Imagine you are scrolling on your phone and you notice a friend’s social media post
about suffering from some gastrointestinal symptoms.
Although it is not too alarming that one of your friends has diarrhea and vomiting…
It is certainly too much information.
Do they really need to share that online?
You shake your head and keep scrolling.
Later, you notice three more of your friends posting about similar problems and you take notice—
that’s more than you would expect in one day!
And when you realize you and your now-sick friends were all recently together,
you might be more concerned.
What is going on?
Why is this happening?
And will you be next?
Your friends might be part of a disease outbreak.
An outbreak is a higher number of cases than expected in a location within a certain time period.
An outbreak often requires further investigation.
During an outbreak, a team of many different public health experts work at identifying the source
and how the disease is spreading,
finding ways of protecting the community from the health threat,
and communicating their findings to people and groups who can use the information
to better protect themselves and others.
Like a team of scientists who use the scientific method to conduct an experiment,
a public health team uses a series of steps to investigate an outbreak
as quickly and efficiently as possible.
These steps often, but not always, follow a specific order.
The steps in an outbreak investigation address key details, like:
“What is the problem?”
“What is the cause of the problem?”
and “What can we do about it?”.
Thinking back to your friends who are sick,
this outbreak investigation would start with “What is the problem?”
Preparing for fieldwork involves doing research.
During this investigation, Ezra, an Epidemiologist,
would research possible causes of gastrointestinal symptoms and generate a list of possibilities,
including known foodborne infectious agents and how they spread.
Along with research, Ezra would collect the required materials and tools,
and assemble the appropriate team to conduct the investigation.
Ezra must also establish, or determine, that an outbreak is occurring
by comparing current surveillance numbers to what is expected.
These surveillance data are collected via reports from hospitals,
healthcare providers, and laboratories.
At any given time, there will be an expected number of people with gastrointestinal symptoms in an area.
But if the disease pattern changes or this number suddenly increases beyond the expected amounts,
this may be the first indication of an outbreak.
So, Ezra needs to know how many people developed nausea and vomiting this week
and compare that with the weekly numbers from previous years.
Other factors that might prompt an outbreak investigation include the speed that the disease is spreading
– especially if it is being passed from person to person -
the severity of symptoms, and the need for immediate prevention strategies to stop the spread of disease.
The next step is to verify the diagnosis.
Laboratorians like Lily can identify the infectious agent causing the illness
and confirm cases with a positive laboratory test.
It is important to test as many suspected cases as possible
to make sure symptoms are being caused by the same infectious agent.
In the case of your sick friends, Lily would conduct laboratory testing on stool samples
submitted to her laboratory from each friend.
If the samples tested positive for a foodborne infectious agent, like E. coli O157:H7,
she would verify a diagnosis of food poisoning for each friend.
Lily can help confirm that an outbreak is occurring when she sees many similar test results,
like If all your friends test positive for the same foodborne infectious agent.
Early on, the outbreak investigation team constructs a working case definition.
Case definitions can vary based on the purpose of why the cases are being identified.
In an outbreak investigation, the case definition is a set of criteria:
who – person, where – place, when – time,
and what – clinical features and laboratory findings,
to consider when determining if a case of disease should be included
as part of the outbreak being investigated.
In this outbreak, the case definition is:
anyone who experienced vomiting, diarrhea, and abdominal cramps,
with a confirmed laboratory diagnosis of E. coli
with illness onset since the 15th day of last month in your community.
“Anyone” is the who, the symptoms and confirmed laboratory test is the what,
since the 15th of last month is the when, and your community is the where.
Case definitions can vary from outbreak to outbreak and can change as the investigation unfolds
– more information helps public health experts refine and improve the criteria as they learn more.
Using that case definition,
the team can systematically search for more cases and record information about them.
The team can contact physicians, laboratories, and sick people
to ask for case information.
At some point in their investigation,
they may also reach out to potentially ill persons through the media or other records.
Pavati, a public health nurse, may contact persons identified as confirmed or suspected cases.
Pavati would ask questions about possible exposures,
such as places they have recently visited, foods they have recently eaten,
and whether they know anyone else who is sick.
Exposures are things that could have caused the infection.
She may also interview people who were not sick
to compare exposures for later analysis and to help find associations.
The team would collect and organize this data into a line list.
Line lists help organize information about infected persons,
like demographics, case information, and possible exposures.
Bella, the team’s biostatistician, analyzes the line list data
as she performs descriptive epidemiology.
This means she is looking for patterns in the data to characterize, or classify, the outbreak by:
person - who was affected, place - where the cases occurred, and time -when they occurred.
Bella also creates maps and tables to help the team identify patterns and trends of the disease.
She also creates an epi curve, a histogram that displays the number of new cases over time.
The team can then use this information to determine the extent of the outbreak
and decide where to apply control measures.
It can also provide clues to what the cause of the outbreak might be.
Although the team has been thinking about possible causes from the beginning of the investigation,
the next step is to develop hypotheses about the source of the outbreak.
What are the usual sources of E. coli O157:H7 food poisoning?
Are the cases clustered, or grouped, in one area?
Or is only one age group affected?
What do the sick people think might have caused their illness?
Analyzing the patterns and answering these questions will help the team determine,
“What is the cause?” of the outbreak.
These hypotheses can be evaluated in different ways.
One way is to use analytic epidemiology to evaluate hypotheses epidemiologically
or quantify the associations among different exposures and disease.
For example, many people who were sick went to the same school
and ate at a new burrito restaurant in the last week.
Using statistical analysis and information from a comparison group of people who did not get sick,
Bella can measure the association between an exposure
- such as going to school or visiting the burrito restaurant-
and disease -such as having E. coli food poisoning.
These calculations can tell us that there is not a strong association
between going to the school and getting sick with the disease.
So, the team may go back and test other hypotheses,
like if people were more likely to get sick with E. coli food poisoning if they ate at the burrito restaurant.
Of course, if Lily can find the same strain of E. coli in a food at the restaurant,
then the lab and epi findings support one another.
This reconciliation of the epidemiology with laboratory and environmental findings
would make support for this hypothesis even stronger!
If people were not more likely to get sick after eating at the burrito restaurant,
then the team will reconsider and refine hypotheses and conduct additional studies as necessary.
Although listed as one of the later steps in an outbreak investigation,
public health experts implement and evaluate control and prevention measures as needed
throughout the course of the investigation.
After confirming that people were more likely to get sick with E. coli food poisoning
after eating at the burrito restaurant, the team, including Ivy, an Infection Prevention Specialist,
contacted the restaurant and made a field visit to conduct an inspection
to identify potential problems with food storage and handling,
collect environmental and food samples for testing, and make recommendations.
The team then looked at the line list
to see what the people ate at the burrito restaurant to look for clues.
Using statistics, Bella identified the likely source of the outbreak to be the lettuce.
Lily confirmed this in the lab using the samples collected during the inspection of the restaurant.
The team contacted the restaurant to let them know that the likely cause was the lettuce.
Ivy had noticed some problems with storage and handling of lettuce and other food items
during their initial visit and had made a series of recommendations concerning those practices.
The restaurant had been closed as soon as it was found to be linked to this outbreak.
Before it could reopen, Ivy made a follow-up visit to ensure that their recommendations were being followed,
that changes in storage and handling were in place,
and that staff would receive periodic teaching in safe food handling.
Prevention strategies can be very different depending on the outbreak.
The food poisoning scenario shows a single point of infection—the lettuce at the burrito restaurant.
But there are other types of outbreaks.
If an outbreak involves person-to-person transmission,
we would need to take into consideration how people interact with each other.
For example, with COVID-19, public health outbreak teams conducted contact tracing,
which includes identifying, monitoring, and supporting close contacts
who have been exposed to, and possibly infected with, the virus
by interacting with someone who has a confirmed case of the disease.
One way COVID-19 is spread is through respiratory droplets,
so prevention measures like wearing masks, socially distancing, and increasing ventilation
were encouraged and implemented in many areas.
Brandon, a behavioral scientist on the team, could provide valuable insights
about human interactions while someone is ill,
and how public health messages could encourage people to take the actions recommended by public health experts.
To ensure prevention measures are working,
the outbreak investigation team will want to initiate and maintain surveillance systems
to continue collecting data to see if the number of cases is decreasing
and if the spread of disease is being controlled.
To keep the public informed, Harper, a Health Communication Specialist,
communicates findings to the public and encourages anyone experiencing symptoms
to contact the health department or a healthcare provider.
They can do this in a variety of ways,
like a spot on the local news as a public service announcement,
through social media posts, or even on the radio.
As a quick recap, we can answer the question, “How is an outbreak investigated?”.
Outbreak investigation requires communication and collaboration of people
with specialized skills to investigate cases, determine the cause,
stop the spread of disease, and keep the public safe.
An outbreak investigation is a multistep process focusing on three key areas:
“What is the problem?” is addressed by using a case definition to collect data on cases–
who, what, where, and when.
“What is the cause?” is addressed by developing and evaluating hypotheses
on how or why the outbreak might be happening.
Finally, “what can we do about it?” is addressed by implementing control
and prevention strategies to control the current problem and prevent the spread of the disease.
Continued monitoring and surveillance are needed to confirm that prevention measures are working,
and disease spread is being controlled.