[Home] [Search] [Discussion]
MODULE 9
Epidemiology:
The Foundation for Community Health
By: Dr Peter Tylee
On successful completion of this module you will be able to:
This module will provide a brief introduction to epidemiology which is widely described as the science of community health. Hopefully you will appreciate that the value and importance of epidemiology should not be discounted simply because it has been left until last. Given that something must occupy the position of final module, it could be seen as a position of prominence, like the back page of a newspaper!
Perhaps this is a completely unnecessary question for readers so far advanced in nursing studies. One could certainly be forgiven for seeking clarification however, so perhaps this will seem a quite reasonable question to which you would appreciate an answer. Some take a quick look at the word and assume that it refers to a branch of dermatology! Some think it is all about the study of the spread of infectious diseases. Others, disappointingly including works as recent as Wass (1994), retain the old-fashioned view that it is essentially confined to the study of disease in populations.
Spradley and Allender (1996, 260) define epidemiology as "the study of the determinants and distribution of health, disease, and injuries in human populations". This definition responds to the major developments within the field of epidemiology which have shifted the emphasis somewhat more in favour of health, rather than its traditional focus on disease. To be fair, the traditional definitions dominated the discipline for a long time, which should not be surprising. It is important to realise though, that epidemiology is not the exclusive domain of the medical profession. Indeed, Lilienfeld and Lilienfeld (1980, 4) provide the following within their rather traditional definition of epidemiology. They say epidemiology is:
| a sequence of reasoning concerned with biological inferences derived from observations of disease occurrence and related phenomena in human population groups. ...[It] is an integrative, eclectic discipline deriving concepts and methods from other disciplines, such as statistics, sociology, and biology, for the study of disease in populations. |
The phrase "integrative, eclectic discipline" is a key to epidemiology and interestingly enough, could be applied equally well to nursing.
In more recent times, perhaps the last ten years or so, with the widening acceptance of positive definitions of health, epidemiology has been influenced to clarify just what health is in populations and then to identify what its determinants are. Since so much was already known about disease, much of this could be derived from inferential reasoning. Still, it must be acknowledged that the vast majority of material documented by epidemiologists has pertained to disease. This documentation has traditionally focussed on three important sets of variables; those related to:
| 1. time | when and over what period |
| 2. place | where (and the characteristics of the place or places) |
| 3. person | who (and the characteristics of those affected or not affected) |
Examples of the types of 'person' variables (or personal characteristics) documented could include:
The remainder of the module extends the answer to the question of just what epideiology is, so let us proceed.
3.0 Traditional Aims & Activities of Epidemiology
The traditional aims and activities of epidemiology can be explored in a variety of ways. One way is to consider the main uses to which epidemiological information is put. These include:
The first of these, aetiological studies, that is studies concerned with identifying the causes of disease, have been extensive and have been conducted in large number. They range from the individual case to whole population levels of focus. Most of what is now understood as part of the so-called natural history of a disease has been learned from these studies. This has important clinical application. For example, consider the following concerning measles.
| Measles is caused by a morbillivirus transmitted by droplets through
conjunctiva or respiratory mucosa. Passive immunity from the mother prevents infection in
the first three months of life, but thereafter the child becomes highly susceptible. About 10-14 days after exposure to infection the prodromal illness develops. This is similar to a common cold, with fever, running nose and eyes, sneezing and cough. Examination of the mouth, however, reveals an eruption of tiny white spots like grains of salt set on a slightly reddened base, usually best seen on the mucose membrane inside the cheeks opposite the molar teeth. These are Koplik's spots and they are diagnostic of measles. If they are overlooked the erroneous impression that the child simply has a cold may appear to be confirmed when the temperature may come down to normal, but on the fourth day the typical rash appears on the skin. Transient prodromal rashes are sometimes seen during the first three days, but the true morbilliform eruption appears on the fourth day as pink macules, about 3-5 mm in diameter, which first appear behind the ears and spread over the face, trunk and limbs over the next two days. Within a day or two the lesions enlarge and become papular, many of them coalesce into large, irregular, blotchy areas, and their colour gradually changes to a darker red. The temperature rises again with the appearance of the rash and continues for several days before finally subsiding as the lesions fade. The Koplik's spots disappear as the rash is developing. The child is infectious from two days before the prodrome until 'staining', when the red rash changes to an orange-brown colour. Lymphadenopathy may occur. Encephalitis is sometimes seen in the prodromal period, and in 0.1 percent of cases around the third day of the rash, when it may leave permanent neurological sequelae. (Rees & Trounce, 1988, 416) |
This very useful natural history of the disease, which charts a course enabling correct clinical and public health management (such as the isolation period) has been developed by epidemiological studies.
Studies involving the cross-reference of epidemiological aetiological hypotheses with clinical sources of data are also revealing of important information. Consider the following study of the link between use of oral contraceptives and death by myocardial infarction.
| ORAL CONTRACEPTIVE PRACTICE | PATIENTS WITH MI | CONTROLS | ||
| N | % | N | % | |
| Never used | 78 | 73.6 | 86 | 84.3 |
| Current users (during month before death) | 18 | 17 A | 7 | 6.9 B |
| Ex-users (used, but > 1 month before death) | 10 | 9.4 A | 9 | 8.8 B |
| Total | 106 | 100 | 102 | 100 |
| Not known | 2 | 8 | ||
A total = 28 (26.4%), B total = 16 (15.7%) X2 = 4.35 p < 0.05 (Lilienfeld & Lilienfeld, 1980, 8)
This study of women aged 40 to 44 years found a significant associative link between use of oral contraceptives and death by myocardial infarction. It lead to an almost immediate modification of the contents of "the pill" and the so-called mini pill was released in its place.
The final activity, related to developing preventative procedures and public health practices, can be illustrated by reference to community trials (so we don't overlook this level of focus). These trials involve whole communities - either animal or human - as the "experimental unit" and therefore don't generally allow randomisation. Consider the following study conducted in California in 1972.
The study sought to modify:
by community education. Three communities were identified and treated as follows.
After two years communities 1 and 2 had significant positive health outcomes. In comparison with the control community there was an estimated decrease in the risk of developing cardiovascular disease of almost 25%. This finding played an important role in the introduction of mass media based public health education.
The aims and activities can be further explored with a review of epidemiological reasoning and types of studies.
4.0 Epidemiological Reasoning & Types of Studies
Epidemiology is a comparative discipline which, as we have already noted, documents data in terms of time, place and person. This documentation is performed during a wide range of types of formal studies which could be classified as one of the following four broad categories.
The last of these is not for the "mathematically challenged". It involves the development of theoretical models, often involving advanced mathematics and analysis requiring the power of super computers. The results however are very practical. Such information as the likely time and place of initial occurrence and pattern of spread of various infectious diseases, such as influenza (remember the recent Hong Kong flu?), together with its predicted virulence plays a vital role in public health planning.
Another way of reviewing epidemiological reasoning is to consider a classification of epidemiological research studies. The following diagram relates this quite succinctly.
Diagram 9.4.1 Classification of Epidemiological Studies
No doubt some of these look familiar to you from your earlier studies of research methodologies.
Arguably the strongest expression of epidemiological reasoning is expressed in the Host, Agent and Environment Model which is virtually synonymous with the discipline. These three components could be thought of as the three essential elements of any disease condition, in ways analogous to the three elements of fire. We know the three elements of fire to be:
If we want to modify or eliminate the fire we can do so by modifying or eliminating any one or more of these three elements. For example, we can put out a fire by cooling it to below flash point (the temperature required for ignition in any given combustible), by starving it of fuel, or by eliminating its oxygen supply. In a similar way, any disease is made up of factors associated with the:
If we want to modify or eliminate the disease we can modify or eliminate any one or more of these. The model has been put to good use and has found its way into nursing practice in various ways. Finnegan and Ervin (in Spradley & Allender, 1997, 186-193) applied it to an approach to community assessment. Lauzon (1977) successfully applied it to an approach to health promotion which is still widely cited (it is reprinted in Spradley & Allender, 1997, for example). Before we go on to further consider the relevance of epidemiology for nurses, complete the following activity.
Activity 9.4.1
Application of model here
(Unfortunately a section was lost from here in this Module. I authored this directly on the university's server from home and failed to keep a completed copy. On leaving the university I omitted to update this copy.)
Hopefully the relevance of epidemiology for nurses is self-evident. We might summarise it as follows:
Epidemiology is especially useful at the primary prevention level in:
ie the epidemiology of health and disease becomes the basis for practice.
Understanding disease in secondary and tertiary prevention.
Performance of one's own studies.
Involvement in data gathering for studies principally conducted by others.
Development and documentation of our understanding of HEALTH.
Further discussion of the application of epidemiology may be addressed in the following activity.
Activity 9.5.1
| Read
either one or both of the following. Chapter 12, Epidemiological Assessment of Community Health Status in Spradley & Allender Chapter 9, The Epidemiological Model Applied in Community Health Nursing in Stanhope & Lancaster |
6.0 Development of Health Models
Another contribution made by epidemiology has been the development and articulation of evolving models of health. We are indebted to Dever (1977) for the basis of much of the following discussion and for the diagrams. In a sense this is a full cycle. We addressed the concept of health early in this subject, noting that it is indeed the central concept in primary health care. We now conclude with a brief review of how the prevailing model of health, which is always very important for setting and reflecting health practices, has changed over relatively recent times.
The first model emerged during the late 1800s and early 1900s. During this period the main problems faced were infectious diseases and the prevailing medical opinions on 'health' reflected the Germ Theory of Disease. At this stage a simple single cause > single effect model was in operation, described as:
The Ecological Model
This model understood health as an ecological balance, where a disturbance of the balance resulted in disease. Health was measured and described in terms of:
By the period of the 1920s to 1940s greater complexity in the model was required and the:
were developed. The result was the development of:
The Social Ecological Model
This model basically replaced the agent (germ) with personal behaviour factors. The same concept of 'balance' was retained and the parameters of health were still:
From the late 1940s through to around 1980 the idea of the illness - wellness continuum grew in significance and various holistic models of health emerged. These models were useful for all diseases and disorders including complex and chronic ones (such as the so-called lifestyle diseases) as well as infectious diseases and accidents and injuries. The model that emerged was:
The Environment of Health Model
This model introduced an interest in measuring health in terms of holistic wellness and stimulated progress in actually identifying and documenting the determinants of health.
It is interesting to note that the relative size of the arrows aiming at wellbeing is intended to reflect the relative contributions of the various factors to achieving health outcomes. Notice that health services contribute less than behaviour, as you might expect, and less than one third the importance of environmental factors. This is food for thought.
There are no further self-study questions or activities to complete. This concludes the modules on Primary Health Care. Practice the principles of PHC and best wishes with your further studies.
Use this link if you want to send e-mail to the Subject Coordinator. (removed)
[Home] [Search] [Discussion]