Disease often passed from one individual to

another• Measles, STDs

FBD investigationThe purpose of the investigation are to stop the outbreak and prevent further exposures by:

• Identifying illnesses associated with the incident and verifying that the causative agent is food borne

• Detecting all cases, the causative agent, the implicated food(s), and the place(s) where the food was contaminated or mishandled

• Determining the source and mode of contamination, processes or practices associated

• Educating stakeholders – prevention• Determining if the outbreak is part of a larger out

break

Foodborne Surveillance System

• Systematically collect data pertaining to FBIs• Investigation protocol• Analysis and interpretation of surveillance and

investigation data• Disseminate consolidated information to

appropriate agencies/partners

Essential Steps in an Outbreak Investigation

Steps of an Outbreak Investigation

1) Assembly Team 2) Establish the existence of an outbreak 3) Verify the diagnosis 4) Define and identify cases 5) Describe and orient the data in terms of person, place and time 6) Develop hypotheses 7) Evaluate hypotheses 8) Refine hypotheses and carry out additional studies 9) Implement control and prevention measures 10) Communicate findings

Establish Team

• Epidemiologists, PHIs, Microbiologist, PHNs, RNs, physicians, Communication Specialist, Medical Technologists etc.

• Free flow of information and coordination critical

• Train staff – interest, education, ability• Outbreak kit/supplies– Forms, equipment, reference library

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

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 1988;10:164-90)

Notifiable Disease

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

Step 1: Establish the existence of an outbreak

Data Sources: Health department surveillance records for a

notifiable disease Sources such as hospital discharge records,

mortality records Physician based reporting Laboratory based reporting Public complaints School illness/absentee records Absentee records from employers Sales on anti-diarrheal drugs

Step 1: Establish the existence of an outbreak

• Two or more person with same disease• Have similar clinical features/same pathogen• Time, place, person association

• Single case of botulism, paralytic shell fish poisoning or rare disease – vibrio vulnificus

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

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• Collect food samples

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.

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

Step 3: Define and Identify cases

• Discussion with health workers• Review case history form• Collect data from cases (non- cases)– Be mindful of cultural/language barrier– Be professional– Exhibit genuine concern for interviewee– Parental consent must be obtained before

interviewing children– Ask open ended questions first– Use menu to jag memory– Obtain specimen

• Line listing

Step 3: Define and identify cases

The following information should be collected from every affected person in an outbreak:

1) Identifying information - name, address, phone2) Demographic information - e.g., age, sex, race, occupation3) Risk factor information4) Clinical information

• Verify the case definition has been met for every case

• Date of onset of clinical symptoms to create an epidemic curve

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 illness 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

– Primarily used to classify exposed persons as cases or non-cases

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)

Can you formulate a case definition?

Component Question Asked Factual Item1. Clinical criteria

2. Time

3. Place

4. Person

What were the predominant symptoms?

When did infection occur?

Where did infection occur?

Who may have been affected?

Acute onset of gastroenteritis

Saturday evening

Wedding reception

Wedding attendee

Case Definition

• All students who consumed food at UTech’s Canteen - Slipe Pen Road on 01/10/12 and who exhibited sign/symptoms of vomiting, diarrhoea and abdominal cramps.

• As scope broadens, may modify/alter definitions

Step 3: Define and identify casesTo increase sensitivity & specificity of reporting, we use three classifications of cases that reflect the degree of certainty regarding diagnosis:

1) Confirmed 2) Probable3) Possible

The case definition is used to actively search for more cases beyond the early cases and the ones that presented themselves.

Confirmed Presumptive Suspected

LaboratoryVerification

ClinicalFeatures

+

+ ++ +

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

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•May stratify epidemic curves by personal traits

– age, gender, race• to assess whether time of onset varies in relation to place or person

characteristics

Example of Epidemic Curves

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

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

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–Most likely agent/illness–Most likely vehicle– Mode of transmission/exposureand should be proposed in a way that can be tested

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 wellObservations 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

Step 6: Evaluate hypothesis

• Attack rates• Specific food attack rates• Cohort Studies - RR• Case Control Studies -OR

Next practical

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

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

– Environmental studies often help explain why an outbreak occurred and is important

– But sometimes laboratory evidence important _PCR

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!

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 control3. Controlling or modifying the host response

to exposure**Immunize the susceptibles; use

prophylactic chemotherapy

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 employedThe report can take several forms:

1) An oral briefing for local health authorities2) A written report to a journal3) Formal presentation of recommendations (a “blueprint” for action)

Conclusion1 ) Assembly Team 2) Establish the existence of an outbreak 3) Verify the diagnosis 4) Define and identify cases 5) Describe and orient the data in terms of

person, place and time 6) Develop hypotheses 7) Evaluate hypotheses 8) Refine hypotheses and carry out additional

studies 9) Implement control and prevention measures 10) Communicate findings