1. INTRODUCTION
Food-borne illness results from the natural, accidental, or
malicious contamination of foods by microbiological or chemical substances. The
impacts of food-borne illness may include morbidity and mortality, increased
health care costs, loss of consumer confidence, economic losses, and lost
productivity to industry.
Changes in food distribution networks and the increased
globalization of trade in food have resulted in larger volumes of raw and
processed products moving across domestic and international boundaries.
Consequently, food-borne illness outbreaks associated with widely distributed
foods result in human illnesses that cross local, provincial/territorial and
international boundaries.
Regulatory bodies responsible for human health and food
safety respond to these events through the development of enhanced food-borne
illness surveillance networks, including the use of molecular sub typing and
other laboratory technology, to enable cluster detection and the linkage of
seemingly unrelated cases to initiate outbreak investigation.
Increased concern and public awareness of food safety has
heightened the demand for swift resolution of food safety issues at a time when
they are becoming increasingly complex, reinforcing the need for collaboration
in multi-jurisdictional outbreak investigations and the active participation of
all partners in centrally led efforts to mitigate risk and prevent further illness.
2. PURPOSE OF THE INVESTIGATION
There are many reasons to do an outbreak investigation.
Those reasons can be listed as,
·
Control
and prevention
The primary reason to investigate
an outbreak is to control the occurrence of disease and prevent further disease.
Therefore, it is necessary to first determine whether the outbreak is ongoing
or is over. If the outbreak is ongoing, the first goal should be to prevent new
cases. If the outbreak has already occurred, the goal should be to determine
the factors or sources that contributed to the outbreak and prevent them from
occurring in the future.
·
Surveillance
Outbreak investigations can add
valuable information to ongoing public health surveillance activities. The goal
of surveillance is not to compile numbers of cases of illness for administrative
purposes, but to provide data that are important to guide public health policy
and action. Continual surveillance adds to existing knowledge regarding the
potential for and occurrence of a disease in a population.
·
Research
opportunities
An important objective of an
outbreak investigation is to gain additional knowledge regarding the natural
history of the disease. Carefully conducted investigations may reveal trends,
new or overlooked disease agents, novel vehicles or transmission modes, groups
at risk or specific risk factors. New knowledge may also be gained by assessing
the impact and effectiveness of control measures.
·
Administrative
concerns
Identifying the cause of outbreaks
may be used to evaluate and improve current health programs in the community,
identify high-risk groups or etiologic agents previously overlooked and guide
future strategies and future allocations in these areas.
·
Political
or legal concerns
There may be overwhelming
pressures placed on the local health departments (LHD) by families of affected
individuals, the media, local politicians and others to determine the source of
an outbreak and whether it may pose a continued or future threat to the
community.
3. STEPS INVOLVE IN INVESTIGATING AN OUTBREAK
Prompt response to food related complaints is the foundation
of a successful investigation. Important steps and information necessary to
determine the initiation and extent of an investigation include examination of
test results and preliminary evidence such as onset times, symptoms and
duration of illness, development of hypotheses, assessment of the magnitude of
the problem, and evaluation of available resources.
The procedure for the investigation and determination of the
existence of an outbreak is reasonably standard regardless of the disease being
investigated. The steps involve in investigation is listed below and they are
not sequential and some contingency planning can be done before an outbreak.
The steps in this procedure include;
- preparation for a detailed epidemiologic investigation
- establish the existence of an outbreak or epidemic
- verify diagnosis
- formulate a tentative hypothesis
- put control measures into operation
- conduct the investigation
- relate the outbreak to time, place and person
- analyze and interpret data
- test hypothesis and formulate conclusions
- prepare a final report of the investigation
3.1 Preparation for a detailed epidemiologic investigation
Planning an epidemiologic investigation may be considered as
the initial step in the process because part of the planning can be done before
an outbreak occurs. The responsible department can begin by training personnel
in how to compile line lists, develop questionnaires, conduct interviews, and
use epidemiologic information.
The department should have 6-8 stool culture kits on hand or
readily available should an outbreak occur because in most cases stool
specimens must be collected within 72 hours of onset of illness to isolate and
identify certain pathogens (e.g., Clostridium
perfringens, Bacillus cereus, and
Staphylococcus aureus).
Lists of contacts such as administrative contacts,
additional personnel, sanitarians, regional contacts, physicians, clinical
laboratories, or other persons who may become involved in outbreak
investigations should be assembled.
Resource materials describing signs and symptoms, incubation
times and specifics regarding specimen collection and appropriate kits to be
used should be maintained and readily available to those processing the initial
calls. This may help in formulating an initial hypothesis.
Once the investigation is underway, the proper clinical
specimens should be collected as soon as possible before patients recover and
become less likely to submit specimens, and before the general interest in the
investigation wanes.
All suspected food borne outbreaks should be examined and a
determination made regarding the feasibility of conducting an investigation
even if the time to collect proper clinical specimens has passed. This is done
in order to determine the source of the outbreak and to prevent similar
outbreaks from recurring.
3.2 Establish the existence of an outbreak or epidemic
Establish the existence of an outbreak by comparing the
incidence of the disease in a specified population during a comparable previous
time period or when point source outbreaks occur in order to be familiar with
disease trends in the community and determine whether there actually is a
higher than expected number of cases in a community.
This can be done through diligent public health surveillance
that provides an accurate assessment of the status of the health of the
community and helps to determine any increases or decreases in communicable
diseases in the local population.
Surveillance data should be reviewed by the responsible
department on a regular basis to become familiar with the status of all
communicable diseases in the area of jurisdiction.
While establishing the existence of an outbreak, have to be
aware of artificial causes of increases such as;
- changes in local reporting,
- changes in case definitions of reportable diseases,
- increased local or national interest in particular diseases,
- new physicians in the area,
- new diagnostic procedures which might identify new or existing infectious agents, and
- Increased populations or new arrivals into the area.
3.3 Verify diagnosis
Analyze clinical histories of cases and have laboratory
tests performed in order to confirm the etiologic agent associated with the
illness. Clinical, laboratory and epidemiologic evidence should be considered.
Verify that laboratory results are consistent with the
clinical evidence as laboratory errors sometimes occur. In verifying the
diagnosis, it is crucial to collect clinical and environmental samples as soon
as possible because many etiologic agents become more difficult to isolate with
time (e.g., Bacillus cereus, Clostridium perfringens, Staphylococcus aureus).
As case-patients begin to recover they may become more
reluctant to submit clinical samples. Also, when delay occurs, environmental samples
are more likely to be discarded or disinfected.
3.4 Formulate a tentative hypothesis
Formulate a tentative hypothesis to explain the most likely
cause of illness, etiologic agent, vehicle, and distribution of cases.
Hypothesis generating is an ongoing process. It may begin with the first phone
call.
This hypothesis may be based on known incubation periods,
symptoms, duration of illness or foods eaten, as well as knowledge about the
various agents responsible for outbreaks. The tentative hypothesis directs the
course of an investigation and control measures, and is tested by data gathered
during the investigation.
A series of hypotheses may evolve during an investigation.
First, facts are examined and broad hypotheses are formulated. As more facts
are gathered, a more specific hypothesis may be formulated.
Confirm the diagnosis if laboratory testing has been
completed.
Examine case histories to determine if there are common
exposures, or if signs and symptoms and onset of illness are consistent with etiologic
agents. Next, additional facts to test the new hypothesis are gathered. The
cycle is continued as necessary.
3.5 Put control measures into operation
The priority during each investigation should be to
implement effective control measures.
This should be done early in the course of the investigation
based on the initial hypotheses. Factors to consider when determining the most
effective control measures include the extent of the illness, who was affected,
when and where did the critical exposure take place, what was the vehicle, how
was the disease transmitted, what is the etiologic agent and whether or not
there is a potential for ongoing or future transmission.
Control measures should focus on specific agents, sources,
or reservoirs of infection and should be targeted to interrupt the transmission
of disease or reduce exposure to disease. These measures should be instituted
as soon as possible to control the current problem and demonstrate to the
community that efforts are being made to control the problem.
Use the information collected during the investigation to
control the current situation and to prevent future problems in the community.
3.6 Conduct the investigation
Conducting the investigation can be listed as;
- Prepare a line list of ill persons listing signs, symptoms, onset times, duration of illness.
- Gather appropriate community and environmental information; investigate potential sources of the responsible agent and factors that may have contributed to the outbreak.
- Obtain clinical specimens (usually stool specimens) from ill case patients for laboratory analysis of enteric pathogens.
- When possible, obtain samples of implicated food or environmental samples for laboratory analysis. Hold these samples under refrigeration until a known etiologic agent has been identified.
3.7 Relate the outbreak to time, place and person
If an outbreak occurs following a common meal or exposure
(e.g., wedding, parties), conduct a survey of known cases to investigate
commonalties, such as onset of illness (time), population characteristics
(e.g., age, gender) (person) and where they could have been infected or exposed
(place).
If an outbreak does not have an established common meal or
exposure (e.g., an increase of cases of illness in a community within a close
time frame), it may be necessary to start with an informational or a more
general survey in order to select case patients.
Develop a questionnaire and perform a case-control study or
cohort study. It is imperative to interview non-ill (control) persons who are
similar or had similar experiences regarding time and place to those ill. Begin
by interviewing and analyzing data from 20-25 ill persons (if available) and
20-25 well persons who had the same exposures but remained well.
Obtain identifying information (name, address, telephone
number, etc.); demographic information (age, sex, race, occupation or group
characteristics); and clinical information (symptoms, onset times, and duration
of illness).
Establish a Case
Definition
Begin with broad or “loose” definitions that may be narrowed
as more cases are defined. Classify cases as “lab-confirmed” or “probable”. Not
all cases need to be lab-confirmed. Make case counts and relate these to the
appropriate population to determine those groups at risk (e.g., same age groups,
same sex, and occupation).
Develop a line listing of cases. Contact those with
information on the illness or environmental circumstances contributing to the
outbreak (e.g., physicians, sanitarians). When attempting to identify cases,
additional contacts may need to be surveyed such as physicians, clinics,
hospitals, laboratories and friends of case-patients.
In some situations the media may be used to solicit
case-patients, but this approach should be considered carefully to avoid
biasing an epidemiologic investigation and damaging the reputation of local
establishments unnecessarily.
3.8 Analyze and interpret data
Summarize field investigations. Compare and interpret all
information collected and results of tests conducted. Construct epidemic curves
to detect the course of the outbreak and to determine if the illness originated
from a single source or is on going, calculate attack rates, develop
appropriate tables and charts, apply statistical tests and interpret the
cumulative data. Define the geographic extent of the outbreak and the population
at risk.
3.9 Test hypothesis and formulate conclusions
Accept or reject the hypothesis on the basis of the
available data and appropriate statistical analysis. For a hypothesis to be
accepted, the patterns of disease must fit the nature of the agent, its source,
its mode of transmission, and the contributory factors that allowed the
outbreak to occur.
If the hypothesis is rejected, another hypothesis should be
developed and additional data gathered in order to test this new hypothesis. A
more systematic study can be conducted as needed to improve the sensitivity and
specificity of the findings, establish the true number of cases, and assist in
arriving at more definitive conclusions.
3.10 Prepare a final report of the investigation
Investigations should be summarized as soon as completed and
a final report sent to the responsible department / authority. These final
reports serve as a record of the rationale and provide documentation for the
activities conducted during the investigation.
The final report can also be used to improve future
investigations and prevention measures. The report should follow the usual
scientific format of introduction, background, methods, results, discussion,
references and recommendations.
Do not use the names of case-patients. The names of
responsible department personnel or authorized personnel involved in the
investigation may be included. The names of facilities or locations where the
food borne outbreak occurred may be included.
The background is a short paragraph describing why the
outbreak investigation was initiated and may include who was affected, how many
people were ill and how many exposed, where the outbreak occurred, the severity
and clinical presentation of the cases. Note whether or not the outbreak
involved a particular setting or social event (e.g., school, restaurant,
wedding, festival) or to particular population (e.g., nursing home, day care
center).
The methods section should list how cases were identified,
how questionnaires were developed, methods used to collect data, as well as
clinical and environmental samples, laboratory tests performed, statistical
methods, control methods instituted, and other features of the investigations
used during the outbreak investigation.
The results section should list what was discovered in the
investigation, results of laboratory testing of clinical or environmental
samples, results of the epidemiologic investigation, the sanitarian’s report,
statistical results, epi-curves, tables, charts and other studies used during
the investigation.
The discussion should briefly summarize the findings of the
investigation. Evaluate the control and methods used in the investigation. Were
they successful? Could they be instituted in similar outbreaks in the future or
how should they be changed? What problems were encountered ? Is the current
surveillance program sufficient to identify and control future outbreaks? ,
etc.
List any important or unique aspects of the outbreak or a
specific disease agent uncovered during the investigation.
4. SUMMARY
Food borne outbreaks have to be investigated for prevention
of further occurring as well as for future requirements. About 10 steps can be
used to investigate an outbreak as described above. Those steps can be carried
out in an order or change of order or skip of steps according to the nature of
food borne outbreak and requirement in investigation.
5. REFERENCES
- Food borne disease outbreaks by Washington State Department of Health, Last Revised on January 2013
- Investigating Food borne Disease Outbreaks by Andrea Ellis
- Guidelines for Food borne Disease Outbreak Response by CIFOR (council to improve foodborne outbreak response)
- FOODBORNE AND WATERBORNE DISEASE OUTBREAK INVESTIGATION MANUAL by Department of Health and Family Services, Wisconsin Division of Public Health, Bureau of Communicable Diseases, Communicable Disease Epidemiology Section.
- Guidelines for Investigating Foodborne Illness Outbreaks by Rhode Island Department of Health
- FOODBORNE ILLNESS AND OUTBREAK INVESTIGATION MANUAL by Kansas Department of Health and Environment, Division of Health
- General Outbreak Investigation / Notification Protocol by West Virginia Department of Health and Human Resources, January 2008
- FOOD BORNE DISEASE OUTBREAKS : guidelines for investigation and control by world health organization (WHO)
- http://doh.dc.gov/service/foodborne-disease-outbreaks
- http://www.phac-aspc.gc.ca/zoono/fiorp-mitioa/index-eng.php
- http://labspace.open.ac.uk/mod/oucontent/view.php?id=451997§ion=1.8
