Epidemiology: Key Concepts
Epidemiology is one of the most important elements in the formal scientific study of disease transmission and prevention, yet few natural health students and professionals know how to maximize the use of this valuable tool. Epidemiology literally means the study of affliction upon a population. It’s the branch of medical science which focuses on the spread of disease within a population, the risks connected to that disease, and the factors which influence those risks.
Epidemiologists are public health professionals. Public health differs from medicine in that a doctor or nurse has a single patient at a time. They work to identify and diagnose illness in that patient, treat and care for the patient, and hopefully, help the patient overcome the disease. In public health, however, the patient is the entire population. Public health professionals work at the macro level to save lives throughout the community, and their responsibilities include providing tools and education for primary care providers to benefit from the latest advancements.
While a career within the field of epidemiology requires advanced post-graduate college training, the basic core of the science can be mastered by individuals and health professionals of all backgrounds to better understand outbreaks within society and to determine personal risks and effective preventive actions.
An Epidemiologist’s Role
Epidemiologists perform a lot of goals to accomplish this task. They engage in public health surveillance where they monitor health conditions within the given population. They conduct field investigation more commonly referred to as shoe leather epidemiology, where they investigate outbreaks or other concerns by gathering specific data about a case. They also conduct analytic studies, where they assess and identify effective prevention measures. They conduct evaluation, where they determine the effectiveness of a program or health intervention. Because epidemiology is a multi-disciplinary field, they work with statisticians, care providers, and laboratory researchers. They also engage in policy development, where the conclusions of their work are used to protect the public through better health related policies.
They engage in public health surveillance, where they gather, analyze, and utilize information about what is happening in a community in terms of disease. They monitor which diseases (both infectious and chronic) are prevalent within a population, and they evaluate these rates over time to catch any deviations from that norm. They are constantly collecting and analyzing data about thousands of health outcomes ranging from emergence of new infectious diseases to general data such as overall mortality rates. This information is gathered from local public health departments, who obtain the information from primary care providers.
History of Epidemiology
The study of disease impact on public health is as old as disease itself. However, the modern epidemiology movement can be traced to a London physician and anesthesiologist by the name of John Snow. In one of the most important medical events to occur, he is credited with establishing the field and the widespread acceptance of the new germ theory of disease in place of the prevalent miasma theory. He is referred to as the Father of Epidemiology. Both of these events can be traced to an incident that occurred late summer 1854 in the Soho area of London.
At the time cholera outbreaks were relatively commonplace and a London based physician/anesthesiologist suspected that the outbreaks were caused by something other than the stench of living in poverty. He proposed that they may be infectious due to transmission of microbes in the water. When the 1854 outbreak occurred, he was successful in documenting the spread of the illness and linking the cases to a certain water pump on Broad Street. He successfully had the handle to the pump removed, preventing further infections and saving the lives of many. While he was mocked by contemporaries, his work continued to document the actual transmission of the illness and his germ theory of disease became accepted over the next decade. Now the events surrounding the Broad Street pump are memorialized in London and the famed London School of Hygiene and Tropical Medicine, one of the most prestigious public health universities in the world, retains the pump handle on display in the lecture hall which also bears his name.
Epidemiologic Triad
John Snow’s achievements utilized the approaches outlined previously, though these specific components of epidemiology clearly did not yet exist. The development of the field continues to follow the basic actions he took, however, particularly in the identification of risk factors and evaluation of disease transmission. This is accomplished through a model known as the epidemiologic triad.
The triad consists of three key components: the agent (generally a microbe), the environment (surrounding the situation), and the host (usually a human being.) This model dominated the field of epidemiology through the late 1800s and early 1900s as infectious disease was the most common cause of mortality. During the mid-1900s, when chronic disease began to become more prevalent, additional models were introduced, which are more applicable. However, the original triad has not changed and is still the best existing model for assessing infectious disease.
The core of the model is that all three factors must be present for disease to take place. Most members of the public focus exclusively on the germ theory of disease, which overemphasizes the agent. However, the agent is powerless without a suitable environment for transmission and a susceptible host. If the host is immune or the environment is hostile for transmission, disease transmission will not occur.
There are many examples where only two of the three factors are present and disease transmission does not occur. If there is a susceptible individual in the right environment, such as a hospital, but microbial exposure does not occur, the individual does not catch the disease. Similarly, if the microbe is present and the environment is right, but the individual is immune, transmission will not occur. Finally, if the microbe is present and the individual is not immune, disease transmission will not occur if the overall environment is not suitable and transmission is not possible.
For infectious disease to spread within a home or within a country, all three factors must be aligned. An infectious microbe must be present, the individual must be susceptible, and the environment for transmission must be right. Therefore, interventions to reduce the spread of illness can include stopping the occurrence by affecting or altering any one of the three components, or a combination thereof.
A significant amount of attention is usually given in conventional health towards elimination of the agent with hand sanitizers or antimicrobials. Yet, this is an unsustainable method. Microbes are throughout the environment and are usually beneficial. A balanced approach to prevention, based upon the most up-to-date epidemiological research, involves modifying all three factors of the triad, whenever possible, to prevent disease transmission.
Outbreaks, Epidemics, Pandemics
When disease does begin to spread, epidemiologists use specific terms to refer to the level the event. An outbreak is the first level of an event. Something is deemed an outbreak if it occurs in an area where it is not expected or when it occurs at rates which are higher than those anticipated based upon historic precedence. For example, when chikungunya (a mosquito borne illness) first appeared in the Western Hemisphere in late 2013, the first cluster of cases were deemed an outbreak. When measles cases appear in larger numbers than usual, it is also an outbreak. So an outbreak can consist of just a few cases or of a seemingly large number of cases, depending upon the population’s norms.
When the outbreak grows to larger numbers, it is termed an epidemic. There are no clear definitions that determine whether an occurrence is an outbreak or an epidemic; it is defined through comparing proportions of cases in the community based upon expected cases, not through a total number of overall cases. Typically, however, to become an epidemic, the outbreak(s) must all be linked and begin to grow outside of the initial region or area. The 2014 – present ebola situation in Africa is an example of an epidemic as it has spread across several countries and is the largest ebola outbreak recorded in history.
If the epidemic expands to a point where it occurs around the world, it becomes a pandemic. An example of a pandemic is the 1918 Spanish Flu. When a pandemic occurs, global leaders must work together to promote public health to reduce transmission throughout the world.
When a disease remains active in an area for several years, it is referred to as endemic. This is a primary cause of concern with the ebola crisis. In previous years, ebola was restricted to small outbreaks which occurred annually. If ebola cannot be maintained and the epidemic cannot be treated, ebola could remain a steady problem as an endemic disease, which simply means that it remains active in the given areas.
Case Definitions
One of the first roles epidemiologists have during an outbreak is to pinpoint the origin of the outbreak or epidemic. This is referred to as the index case. For example, the cholera outbreak which John Snow researched was ultimately linked to a sick baby whose dirty diaper had been rinsed and dumped into the ground near the Broad Street pump. Through a leak in the pump casing, the microbes spread into the large water source and infected hundreds of individuals throughout SoHo within a matter of days. Identifying the index case provides important information which is used for future prevention. The index case may also be referred to as the primary case or patient zero. (hint: Index case is more common among epidemiologists; patient zero is more commonly used in the media and in popular entertainment.)
From there, the index case’s personal contacts are identified. In the early ebola outbreak, for example, epidemiologists scrambled to identify each person who had contact with the patient from the earliest point in which he could have been contagious. These are referred to as contacts. If contacts become sick, they are secondary cases because they became sick as a result of contact with the primary case.
It’s also important during an investigation to have a clear definition for cases. Outbreaks don’t take breaks to allow researchers to catch up with their progress, so epidemiologists must distinguish between confirmed cases, suspected cases, and probable cases. Not everything which resembles the disease is going to be the disease and one of the first rules of any scientific investigation is to assume nothing. It may be easy to assume that the individual who is sneezing and coughing is a pertussis secondary case if they are siblings of the index case, but it is also possible that the sibling simply has a cold. Epidemiologists can’t run the risk of classifying all colds as pertussis, so different terms must be used to ensure accuracy.
To begin, a case definition is developed, if one does not exist already. This clarifies which criteria are used to distinguish between the cases. If the patient meets all of these criteria, they are confirmed cases. In most infectious disease studies, these require laboratory confirmation. A suspected case is simply one in which the disease is possible, and a probable case is one which meets many of the criteria but not necessarily all. For example, in an infectious disease outbreak, a confirmed case likely has all of the symptoms of the illness plus laboratory confirmation of the microbe. Someone who has all of the symptoms, but has not yet been checked for laboratory confirmation may be a probable case. It is likely that the tests will confirm the illness and they will become confirmed cases. Someone with most of the symptoms, but not all, may be a suspected case. This may turn out to be something else, it may be the illness in its early stages, or it may be a milder case. These individuals can turn into probable or confirmed cases, but they may not.
These definitions enable epidemiologists to clearly communicate the severity of a situation with accurate details. Remember that suspected and probable cases are not to be discarded; epidemiological research is ongoing, so cases must be reported factually and this includes reporting on information which is not yet completed.