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Unformatted text preview: COMMUNICABLE DISEASE MANAGEMENT PROTOCOL MANUAL Epidemiological Investigation
Definition of an Outbreak
An outbreak is commonly defined as:
The occurrence in a community or region of cases
of an illness with a frequency clearly in excess of
normal expectancy. The number of cases
indicating presence of an outbreak will vary
according to the infectious agent, size and type of
population exposed, previous experience or lack of
exposure to the disease, and time and place of
occurrence. Therefore, the status of an outbreak is
relative to the usual frequency of the disease in the
same area, among the same population, at the
same season of the year.1 Epidemiological Investigation
Regardless of the communicable disease, the procedure
for the investigation of a suspected outbreak is the
1. Establish the existence of an outbreak
Compare current information with previous
incidence in the community during the same time
of year to determine if the observed number of
cases exceeds the expected. Compare available
information about new cases with a
predetermined definition of an outbreak.
2. Confirm the diagnosis
Analyze clinical histories of cases and have
standard laboratory tests performed to confirm or
reject the suspected diagnosis and to determine
the type of agent associated with the illness (e.g.,
bacterial, viral, other).
3. Establish the case definition and count cases
Develop a case definition and establish methods
for identifying and counting cases. Manitoba
Public Health Communicable Disease Control Unit 4. Relate the outbreak to time, place and person
Characterize the outbreak according to person,
place or time by interviewing known or selected
cases to determine common experiences, such as
when they became ill (time), where they became
infected (place) and who they are (person).
5. Determine who is at risk of becoming ill
Count cases and relate these counts to the
appropriate population to find the group(s) at
risk. Contact those who can provide information
on the illness or about the environmental
circumstances that contributed to the outbreak.
6. Formulate a tentative hypothesis
Formulate a tentative hypothesis to explain the
most likely cause, source and distribution of cases.
7. Compare the hypothesis with the established
The hypothesis will direct the course of the
investigation, and will be tested by the various
data gathered during the investigation. Several
successive hypotheses may be required.
8. Plan a detailed epidemiologic investigation
Determine from the collected data what other
information is needed and what resources are
available to test the hypothesis. Develop or obtain
interview forms, gather sampling equipment, and
alert people who will take part in the
Carry out the investigation. Interview people who
are similar or who had similar experiences to the
ill with respect to time or place but who are not ill
(controls); gather appropriate community and
environmental information; investigate potential
sources of the responsible agent and factors that
contribute to the outbreak; and obtain specimens
and samples. 1 COMMUNICABLE DISEASE MANAGEMENT PROTOCOL MANUAL Summarize and interpret all of the information
that has been collected and examine the results of
laboratory tests that have been conducted.
Construct epidemic curves, calculate rates,
develop appropriate tables and charts, apply
statistical tests to the data, and interpret the data.
On the basis of the available data and the
appropriate calculations, accept or reject the
hypothesis. For a hypothesis to be accepted, the
patterns of disease in the host must relate to 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, a second hypothesis must be developed
and additional information must be gathered.
9. Prepare a written report
A written report should summarize the
investigation and be prepared as soon as the
investigation is completed. This report should
include the various information listed in the
example referenced (refer to “Report of an
Investigation of an Outbreak”).
10. Implement control and prevention measures
Effective control measures should be developed
using the evidence that has been uncovered. Use
the information that has been collected during the
investigation to control the current outbreak and
also to prevent future problems in the community.
Initiate or intensify surveillance of the disease and
agent. If imminent danger exists, control measures
should be initiated after a tentative hypothesis has
been formed. References:
1. Chin, James (Editor). Control of Communicable
Disease Manual. American Public Health
Association, Washington DC, 2000.
2. Greg, Michael B. (Editor). Field Epidemiology.
Oxford University Press, New York, 1996. 2 Practical Considerations
• Each Regional Health Authority (RHA) or other
health care jurisdiction should carry out routine
surveillance to identify outbreaks early.
• Each RHA or other health care jurisdiction should
have a plan for the investigation of outbreaks, based
on the above model. The key public health officials
should be clearly identified. In Regional Health
Authorities, this will include, at a minimum, the
Medical Officer of Health or designate and the
communicable disease control contact person.
Many investigations, including all suspected foodand water-borne outbreaks will require the services
of the public health inspector.
• Each RHA or health care jurisdiction should
establish direct contact with Cadham Provincial
Laboratory (CPL) to ensure that the appropriate
specimens are obtained, processed and results
reported using an assigned outbreak code, as
quickly as possible.
• At minimum, a line-listing of cases and contacts
with other relevant data should be maintained
electronically. The computer software, EPI INFO,
is available in most Regional Health Authorities
and is ideal for collecting and analyzing data from
• It is a requirement that outbreaks be reported to
the Director of Comminicable Disease Control
using the outbreak reporting form (see Forms
section of this manual).
• If media interest is likely, the RHA or other health
care jurisdiction should establish one contact
person as quickly as possible. Ideally, the Medical
Officer of Health will be the contact if the outbreak
is occurring within the community. Assistance is
available from the communications staff at
• Refer to the Protocol to Investigate Food Borne
Illness and Suspected Food Borne Illness if COMMUNICABLE DISEASE MANAGEMENT PROTOCOL MANUAL food/water appears to be the source of an outbreak.
This manual includes a section on investigation of
outbreaks, with samples of forms which may be
used to collect and summarize information.
• The RHA or other health care jurisdiction may
request assistance from Manitoba Health with the
investigation of an outbreak. Where required, the
Manitoba Provincial Outbreak Response Plan
(ORP) will be implemented (refer to “Manitoba
Provinvial Outbreak Response Plan (ORP) in the
intro section of this manual). – – Management of Outbreaks in Institutions
• All of the above considerations apply to the
investigation of outbreaks in health-care or
residential-care facilities. However, there are several
additional items for consideration.
– Each facility should have a plan in place for
routine surveillance and for outbreak
investigation. The Regional Health
Authority should be aware of this plan and
be available to assist where required. In most
facilities, the infection control practitioner is
responsible for surveillance, and initiation
and execution of outbreak investigations.
– The successful control of outbreaks within
institutions requires a team approach by the
facility staff and Regional Health Authority
(usually the Medical Officer of Health or
delegate). – The Provincial Infection Control Nurse will
provide training to health-care facilities and
assist them in developing infection control
policies and procedures. An integral part of
the team, the Provincial Infection Control
Nurse should be informed of and involved
in the outbreak investigation.
It may be difficult to recognize an outbreak
in a health-care facility. Diarrhea, for
example, is not uncommon in residents of
personal care homes. The Regional Health
Authority or other health-care jurisdiction
should establish circumstances under which
they are to be notified. Circumstances
should include diseases which have the
potential for involvement beyond the facility,
and those where public health intervention
or expertise may be required (e.g., foodborne illness, influenza).
It is very important that food-borne
outbreaks in institutions be recognized
immediately since the potential for spread is
great in these facilities. In these instances,
early consultation with a public
health inspector, Provincial Infection
Control Nurse and staff within the RHA or
other health-care jurisdiction responsible for
communicable diseases is strongly
recommended. 3 COMMUNICABLE DISEASE MANAGEMENT PROTOCOL MANUAL Example
Report of an Investigation of an Outbreak
Reason for Investigation
• Include a statement describing how the outbreak
was identified (i.e., unusual number of cases). Investigation
• Present a general summary characterizing the
outbreak by person, place and time.
– Include the date the investigation began and
– State the number of cases ill and number of
individuals at risk.
– If the number of cases is small (e.g., 25 or
less), a table may be constructed such that
relevant data (e.g., age, gender, date of onset)
are presented for each individual.
– If the number of cases is large, the data may
be summarized and presented in tabular or
graphical format (e.g., age distribution,
– Compare the observed number of cases with
that expected for the same population
during the same time period.
• Present a clinical description of cases
– Describe the symptoms and physical
findings, including the results of laboratory
– If a table of individual cases is included,
incorporate the more important clinical and
laboratory findings in the table; otherwise,
summarize the findings numerically.
– Present a summary of the epidemiologic data
and findings. 4 – State pertinent information describing the
distribution of cases which would indicate
the source (e.g., outbreak confined to
customers of a particular restaurant).
– If an outbreak is localized to a group, such as
an institution or in connection with a
dinner, provide the total population at risk
(total population potentially exposed).
– Present in tabular form any associated
individual characteristics (e.g., age, history of
previous attack, immunization) with the
cases. Where possible, compare these case
characteristics with unaffected individuals.
– Present in tabular form, the association of
cases with various environmental factors
such as water, milk supply, foods, etc.
Compare these cases with the general
population when possible.
• Summarize results of an investigation of common
food and drink supplies
– If the outbreak investigation required data
from the water or milk supply, summarize
the findings of the engineer, public health
inspector or veterinarian.
– Summarize the results of bacteriologic
examination of any other food suspected as
– Provide details of preparation, handling and
storage of suspected foods, brands of these
foods, and places from which they were
– State information as to illnesses or infections
among those handling the suspected food. Discussion as to Source
• State reasons for suspecting or excluding milk,
water, and each article of food or other possible
medium of infection. COMMUNICABLE DISEASE MANAGEMENT PROTOCOL MANUAL Conclusions
• State your opinion regarding:
– Nature of the illness.
– Source of the outbreak.
– Method of transmission. Summary of Control Measures Implemented or
• State control measure for immediate control.
• State control measures for future prevention. 5 ...
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- Spring '08