D028 Communicable Disease Outbreak Assessment Presentation

Grace J.
September 25, 2026September 25, 2026
D028, MSN, Western Governors University

Student Name

Western Governors University

D028 Advanced Health Assessment for Patients and Populations

Prof. Name

Date

Introduction

A systematic epidemiologic outbreak investigation helps public health professionals identify cases, determine the likely source of infection, understand transmission patterns, measure disease occurrence, and implement measures to prevent additional illness. In the fictional Crab Apple Valley outbreak, investigators used case definitions, surveillance, person-place-time analysis, epidemiologic calculations, an epidemic curve, and an observational study design to investigate an outbreak of plague caused by Yersinia pestis. The scenario suggests that the initial infections were associated with flea exposure and that later infections may have resulted from respiratory exposure after the disease progressed to pneumonic plague.

The Crab Apple Valley investigation provides a practical example of how epidemiologic principles are applied during a communicable disease outbreak. It demonstrates the importance of case identification, diagnosis verification, surveillance, descriptive epidemiology, transmission assessment, cumulative incidence, attack rates, case fatality rate, epidemic curves, study design, outbreak classification, infection-control measures, public communication, and nursing responsibilities.

Background of the Crab Apple Valley Outbreak

The Crab Apple Valley scenario describes a localized outbreak of plague caused by Yersinia pestis. The initial patient, Jack Kalani, worked on an orchard farm where rodent activity created an environmental setting associated with potential exposure to infected fleas. After becoming ill, he developed symptoms consistent with plague and subsequently progressed to pneumonic disease.

Once pneumonic plague developed, people who had close contact with the patient could potentially have been exposed to infectious respiratory droplets. This prompted investigators to examine individuals who had contact with Jack at the Crab Apple Valley farmers’ markets, Community Hospital, and other relevant settings.

Healthcare workers and emergency medical personnel who participated in patient care or resuscitation were also considered potentially exposed. Investigators used descriptive and analytic epidemiology to examine the outbreak according to person, place, and time.

Case Definition for the Plague Outbreak

A case definition is a standardized set of clinical, laboratory, and epidemiologic criteria used to determine whether an individual should be counted as a case during an outbreak. Establishing the definition early helps investigators apply the same criteria to everyone and maintain consistent case counts.

An outbreak case definition may include several components:

Clinical criteria: Signs, symptoms, and examination findings.
Laboratory criteria: Test results indicating possible or confirmed infection.
Epidemiologic criteria: Relevant exposure, contact, location, and timing.
Case classification: Suspected, probable, or confirmed categories.
Population and setting criteria: Geographic areas, facilities, workplaces, or other defined settings.
Exclusion criteria: Circumstances that prevent an individual from meeting the case definition.

In the Crab Apple Valley scenario, suspected cases included individuals with relevant exposure on or after August 1 at the farmers’ markets or Community Hospital who subsequently developed symptoms such as fever, chills, cough, weakness, or painful and swollen lymph nodes.

A standardized definition allowed investigators to identify potential cases systematically, compare cases consistently, and monitor the progression of the outbreak.

Ten Steps of an Epidemiologic Outbreak Investigation

A communicable disease outbreak investigation follows a structured process. Some steps may occur simultaneously because investigators often need to begin control measures before the investigation is completely finished.

1. Establish an Outbreak Investigation Team

The first step is to assemble a multidisciplinary team with the expertise necessary to investigate and control the event. The Crab Apple Valley team included healthcare professionals, epidemiologists, laboratory personnel, pharmacists, nurses, public health representatives, administrative personnel, a statistician, and a public information officer.

Each professional contributes a different area of expertise. Epidemiologists evaluate disease patterns and exposures, laboratory personnel support diagnostic testing, nurses contribute to surveillance and patient care, clinicians assess affected individuals, and communication specialists help provide accurate information to the public.

2. Establish the Existence of an Outbreak

Investigators must determine whether the observed number of illnesses is greater than the number normally expected for a particular population, location, and time period.

In Crab Apple Valley, the appearance of multiple people with similar symptoms represented an unusual pattern that warranted further investigation. Establishing that an outbreak exists provides the foundation for determining its source, magnitude, and transmission pathway.

3. Verify the Diagnosis

Investigators should verify the diagnosis before attributing illnesses to a particular pathogen. Many infectious and noninfectious conditions can cause similar symptoms, so laboratory and clinical evidence are essential.

In the scenario, diagnostic testing supported infection with Yersinia pestis. Chest X-ray findings also indicated pulmonary involvement in affected patients, supporting the diagnosis of pneumonic plague.

Diagnosis verification helps ensure that subsequent case counts, epidemiologic analyses, and control measures are directed toward the correct disease.

4. Develop a Case Definition

After verifying the suspected disease, investigators establish criteria for identifying suspected, probable, and confirmed cases.

The definition may include symptoms, laboratory findings, exposure history, geographic location, and a specified period. Applying these criteria consistently makes case identification more reliable and improves the accuracy of epidemiologic calculations.

5. Identify and Find Cases

Case finding involves locating individuals who meet or may meet the case definition. Investigators can review hospital records, laboratory reports, emergency department documentation, urgent care records, and exposure histories.

Potential cases may be identified through hospitals, laboratories, emergency medical services, healthcare providers, public health agencies, contact tracing, and direct interviews with exposed individuals.

Rapid case finding is especially important when an infectious disease can cause severe illness or spread through close contact.

6. Describe the Outbreak by Person, Place, and Time

Descriptive epidemiology examines who became ill, where cases and exposures occurred, and when illness developed.

Investigators may use line lists, patient interviews, geographic information, exposure histories, and epidemic curves to identify patterns. For example, cases concentrated around a particular location may suggest a common exposure, while cases appearing in successive groups may indicate person-to-person transmission.

Person-place-time analysis helps investigators develop hypotheses that can subsequently be examined using analytic epidemiology.

7. Develop and Evaluate Hypotheses

After identifying epidemiologic patterns, investigators develop hypotheses about the source of infection and how transmission occurred.

In the Crab Apple Valley scenario, possible exposures included attendance at farmers’ markets and subsequent contact with infected individuals at the hospital. Investigators can compare disease occurrence among exposed and unexposed groups to determine whether a particular exposure is associated with illness.

8. Implement Control Measures

Control measures should begin as soon as sufficient evidence is available to reduce the risk of additional transmission. Investigators do not always need to wait until every aspect of an outbreak has been resolved.

Potential measures in the scenario include patient isolation, respiratory precautions, appropriate antimicrobial treatment, prophylaxis for relevant contacts, and vector and environmental control.

9. Communicate Findings

Outbreak information should be communicated to healthcare professionals, public health agencies, affected communities, patients, and other stakeholders.

Communication should explain what is known, what remains uncertain, which symptoms require attention, how exposure can be reduced, and when medical evaluation should be sought. Updates should be provided as investigators obtain additional information.

10. Maintain Surveillance

Surveillance should continue after control measures are implemented. Monitoring new cases allows investigators to determine whether transmission is decreasing and whether additional interventions are necessary.

Continued surveillance can also identify previously unrecognized cases and provide early warning if disease activity increases again.

Active and Passive Surveillance During an Outbreak

Surveillance provides the information needed to detect cases, monitor disease trends, identify changes in transmission, and evaluate interventions.

Active Surveillance

Active surveillance occurs when public health personnel actively search for cases rather than relying only on routine reporting.

For the Crab Apple Valley investigation, active surveillance could include reviewing hospital records, contacting healthcare facilities, interviewing exposed individuals, conducting contact tracing, and monitoring healthcare workers following potential exposure.

Active surveillance can improve the completeness and speed of case detection. However, it generally requires more personnel, time, coordination, and financial resources than passive surveillance.

Passive Surveillance

Passive surveillance relies on routine reports submitted by healthcare providers, laboratories, hospitals, and other reporting organizations.

For example, healthcare facilities may report suspected or confirmed plague cases to the appropriate public health authority. These reports can help officials recognize unusual increases in disease occurrence.

Passive surveillance typically requires fewer resources, but incomplete or delayed reporting can result in missed cases.

Suspected Cases in the Crab Apple Valley Scenario

A suspected case is an individual who meets specified clinical and epidemiologic criteria but does not yet have sufficient evidence for classification as probable or confirmed.

In the Crab Apple Valley scenario, 73 individuals presented to urgent care facilities with flu-like symptoms after potential exposure. Reported symptoms included fever above 38°C, chills, weakness, and tender or swollen lymph nodes.

Emergency medical personnel and hospital employees who had contact with the initial patient during resuscitation or clinical care were also considered potential cases or exposed individuals.

Identifying suspected cases early allows investigators to begin diagnostic testing, monitoring, contact investigation, and appropriate infection-control procedures.

Probable and Confirmed Plague Cases

Case classification depends on the criteria established for the particular outbreak. A probable case generally meets defined clinical and epidemiologic criteria and may have supporting laboratory evidence. A confirmed case meets the definitive laboratory or other criteria established by the case definition.

In the Crab Apple Valley scenario, one patient had a temperature of 103.2°F, chills, weakness, tender and swollen lymph nodes, and a positive rapid diagnostic test. Under the scenario’s classification system, these findings supported classification as a probable case.

Laboratory confirmation is particularly important because symptoms such as fever, weakness, chills, and cough can occur with numerous illnesses. Diagnostic testing helps determine whether suspected cases are actually associated with Yersinia pestis.

How Plague Is Transmitted

Understanding transmission is central to outbreak investigation and control. Transmission describes how an infectious agent moves from a source or reservoir to a susceptible host.

Direct transmission occurs when an infectious agent passes directly from an infected person, animal, or source to another susceptible individual. Depending on the disease, this may involve physical contact or exposure to infectious respiratory secretions.

Indirect transmission occurs through an intermediate mechanism. Major categories include:

Vector transmission: A living organism, such as a flea, mosquito, or tick, transmits the pathogen.
Vehicle transmission: A contaminated substance or material, such as food, water, or equipment, contributes to transmission.
Fomite transmission: Contaminated inanimate objects serve as an intermediate source of exposure.

Transmission Pathways in Crab Apple Valley

The scenario suggests that the initial infections were associated with vector-borne transmission. Jack Kalani’s work in an orchard environment with rodent activity created an opportunity for exposure to infected fleas carrying Yersinia pestis.

The later development of pneumonic plague introduced a potential respiratory transmission pathway. Pneumonic plague can spread through respiratory droplets during close contact with an infected person. This provides a possible explanation for secondary infections among healthcare personnel who had close contact with affected patients.

The scenario demonstrates how more than one transmission pathway can contribute to different stages of a communicable disease outbreak.

Cumulative Incidence in Epidemiology

Cumulative incidence measures the proportion of an initially disease-free population at risk that develops a disease during a specified period.

The basic formula is:

Cumulative incidence = Number of new cases during the period ÷ Number of people at risk at the beginning of the period

The result may be multiplied by 1,000, 10,000, or 100,000 to make the measure easier to interpret.

Cumulative incidence is useful for estimating the risk of developing disease within a defined population and time period. During an outbreak, it can also help investigators compare disease occurrence among different populations or exposure groups.

Cumulative Incidence Calculation for Crab Apple Valley

The scenario reports 93 new cases between August 1 and August 7 in a population of approximately 125,000 people.

Using a multiplier of 1,000:

Cumulative incidence = (93 ÷ 125,000) × 1,000

Cumulative incidence = 0.744 cases per 1,000 people

Rounded to two decimal places, the cumulative incidence is approximately 0.74 cases per 1,000 people during the specified period.

This means that approximately 0.74 new cases occurred for every 1,000 people in the population during the specified outbreak period. Population-level incidence should be interpreted alongside the seriousness of the disease, the characteristics of affected groups, and the circumstances of the outbreak.

Purpose of the Crab Apple Valley Outbreak Investigation

The primary purpose of the investigation is to identify the likely source of the plague outbreak, determine how Yersinia pestis was transmitted, measure the extent of disease, and implement interventions that can prevent additional infections.

Investigators need to determine who became ill, where and when exposure occurred, whether transmission occurred between people, and which interventions can interrupt the identified transmission pathways.

The investigation can also provide information for future prevention. Findings may guide environmental monitoring, vector management, infection-prevention practices, surveillance systems, community education, and public health preparedness.

Observational Study Design for the Outbreak

The appropriate epidemiologic study design depends on the research question, available data, population characteristics, and circumstances of the outbreak.

An experimental study would not be appropriate for determining whether an exposure causes plague because intentionally exposing people to a potentially severe or fatal infectious disease would be unethical. Instead, investigators can use observational designs.

A retrospective cohort study may be appropriate when investigators can identify a defined population and determine whether individuals experienced a particular exposure before comparing disease occurrence between exposed and unexposed groups.

For example, investigators could retrospectively examine people who attended the farmers’ markets or had contact with infected individuals and determine whether particular exposures were associated with subsequent illness.

A retrospective cohort study can allow investigators to calculate attack rates and measures such as relative risk, helping assess associations between specific exposures and disease occurrence.

Epidemic Curve and Histogram

An epidemic curve, commonly called an epi curve, displays cases according to the date or time when symptoms began. It is one of the most useful tools for understanding the timing and potential transmission pattern of an outbreak.

In the Crab Apple Valley scenario, the largest number of cases occurred around August 3. The early cluster among people associated with the farmers’ markets suggests a possible common exposure, whereas later cases associated with the hospital may indicate subsequent person-to-person transmission.

An epidemic curve can help investigators evaluate:

The likely timing of exposure.
The incubation period.
Whether cases are consistent with a common-source exposure.
Whether person-to-person transmission may have occurred.
Whether transmission continued through successive generations.

The curve in this scenario therefore helps illustrate a possible transition from an initial community exposure to secondary transmission in a healthcare setting.

Incubation Period and Its Role in Outbreak Investigation

The incubation period is the interval between exposure to an infectious pathogen and the onset of symptoms. Investigators use incubation-period information to determine whether the timing of illness is consistent with a suspected exposure.

The incubation period varies according to the pathogen and clinical form of disease. Bubonic plague generally develops several days after exposure to an infected flea, while pneumonic plague may have a shorter incubation period.

In the Crab Apple Valley scenario, Jack Kalani’s initial illness is consistent with possible flea-associated exposure. His subsequent pulmonary involvement created a potential opportunity for respiratory transmission to individuals who had close contact with him.

Incubation-period information can help investigators estimate exposure windows, interpret epidemic curves, identify contacts who may need monitoring, and evaluate competing hypotheses about the source of infection.

Endemic, Epidemic, Outbreak, and Pandemic Disease Patterns

Public health professionals use specific terms to describe the frequency and geographic distribution of disease.

Endemic refers to the usual or expected presence of a disease within a particular population or geographic area.

Epidemic refers to disease occurrence that exceeds the expected level within a defined population, geographic area, or time period.

Outbreak commonly describes a localized occurrence of disease. Depending on the public health context, the terms outbreak and epidemic may sometimes be used interchangeably.

Pandemic refers to an epidemic that spreads across multiple countries or continents and affects large populations.

The Crab Apple Valley scenario represents a localized outbreak or epidemic because plague cases increased above the expected level within a defined community and period. It does not represent a pandemic because the scenario describes a localized event rather than widespread international transmission.

Case Fatality Rate for the Outbreak

The case fatality rate (CFR) is the proportion of identified cases that result in death from the disease during a specified period.

The formula is:

CFR = (Number of deaths among cases ÷ Number of cases) × 100

The scenario reports one death among 93 cases:

CFR = (1 ÷ 93) × 100

CFR = 1.08%

Therefore, the case fatality rate for the scenario is approximately 1.08%.

CFR should be interpreted carefully because it can be affected by case detection, diagnostic practices, treatment availability, disease severity among identified patients, and the timing of the analysis. It should not automatically be interpreted as the mortality risk for every person who develops plague.

Primary Attack Rate

An attack rate is a form of cumulative incidence commonly used during outbreak investigations to describe disease occurrence within a defined population over a specific period.

The primary attack rate describes illness associated with an initial exposure or exposure event.

The scenario reports 62 cases among 200 people exposed at the farmers’ markets.

Primary attack rate = (62 ÷ 200) × 1,000

Primary attack rate = 310 cases per 1,000 exposed people

Expressed as a percentage:

(62 ÷ 200) × 100 = 31%

Therefore, the primary attack rate was 31%, or 310 cases per 1,000 exposed individuals.

This measure helps investigators determine the proportion of people who became ill after the suspected initial exposure.

Secondary Attack Rate

The secondary attack rate describes disease occurrence among susceptible individuals who were exposed to a primary case or cases within a defined contact group and period.

The scenario reports 31 cases among 480 hospital employees who were exposed.

Secondary attack rate = (31 ÷ 480) × 1,000

Secondary attack rate = 64.58 cases per 1,000 exposed people

Expressed as a percentage:

(31 ÷ 480) × 100 = 6.46%

Therefore, the secondary attack rate was approximately 6.46%, or 64.58 cases per 1,000 exposed employees.

Comparing primary and secondary attack rates allows investigators to describe disease occurrence associated with the suspected initial exposure separately from disease occurring after contact with infected individuals.

Primary and Secondary Transmission in the Outbreak

In the scenario, primary cases were associated with exposure at the Crab Apple Valley farmers’ markets and environmental conditions surrounding the suspected initial source. Flea-borne transmission involving infected rodents is identified as the suspected mechanism for these initial infections.

Secondary transmission was associated with the later development of pneumonic plague. Healthcare workers who had close contact with infected patients, including personnel involved in resuscitation and direct patient care, could have been exposed to infectious respiratory droplets.

Distinguishing between primary and secondary transmission is important because each pathway may require different control strategies. Vector and environmental measures address potential flea and rodent exposure, while isolation, respiratory precautions, contact monitoring, and appropriate prophylaxis can help reduce subsequent person-to-person transmission.

Outbreak Control Measures

Outbreak control measures should address the suspected or confirmed routes of transmission and should be based on current public health and clinical guidance.

Isolation and Respiratory Precautions

Patients with suspected or confirmed pneumonic plague should receive appropriate isolation and infection-prevention measures. Healthcare professionals should follow current facility and public health guidance regarding respiratory precautions and personal protective equipment.

Reducing exposure to infectious respiratory secretions is particularly important when pneumonic disease and person-to-person transmission are suspected.

Antibiotic Treatment and Post-Exposure Prophylaxis

Prompt clinical evaluation and appropriate antimicrobial treatment are important components of plague management. Antibiotic selection should follow current clinical and public health recommendations while considering the patient’s clinical condition.

People who have experienced relevant close exposure may require post-exposure prophylaxis depending on the nature of the exposure and applicable public health recommendations.

Vector and Environmental Control

Because the scenario identifies possible flea and rodent exposure, environmental investigation is an important part of outbreak control.

Public health authorities can assess rodent activity and flea populations and implement appropriate measures to reduce human exposure. Vector-control activities should be carefully planned because disturbing infected rodents may increase exposure to infected fleas.

Communication Plan for the Plague Outbreak

Clear risk communication is an essential component of outbreak response. Healthcare professionals, public health agencies, patients, and community members need timely and accurate information to respond appropriately.

Public health messages should explain relevant symptoms, potential exposure risks, prevention strategies, and when medical evaluation should be sought. Healthcare organizations should also receive updated guidance concerning diagnosis, reporting, infection prevention, and patient management.

An effective outbreak communication strategy may include:

Public health alerts and official announcements.
Notifications for hospitals and healthcare professionals.
Coordination among local, state, and national public health agencies.
Community education through reliable communication channels.
Regular updates as new epidemiologic information becomes available.

Communication should clearly distinguish confirmed information from preliminary findings. When recommendations change because investigators obtain new evidence, the reason for the change should be communicated clearly.

Nursing Implications and Application to Practice

The Crab Apple Valley scenario demonstrates how epidemiologic methods support the detection, investigation, and control of communicable diseases. Case definitions, surveillance, epidemic curves, incubation periods, cumulative incidence, attack rates, and case fatality rates provide different types of information about an outbreak.

Nurses are often among the first healthcare professionals to recognize unusual symptoms, clusters of illness, or possible exposure patterns. Early recognition and reporting can support timely public health investigation and intervention.

Effective outbreak response also requires interdisciplinary collaboration. Nurses, physicians, epidemiologists, laboratory professionals, infection-prevention specialists, pharmacists, administrators, and public health professionals each contribute specialized knowledge.

Nurses can contribute to outbreak management by:

Recognizing and reporting suspected cases.
Following isolation and infection-prevention procedures.
Participating in surveillance and contact investigations.
Collecting and communicating relevant patient information.
Educating patients and families about prevention.
Supporting appropriate use of personal protective equipment.
Reporting changes in patient condition to the healthcare team.
Participating in public health preparedness and response activities.

A strong understanding of epidemiology helps nurses connect individual patient care with population-level disease prevention. It also supports evidence-based decision-making during communicable disease outbreaks.

Key Takeaways From the Crab Apple Valley Outbreak

The Crab Apple Valley scenario demonstrates how multiple epidemiologic concepts work together during a communicable disease investigation.

A case definition establishes consistent criteria for identifying and classifying cases.
Active and passive surveillance provide complementary approaches to disease detection.
Person, place, and time analysis helps investigators recognize epidemiologic patterns.
An epidemic curve provides information about the timing and potential pattern of transmission.
The incubation period helps investigators estimate potential exposure windows.
Cumulative incidence measures the occurrence of new disease within a population at risk during a specified period.
Attack rates describe disease occurrence among defined exposed groups during an outbreak.
The case fatality rate describes the proportion of identified cases that result in death.
Isolation, respiratory precautions, antimicrobial treatment, prophylaxis, and environmental interventions can help interrupt transmission when appropriate.
Effective risk communication supports coordinated outbreak response.
Nurses contribute to surveillance, early recognition, infection prevention, patient education, reporting, and interdisciplinary outbreak management.

Understanding these concepts enables healthcare professionals to interpret outbreak data and participate effectively in evidence-based public health responses.

Frequently Asked Questions About Epidemiologic Outbreak Investigations

What is the purpose of an epidemiologic outbreak investigation?

The purpose is to identify cases, determine the likely source and transmission pathway, measure the extent of disease, identify people at risk, implement control measures, and prevent additional cases.

What are the three components of descriptive epidemiology?

Descriptive epidemiology examines person, place, and time. These components help investigators determine who is becoming ill, where cases or exposures are occurring, and when disease develops.

What is a case definition?

A case definition is a standardized set of clinical, laboratory, and epidemiologic criteria used to determine whether an individual should be classified as a suspected, probable, or confirmed case.

What is the difference between active and passive surveillance?

Active surveillance involves public health personnel actively searching for cases, whereas passive surveillance relies primarily on routine reports from healthcare providers, laboratories, and healthcare facilities.

What is an epidemic curve?

An epidemic curve is a graph that displays cases according to the time or date of symptom onset. It can help investigators identify exposure patterns, estimate timing, and evaluate whether transmission may have occurred through a common source or between people.

How is cumulative incidence calculated?

Cumulative incidence is calculated by dividing the number of new cases occurring during a specified period by the number of people at risk at the beginning of that period.

Cumulative incidence = New cases ÷ Population at risk

What is the case fatality rate?

The case fatality rate is the proportion of identified cases that result in death from the disease during a specified period.

CFR = (Deaths among cases ÷ Total cases) × 100

What is a primary attack rate?

A primary attack rate describes the proportion of people who become ill after an initial exposure or exposure event.

What is a secondary attack rate?

A secondary attack rate describes disease occurrence among susceptible people who were exposed to a primary case or cases within a defined contact group and period.

Why is epidemiology important in nursing?

Epidemiology helps nurses recognize disease patterns, report suspected cases, participate in surveillance, apply infection-prevention measures, educate patients, support contact investigations, and contribute to population-level disease prevention.

References

Centers for Disease Control and Prevention. (2012). Principles of epidemiology in public health practice (3rd ed.). U.S. Department of Health and Human Services. https://www.cdc.gov/csels/dsepd/ss1978/index.html

Centers for Disease Control and Prevention. (2024). About plague. https://www.cdc.gov/plague/about/index.html

Centers for Disease Control and Prevention. (2024). Plague. https://www.cdc.gov/plague/