Difficulty: Intermediate | Prerequisites: Part 1 (HBV Fundamentals and Nursing Home Outbreak Risk), basic understanding of epidemiological concepts.
Outbreak investigation is a structured, step-by-step process that public health teams use to identify the source of disease transmission, control ongoing spread, and prevent future occurrences. This set of notes walks through that process using a real-format case study: an HBV outbreak in a nursing home, investigated over the course of several days. Each phase of the investigation introduces distinct skills, from initial notification and case definition through to active case finding, data analysis, and source identification. If you are studying public health nursing or epidemiology, this is core material for understanding how field investigations unfold in practice.
An outbreak investigation follows a logical sequence: verify the diagnosis, establish a case definition, identify and interview cases and contacts, conduct active case finding through serological testing, perform descriptive and analytic epidemiology, identify the source, and implement control measures. In the case study, the team moved from a phone report of two confirmed HBV cases to the identification of 30 total cases, one death, and a contaminated alcohol dispenser as the probable source.
Case definition
A set of criteria used to classify whether a person's illness counts as a "case" in an outbreak. Includes person (who is affected), place (where it is happening), and time (when cases occurred). Think of it as the rulebook for deciding who goes on the case list and who does not.
Active case finding
The process of proactively seeking out additional cases through testing, screening, or surveillance rather than waiting for cases to be reported. In simple terms, instead of waiting for sick people to show up, you go looking for them.
Epidemiologic team (epi team)
A multidisciplinary group assembled to investigate an outbreak. Typically includes epidemiologists, public health nurses, and sometimes laboratory staff or environmental health officers.
Case-patient
A person who meets the case definition in an outbreak investigation. In the case study, the initial case-patients were Mrs. Johnson (84, nursing floor) and Mr. Sanchez (75, assisted living floor).
Differential diagnosis
A list of possible conditions that could explain a patient's symptoms, considered before a definitive diagnosis is made. In this case study, HBV was not immediately suspected because the presenting symptoms (nausea, vomiting, rash) overlapped with other conditions.
Case-control study (unmatched)
An analytic epidemiological study that compares people who have the disease (cases) with people who do not (controls) to identify exposures associated with illness. "Unmatched" means cases and controls are not paired by age, sex, or other variables. This was the study design used to link blood glucose monitoring to HBV infection in the case study.
Close contacts
People who have had significant direct or indirect exposure to a confirmed case. In a nursing home, close contacts may include roommates, staff who provide hands-on care, and residents who share medical procedures or common areas.
Serological investigation
Blood testing to detect markers of current, past, or resolved infection (or vaccination status). In the case study, 96 residents and 45 staff were tested for serological markers of HBV.
The outbreak begins with a phone call from a lab worker reporting two confirmed HBV cases at a local nursing home.
This is unusual: the local health department has never experienced an HBV outbreak in a nursing home population. Previous community HBV cases (five the prior year) were all linked to intravenous drug use.
The public health nurse contacts the nursing home and is referred to the on-call physician, Dr. Roberts.
Key information to obtain from the physician at this stage:
Confirmation of the diagnosis and lab results
Number of suspected and confirmed cases
Onset dates and symptoms for each case
Demographic details (age, sex, floor/unit)
Any known common exposures or risk factors
Vaccination status of affected residents
Whether any infection control measures are already in place
Dr. Roberts explains she is the on-call physician, not the residents' primary doctor. Each resident can choose their own private physician, which is why HBV was not initially suspected and lab testing was delayed.
Four additional residents are exhibiting symptoms (nausea, vomiting, rash) but lab results are pending.
With at least two confirmed cases sharing a potential common exposure, the threshold for assembling an epidemiologic team is met.
Before visiting the site, the team meets to discuss what they know and plan their approach.
The epi team visits the nursing home and meets the head of nursing to understand the facility layout, staffing, and care structure.
Building a case definition requires information from nursing staff about:
Person: Who is affected? Age, sex, medical conditions, floor/unit, vaccination status.
Place: Where in the facility are cases occurring? Are they confined to one floor or spread across multiple units?
Time: When did symptoms begin? When were cases confirmed? What is the likely exposure window?
The case definition allows the team to systematically classify confirmed, probable, and suspect cases going forward.
Known risk factors for HBV infection include:
Contact with infected blood or body fluids
Sharing needles or medical equipment
Unprotected sexual contact
Living with someone who has HBV
Receiving injections, blood transfusions, or medical procedures with contaminated equipment
Occupational exposure (healthcare workers)
From nursing staff, the team collects information about:
Types of medical procedures performed on residents (B12 shots, iron supplements, blood glucose monitoring)
How supplies are stored and whether they are shared across floors
Staff rotation patterns and whether any staff work across multiple units
Visitors and any recent changes to routine
Open-ended questions are important because they avoid leading the respondent toward expected answers and can reveal unanticipated risk factors or exposures the investigator had not considered.
Case-Patient #1 (Mrs. Johnson, 84, nursing floor): has senile dementia and paranoia, lives on the nursing floor. Recalls little but mentions "a man who is stealing things from her room."
Case-Patient #2 (Mr. Sanchez, 75, assisted living floor): admitted to the hospital ICU, unavailable for questioning.
Even when interviews yield little information, they remain valuable. They can confirm timelines, reveal unexpected contacts, and sometimes provide leads that only make sense later in the investigation.
When patient interviews are unproductive, alternative information sources include:
Medical records (treatments, procedures, medications, practitioner visits)
Nursing logs and daily activity records
Staff accounts of patient routines and movements
Visitor logs
Medical records reveal that both case-patients share the same practitioner, both receive vitamin B-12 injections, and both are diabetic.
The team launches a serological investigation of staff and close contacts of all case-patients (suspected and confirmed) as part of active case finding.
Active case finding is essential here because many acute HBV infections go clinically unrecognised, and waiting for symptomatic cases to present would undercount the outbreak.
96 residents and 45 staff members are tested.
Results: 9 new acute infections among residents, 0 among staff.
The absence of staff infections is a meaningful finding: it suggests the transmission route is specific to something residents experience and staff do not (e.g. a patient-side medical procedure), rather than a general environmental exposure.
Next steps include:
Interviewing and reviewing records for the 9 new cases
Looking for commonalities in procedures, timing, and location
Conducting descriptive epidemiology (epidemic curve, mapping cases by time and place)
Moving toward analytic epidemiology (case-control study) to test hypotheses
Over the following week, 30 total case-patients are confirmed. One death is attributed to HBV.
An unmatched case-control study statistically links cases to blood glucose monitoring for patients with diabetes mellitus on a specific day.
Environmental sampling identifies the alcohol dispenser near the blood glucose monitoring area as the most likely source of contamination.
The conclusion: even when direct procedural infection control (needle care) was adequate, a shared environmental surface in the immediate procedure area was the vector.
Attack rate (useful in outbreak settings):
Attack rate = (Number of new cases during the outbreak / Population at risk) x 100
In this case study, 30 confirmed cases among 160 residents gives an attack rate of 18.75% among all residents (though the denominator for those truly at risk, i.e. unvaccinated diabetic residents, would be smaller and the rate correspondingly higher).
This step-by-step methodology is the standard approach used by health departments and the CDC when investigating outbreaks of any kind, from foodborne illness to respiratory infections to healthcare-acquired bloodborne pathogens. The specific lesson about environmental surfaces (the alcohol dispenser) has informed updated CDC guidance on blood glucose monitoring in long-term care settings, emphasising that every shared item at the point of care must be decontaminated or dedicated to a single patient.
"If the two initial cases are on different floors, it probably is not an outbreak." Two cases of a disease not normally seen in this population, occurring close together in the same facility, meets the threshold for investigation regardless of physical separation within the building.
"Staff interviews are less important than patient interviews." In this case, patient interviews yielded almost nothing. Staff interviews and medical records provided the critical leads.
"No cases among staff means the investigation can be narrower." The absence of staff cases is itself a clue. It narrows the transmission pathway to something patient-specific, which helped point the investigation toward blood glucose monitoring.
"A case-control study requires matched cases and controls." Not necessarily. An unmatched case-control study was used here and was sufficient to identify the risk factor.
⚠️ Be able to list the sequential steps of an outbreak investigation: verify, define cases, identify/count cases, collect and analyse data, develop and test hypotheses, implement control measures, communicate findings.
⚠️ Understand why a case definition must include person, place, and time.
⚠️ Know the difference between active case finding (proactive screening) and passive surveillance (waiting for reports).
⚠️ Be prepared to explain why open-ended questions matter in outbreak interviews and what alternative data sources exist when patient interviews are unproductive.
⚠️ The unmatched case-control study design used in this case is a commonly tested concept in epidemiology courses.
True or false: An outbreak investigation should only begin once a minimum of five cases have been confirmed. False. Two confirmed cases with a potentially common exposure in an unusual setting is sufficient to begin an investigation.
Fill in the blank: A case definition includes information about _____, _____, and _____. Person, place, and time.
True or false: Active case finding means waiting for symptomatic patients to seek care. False. Active case finding means proactively seeking out cases through testing and screening.
True or false: In this case study, staff members were also found to be infected with HBV. False. Zero staff cases were identified.
Q: Why was the HBV diagnosis delayed for the initial two case-patients?
A: Because each resident had their own private physician and HBV was not immediately suspected. The presenting symptoms overlapped with other conditions (differential diagnoses), so lab testing for HBV was not ordered promptly.
Q: What information sources did the epi team rely on when patient interviews were unproductive?
A: Medical records, nursing logs, staff interviews, and later serological testing of contacts and residents.
Q: Why is it important to ask open-ended questions during an outbreak investigation?
A: Open-ended questions avoid leading the respondent and can reveal unexpected risk factors, exposures, or contacts that the investigator may not have anticipated.
Q: What study design was used to identify the risk factor in this outbreak, and what did it find?
A: An unmatched case-control study, which statistically linked cases to blood glucose monitoring for patients with diabetes mellitus on a particular day.
Q: What was the significance of finding zero cases among staff?
A: It suggested the transmission route was specific to a procedure or exposure experienced by residents but not by staff, narrowing the investigation toward patient-side procedures like blood glucose monitoring.
This material links to biostatistics (calculation of attack rates, case-control study design and odds ratios), to infection control and standard precautions (particularly around point-of-care equipment), and to health law and ethics (informed consent, mandatory reporting). The concept of differential diagnosis bridges to clinical reasoning in nursing and medical education. The role of the public health nurse as the lead investigator connects to community health nursing practice and scope of practice.
Outbreak investigation, case definition, active case finding, passive surveillance, epidemiologic team, case-patient, case-control study, unmatched case-control, attack rate, differential diagnosis, serological markers, medical record review, open-ended questions, epidemic curve, descriptive epidemiology, analytic epidemiology, risk factor identification, HBV outbreak, nursing home investigation, public health nursing