1.2: Purposes of Epidemiology

The major purpose of epidemiology is a pragmatic one; namely, to provide data on which a rational decision for the prevention and/or control of disease in animal populations can be based. In domestic animals this involves optimizing health (productivity) and not necessarily minimizing the occurrence of disease. Many medical disciplines have a similar general purpose. The special contribution of epidemiology is providing information describing the frequency and distribution of health and disease, identifying factors influencing the occurrence and severity of disease in the population of concern (in its natural setting), and quantitating the interrelationships between health and disease.

To fulfill these purposes, an epidemiologic study might be carried out to estimate the frequency of disease (e.g., the rate of infertility in dairy cows) or to identify factors that might cause the disease of concern (e.g., whether the type of ration is associated with the rate of respiratory disease in feedlot cattle). The former activity is known as descriptive epidemiology because its primary purpose is to describe what the syndrome is, who is affected, where the disease occurs, and when it occurs. The latter activity is called analytic epidemiology because the primary emphasis is on the collection and analysis of data to test a hypothesis; that is, to provide answers to why the disease occurred.

The relationship between development of disease and the operational purposes of epidemiology is shown in Figure \(\PageIndex{1}\). These operational purposes include primary, secondary, and tertiary prevention of disease. (This ordering not only represents a convenient way of differentiating among these purposes, but also reflects their inherent utility in the health care of populations. That is, society should emphasize primary rather than tertiary prevention as a means of improving health status. Health will improve only marginally by killing weeds and treating disease.) Primary prevention includes those activities directed toward preventing exposure to causal factors, particularly the complexes of factors that are sufficient to produce disease. Quarantine and vaccination arc examples of primary prevention. Vaccination does not prevent exposure to the agent but can prevent a sufficient cause from forming by rendering the animal immune to the level of challenge by the agent under field conditions.

Figure \(\PageIndex{1}\): Note: Disease may be studied with the primary objective of increased understanding or pragmatically. with intervention as the objective. Good understanding is an aid to control. but it is neither necessary nor sufficient, for successful intervention. •Monitoring productivity changes may be used to indicate the precense of a pathologic process, in the Absence of Clinical disease. Also, productivity may ilaett be a component of a sufficient cause. A sufficient cause is one that always produces the disease 1- 5.6.2).  (Copyright; author via source)

Secondary prevention includes those activities designed to detect disease processes as early as possible before clinical disease occurs. The underlying and biologically reasonable principle is that early detection will allow treatment and hence increase the probability of restoring the individual to full health and reducing production losses. Despite the reasonableness of this argument, its basis should be formally evaluated whenever possible. Screening tests to detect brucellosis and tuberculosis, somatic cell counts to detect mastitis, regular examinations of the postpartum cow, and metabolic profiles are examples of tests used in secondary prevention.

Tertiary prevention is more commonly known as therapeutics. It has been noted that for economic reasons tertiary prevention, especially in domestic animals, is somewhat of a salvage operation. However, despite the best efforts to prevent disease, it will occur (it is hoped much less frequently), and many veterinarians will continue to be employed primarily in the therapeutic role. At present, much of the time spent during a veterinarian's education is devoted to understanding the pathogenesis of disease, diagnosing disease, and instituting an adequate therapeutic (including surgical) regime. Yet, epidemiologic skills can increase the clinician's abilities at tertiary prevention. The concepts of field trials (Chapter 7) are applicable to clinical trials and the evaluation of therapeutic regimes. In terms of diagnosing disease, various forms of decision analysis (see Chapter 9) are becoming more widely used as an aid to understanding the process of differential diagnosis as well as for evaluation of alternative therapeutic strategies. Epidemiologic studies are used infrequently to study the pathogenesis of disease; nonetheless, the results of epidemiologic studies often provide indirect but useful clues about the nature of the disease process.

As shown in Figure \(\PageIndex{2}\), the period between exposure to an agent (infection) and the occurrence of clinical disease is referred to as the incubation period. Infectious agents often have different incubation periods, and this knowledge can be of value when investigating or predicting disease outbreaks. The latent period for infectious diseases refers to the period between infection and shedding of the organism and is usually shorter than the incubation period. For noninfectious diseases, it is the period between exposure to the agent and the occurrence of detectable pathologic changes.

Figure \(\PageIndex{2}\): Hypothetical relationship between production monitoring and disease states and other events. Production level can be measured in calves by weight gain or feed efficiency and after lira! calving in milk production per day. (Copyright; author via source)

As previously mentioned, high production can be a cause of disease as well as being affected, usually adversely, by the occurrence of disease (Figure 1.1 ). Monitoring productivity at the herd and the individual animal level often provides the first clue that something is wrong biologically. Hence production monitoring should be an integral component of a health management program, a feature that will be elaborated in subsequent chapters. A simplified concept of production monitoring is shown in Figure 1.2. By monitoring production, disease may be detected at an early stage; hence production monitoring is a form of secondary prevention. For instance in Figure 1.2 production decreases could have been used to predict the subsequent occurrence of calfhood diseases and/or those occurring at the second calving. The diagram also implies that level of production could be used to detect subclinical diseases (e.g., mild metritis at the first calving) as well as the occurrence of other events such as estrus.