Today we are going to discuss the epidemiology of human disease. This lecture will be a little different from previous lectures, in that it is much more technical and descriptive. We are primarily going to focus on the fundamentals of epidemiological studies. What are they? How are they conducted? Why are they useful for assessing human variation? There is a great deal of historical accounts that have gained much attention from epidemiologists assessing the spread of disease throughout the human population. The actual field of epidemiology began in the 20th century when epidemics were rampant in western Europe.
The discipline or epidemiology is defined as the study of how, when and why diseases occur. So, epidemiologists try to determine what causes a particular disease, who is most likely to contract a disease or a condition, why a condition occurs in a particular area, and if it is contagious then how does it spread? What are the best ways to control the disease? Anthropologists have a different role in assessing diseases in that they are more concerned with the interplay of biology, culture, and environment. This is really what we are more focused on for this lecture, that is how culture shapes disease. For example, some questions that anthropologists may be investigating is How does culture shape disease? How do different activities and social networks between population groups impact the transmission of disease? An example would be anthropologists investigating a Southeastern Asian population that interacts with domesticated fowl and how it impacts their chances of contracting influenza? There is of course an interaction between the disciplines of epidemiology and anthropology that dates back to at least the 1950’s when medical anthropologists worked with epidemiologists to understand psychiatric disorders. In general though, most epidemiologists have a stronger training in biology and public health. They typically approach their assessment of human disease through a statistical analysis and other quantitative approaches. On the other hand, the training of most medical anthropologists focuses more on culture and the health-related behaviors of humans within different cultures. Anthropologists rely on qualitative ethnographic understandings.
So, while I want you to recognize the differences between these 2 disciplines, also keep in mind that often times it is these 2 disciplines working together that provides the best assessment of disease in the human population. So for today we are going to cover two major areas of discussion. First of all, I want to discuss some fundamentals of epidemiological research.
Here, we will discuss some terminology, how diseases are classified, and the various recognized modes of transmission for diseases. We will then discuss epidemiological data. How are epidemiological studies conducted and/or designed? What are the different types of epidemiological studies? We will end the lecture with a discussion of the various ‘categories’ of studies and I’ll provide a brief example for each. We are going to start with some basic terminology. The most important terminology for epidemiologists are infectious and noninfectious disease. Infectious disease is often referred to as communicable diseases and is caused by specific infectious agents or their toxic counterparts. Infectious diseases are recognized as those that can be transmitted from one person to another through the agents or toxins, whether directly or indirectly. Many infectious diseases are also understood to be acute and relatively short-lived, however, this isn’t a criteria that can be used to separate infectious diseases from others, as there are definite examples that are recognized as chronic (AIDS, TB).
However, more often than not, many infectious diseases are acute and have a short duration. Noninfectious diseases are those that that have an environmental or genetic cause. These include most kinds of cancers, genetic diseases (like Tay-sachs or sickle-cell anemia), nutritional diseases (like vitamin D deficiency, iron deficiency) and allergies. What is interesting is that a lot of conditions that were once thought to be noninfectious in origins are now understood to be caused or at least strongly influenced by infectious organisms. For instance, many viruses have been implicated in the developmental of many leukemia’s. Another example of this is the human papillomavirus (HPV) which has been linked to cervical cancer. So, the boundaries between defining infectious versus noninfectious diseases has become blurred through the years.
Just to give you more examples of noninfectious and infectious diseases, I wanted to show this chart. You will notice that noninfectious diseases are all those that are not related to some type of infectious organism or toxin. There are many different causes for noninfectious diseases, but it is often the case that the disease is caused by the interaction of two or more causes. Also, keep in mind that even though a disease is currently viewed as one that is noninfectious, this doesn’t mean that in the future there will be an infectious cause for them identified (which has often happened in the past). For the most part, though, you can see that many noninfectious diseases are related to environmental factors, while infectious diseases are due to some type of toxin or infectious organism, whether a bacteria or virus. This is a table that shows various risk factors that can influence the existence of infectious and noninfectious diseases. Obviously, the concept of ‘risk’ will differ according to who you are talking to (a doctor versus an individual in the population). Risk factors are divided into those that are considered to be ‘host’ factors, particular to the individual.
In other words, these are things related to differences in age, sex, culture, or the individual’s underlying level of health. The other category of factors are environmental, those risks factors that are related to environmental differences. When a disease, whether infectious or noninfectious, is present in a population at a relatively constant level at all times, it is referred to as endemic. The term endemic refers to this occurring in a population at low levels.
When the number of cases increases above the expected normal level, then the disease is said to be epidemic. So, then the extreme of this is when the number of cases occurring worldwide suddenly increase, then the disease becomes pandemic. The total number of cases of disease in a given population during a particular time period is referred to as the prevalence of the disease. The number of “new” cases during a particular time period is referred to as the incidence of disease. So, going back to the last slide of terms, an epidemic or pandemic occurs when the incidence of a disease suddenly increases. For an endemic disease, there is little or no change in the incidence over time. Prevalence rates, though, can be high or low in all three situations (an endemic, epidemic, and pandemic). There are also various terms that are associated with the progression of infectious diseases.
There are several stages, that very in terms of length of time, depending on the particular disease. The incubation period is marked by the time from infection to the development of symptoms. The latent period is defined as the length of time between infection and the ability to infect someone else.
Typically, the incubation period is longer than the latent period and will usually end before symptoms actually occur. This is why disease can easily spread, because the incubation period (when you contract the disease and your body takes a few day to be impacted by it) will mask any symptoms, so that by the time you know you are even aware that you are infected, you have likely passed it on to someone else. It’s also interesting though that often times, depending on the disease, the infectious period may end before symptoms disappear. In other words, the individual could be portraying symptoms, but no longer able to pass the infection on to someone else. So, the end of the latent period signals the beginning of the infectious period, while the end of the incubation period signals the beginning of the period of symptomatic illness. Sometimes, a person may enter a state of immunity (temporary or permanently) at the end of the infectious period. Another thing not evident in this figure is that death can occur at any time during this process.
There are 2 recognized modes of transmission for infectious diseases. Direct modes are those where transmission occurs directly from one primary host to another. The most familiar direct mode of transmission is respiratory transmission. This is when infectious organisms are spread through the air (cough, sneeze, or even breathes). It only occurs if the person is in close enough contact with an infectious person to breathe the droplets released by that person. This, of course, is directly related to differences between cultures simply because respiratory infections are more likely to occur when large numbers of human live in close association with one another on a regular basis. There are some diseases that can be transmitted from a mother to her unborn offspring (either in utero or at time of delivery).
This is referred to as vertical transmission. When these diseases attack a newborn or an unborn fetus, they will often be fatal and can sometimes impact the child indefinitely. For instance, if a mother contracts rubella during pregnancy, the child will be born with congenital rubella syndrome (which has a number of symptoms including blindness, deafness, and mental retardation). The other mode of transmission is indirect. Indirect modes include an intermediate host or agent to facilitate transmission between hosts. These intermediate agents can include things like needles (through vaccination or drug use), water, soil, or food (which is often shared among individuals). These can also include living vectors like mosquitos, ticks, fleas, and lice.
So, as an example, one thing we have talked about this semester is malaria, which is transmitted through mosquitos. Now that we have covered some of the basic terminology of epidemiological studies, I want to now move into a discussion of the data, methods, and applications within epidemiology. There are currently two sources of existing epidemiological data.
The first source is that of vital statistics on morbidity. Vital statistics is defined as the systematically tabulated information on the number of vital events in a population. This includes births, deaths, marriages, and divorces/separations and is based on official registration of these events. Obviously, epidemiological studies wouldn’t be possible without knowing this type of information for a population. The earliest evidence of the actual collection of this data began in the 1500s in Western Europe, and we don’t really see it in other parts of the world until the mid-20th century. The earliest types of registration of births, deaths, and marriages comes from church membership records. For the most part, these types of records are now kept by the World Health Organization (WHO).
The other type of source of epidemiological data are surveys and surveillance. Surveillance is defined as the regular and ongoing collection of data on the occurrence and spread of disease. These are usually large-scaled surveys that are conducted by the CDC (U.S. Centers for Disease Control) and the WHO. The CDC focuses on diseases here in the U.S., while the WHO tracks diseases throughout the world. So, as you can see, the current data is not collected by epidemiologists, but is rather a product of nurses, doctors, and religious authorities who are responsible to properly collect and record the data on appropriate forms and submit to the appropriate agency.
This also means there are areas for sources of error and also there are regions of the world where records are scarce and the recordings of diseases or epidemiological data just doesn’t occur. So, for any type of epidemiological study, the quality of the data must be taken into account before beginning any research on particular populations. There are 2 major types of epidemiological studies. The first is an observational study. This is kind of self-explanatory, but this is a type of study that relies on careful observations of the development of a disease or observations of the presence or absence of risk factors. Observational studies can be further divided into descriptive studies or analytical studies. Descriptive studies are concerned with identifying and describing the risks associated with disease and/or the exposure to disease. So, these type of studies do not have any specific hypotheses that are being tested, but are only concerned with describing what exists, hopefully generating hypotheses that can be tested.
Analytical studies are concerned with the actual analyzation of data to investigate whether associations between exposures and disease. So, analytical studies are those that are testing specific hypotheses. The other type of epidemiological study is experimental. This is when the researcher will manipulate one or more factors to see how it impact the development of disease. You can see here, that these 2 major types of studies (observational and experimental) produce different types of research (ecological, cross-sectional, cohort studies, and case-control studies). I want to briefly describe each of these types of studies and then we will move onto discussing methodology. Ecological Studies: Ecological studies are those that focus on groups or subgroups (which is referred to as the ecological unit) rather than focusing on the individual. The end-product of this type of study is one that provides summary measures of exposure and disease for each group under study.
An example of this would be a study that assesses the level of sanitation in different parts of a city to see if or how it is related to the prevalence of diarrheal illnesses. One important thing to keep in mind about ecological studies is that the finding for a ‘group’ cannot be applied at the individual level. In other words, one can’t assume that if there is a group correlation between level of sanitation and prevalence of illness that this means that all individuals in one group follow that pattern. So, not every individual that lives in a poorly sanitized neighborhood will portray a diarrheal illness. So, the value in conducting an ecological study is that it can reveal potential patterns or relationships between risk factors and disease. Cross-Sectional Studies: Cross-sectional studies are those that assess risk factors and disease in an entire population at one particular time. In other words, it is like taking a snapshot of a population at that time. An example of this would be the CDC’s National Maternal & Infant Health Survey that was conducted in 1988.
This was a survey administered to mothers who experience livebirths and stillbirths during that particular time period (1988). The study was designed so that factors related to poor pregnancy outcomes could be assessed. This is essentially helping to identify any risk factors for a disease, or in this example possible risk factors associated with poor pregnancy outcomes. Cohort Studies: A cohort refers to any group of people that share a common characteristics or interest. For these types of studies, a cohort is divided into 2 or more groups. Most commonly, there are 2 groups: one consists of individuals exposed to a factor thought to be related to a disease under study and the other group consists of non-exposed individuals. So, lets just use the NMIHS again (the survey of mothers and infants conducted in 1988 by the CDC). If were were conducting a cohort study, then we could assess the ‘disease’ of infant death by studying the women in 2 different groups, those that adequate prenatal care and those that had inadequate prenatal care.
These 2 groups of women would be followed over time and the incidence of death for the infants would be recorded. If the factor or prenatal care is at all related to the ‘disease’ of infant death, then more people within the 2nd group would show higher exposure to the disease (in other words, there would be higher occurrences of infant death). One of the biggest issues with this type of study is that it is long-term, and therefore, many of the people that begin the study may drop out or not want to complete the study. Case-Control Studies: In case-control studies, these are essentially set-up opposite of how cohort studies are conducted.
Cohort studies use exposure forms for the basis of group membership, while case-control studies use the disease of interest as the basis for group membership. Therefore, for case-control studies the two groups under study include cases (or those with the disease) and controls (individuals that do not have the disease). So, if a particular risk factor exists, a case-control study is set-up to reveal this because the case group should have a higher history of the risk factor or it should occur significantly more in the cases than in the controls. So, for example, if we were to assess the diabetes, we would have a case group of individuals that have been diagnosed with diabetes and a control group of individuals that have not been diagnosed with diabetes.
One risk factor may be obesity. If it is a risk-factor, it would be observed more in the case group than in the control group. So, the primary purpose of case-control studies is to identify potential risk factors that are linked with a disease.
Randomized Trial Studies: The last type of study is randomized trial studies. These are most often used in experimental studies. Here, individuals are put into groups randomly, creating groups that don’t vary in their underlying characteristics. The groups are then treated differently and the reactions to treatments are observed. An example for this type of study would be looking at the effectiveness of an influenza vaccine. 2 groups would be created randomly and both groups would be exposed to influenza. However, only one group would receive the vaccine and the other would not. The value of this type of study is that it will reveal how the development of a disease progresses under certain influences.
It will also reveal if and how the disease reacts to the medication or factor being tested. I know this lecture has differed a lot from others in that it has been more technical and not primarily concerned with the ‘human variation’ relationship. However, you will be reading an article that does discuss the relationship of genetic variability to medically important differences in disease.
Just as I briefly mentioned in the beginning of this lecture, there are cultural differences between populations that have led to differences in the causes, expression, and prevalence of various diseases. So, this idea of racial and ethnic differences in the expression of various diseases will be discussed in your assigned reading for the week, and all of the details of epidemiological studies that we have discussed will apply to the article.
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