How outbreaks unfold 

ELIZABETH BRICKLEY: The first thing that typically happens in an outbreak is that we’ll notice a cluster of illness. And a cluster, we’ll define that as a higher frequency of cases than we would typically expect to see in that geographic location and over that period of time. Now, we may notice clusters either through formal or informal measures. A formal measure would be something like a clinician notices an increase in the number of cases coming into their clinic and formally reports that to the public health services. An informal notification of a cluster could arise when we have a community member who attended a dinner and noticed that a lot of people who were also at that dinner developed a certain illness afterwards.

When we think that there might be a common cause to that cluster of illness that we’re observing, we’ll think of that then as an outbreak.

When we’ve identified an outbreak, our first step is to undertake what is known as an outbreak investigation. For an outbreak investigation, we’ll want to assemble a multidisciplinary team that uses clinical, epidemiological and laboratory data to identify the source of the outbreak. You’ll learn about that later on this week in the course. One of the really important things is when we’re starting out with an outbreak is we can’t assume that it’s necessarily due to an infectious disease. We also have to consider other potential exposures including environmental contaminants and nutritional deficiencies among other potential sources.

In conducting an outbreak investigation, one of our first steps is to establish our case definition. A case definition is a specific set of criteria that we use to define whether or not the illness that we’re seeing is attributable to the potential outbreak that we are investigating. An example of this is with cholera. For example, you may have individuals who are over the age of five who develop acute watery diarrhoea that is or is not accompanied by vomiting.

When designing a good case definition, we want to make sure that our criteria are sensitive enough that we capture all of the cases that are likely attributable to the source of the outbreak but that are specific enough that we are excluding individuals who have become ill for another reason. In thinking about the specific criteria that we include in our case definition, we’ll usually start with the clinical signs and symptoms. We may, however, add additional criteria that are related to time and place. For example, you may want to exclude individuals who meet your case definition, but their cases occurred before the event where you think the outbreak may have begun.

Another common approach with case definitions is you may want to include multiple definitions within that case definition. For example, we may have individuals that we call confirmed cases if they have a positive laboratory test for the causal agent. For example, our confirmed cases could be individuals with acute watery diarrhoea from whom we were able to isolate Vibrio cholerae from their stool. We may have other individuals that we consider to be probable cases because their illness and clinical presentation is typical of the disease. However, we have been unable to confirm that diagnosis using our laboratory methods.

Once we’ve established our case definition, our next job is to find cases. We have several approaches for doing this. Our first option is to look within the public health surveillance system to see if there may be other cases that we haven’t yet detected that are connected to the outbreak. In regions with fragile health systems, we may not have reliable surveillance data, in which case, we may need to contact local hospitals, and depending on the illness, emergency or pathology departments, and ask to review their health records to see if they have come across any other suspected cases. A third option is to try to investigate other individuals who may have been exposed. We can call this contact tracing.

For example, we may know that someone who developed measles, first took a journey on a bus. So we may want to follow up with the other passengers on that bus ride to see if any of them have also become cases. After we have identified and collected data related to our cases, we will then generate hypotheses about the potential source of the outbreak and test these hypotheses using epidemiological and laboratory studies. Two of our common approaches in epidemiological studies are to use a retrospective cohort study or a case control study.

In a retrospective cohort, we are using data from a well-defined population and following them up over time to see who becomes exposed, and then who of that group that was exposed and unexposed goes on to become a case. For case control studies, we are starting with a group of cases with the disease and comparing them to a control group who did not develop the disease and trying to discern whether there is an association between the case outcome and the odds of having an exposure. For the laboratory testing, we may want to first collect human specimens.

We may also want to collect environmental samples, such as from a water source, or from a point source we may consider testing a food that we suspect could have been contaminated.

Once we’ve identified the source of an outbreak, all of our energies then go towards control measures. Our control measures will be specific to the causal agent, the epidemiologic population at risk, and what we know about its roots of transmission. For vector borne outbreaks, such as malaria, we may implement insecticide spraying. For vaccine preventable diseases, such as measles, we may set up immunisation campaigns. For outbreaks of highly transmissible or virulent pathogens, such as Ebola, we may consider isolating or even quarantining potential cases. One aspect that is common to all control efforts is communication. When we communicate with the public, we want to be very clear and specific.

We want to provide messages that tell the public how to protect themselves and what to do if they feel ill. For example, when the cluster of microcephaly cases arose in Latin America during the recent Zika virus outbreak, social scientists played a critical role in conveying to pregnant women about how they may reduce the risk of exposure to mosquito bites.

After an outbreak has been successfully controlled, our next priorities lie in surveillance and in epidemic preparedness. Epidemiological surveillance is, according to the World Health Organisation, the continuous and systematic collection, analysis, and interpretation of health related data needed for the planning, implementation, and evaluation of public health practice. Surveillance systems can either be passive systems that rely on health care providers to report cases, or they can take an active approach and look for cases that can arise in the public health system and laboratories. Surveillance is important to demonstrate that an outbreak has truly ended.

But it also serves another purpose, which is to be an early warning system and to alert us if another outbreak is on its way so that we can respond quickly. Whenever one outbreak ends, we immediately begin to think about what are the lessons that we have learned and how can we use those for preparedness. And you’ll learn a lot more about preparedness over the next two weeks of this course.