Epidemiology: Why Should I Care?

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While the following article was written with the equine veterinarian in mind, horse farm and stable owners and managers need to take heed to these wise words about epidemiology. Noah Cohen, VMD, MPH, PhD, DACVIM, of Texas A&M University, wrote this article for the University of Kentucky College of Agriculture's Equine Disease Quarterly, which goes out to veterinarians and lay persons in the horse industry.Using your knowledge of the breed, discipline, area, and even your own farm's equine population can help you and your veterinarian better understand problems that arise, and perhaps even avoid problems.

For many equine practitioners (and horse owners), the topic of epidemiology is an antidote to insomnia. Epidemiology is concerned with understanding the distribution of disease (and health) or the determinants of disease (and health) in populations. Because of the context in which it is often encountered, epidemiology generally is perceived to be about regulatory requirements, food safety and zoonotic diseases. These areas, though important, hold little appeal for practice-minded veterinarians and students. In fact, the science of epidemiology extends beyond these areas and has direct relevance to all aspects of equine practice. Epidemiology is important to veterinarians and horse owners for at least three reasons.

First, the veterinarian's (and horse owner's) best source of clinical evidence comes from studies of patients rather than from experimental models of disease (which rarely mimic the complex circumstances and manifestations of naturally occurring disease) or studies of experimental animals (which also are poor mimics for the target species). Patient-based studies may be observational or experimental; the latter are known as randomized clinical trials. The design, conduct, analysis and interpretation of patient-based studies are principal activities of modern epidemiology. Thus, epidemiology is a--if not the—basic science of evidence-based equine practice.

Second, equine practitioners, scientists and horse owners deal with communities or populations of horses on a daily basis. These populations may be defined by their location, such as farms, barns, racetracks, showgrounds or fairgrounds, etc. Communities also may be defined on the basis of breed or activity; for example, we may consider roping horses or Hanoverian horses to be a definable population that experiences health disorders that are particular or particularly common to their group. Finally, we may consider horses with a given disease status to be a community (e.g., horses with recurrent airway obstruction, those with metabolic syndrome, those with degenerative joint disease of the pastern joints, etc.). Thus, a population-based approach is inherent in daily practice.

Third, whenever veterinarians treat an individual horse (or owners manage an individual horse), we apply what we have learned about similar cases from our experiences and from the experiences of others. When I was a student, I remember thinking there was something magical about how my teachers knew that retinal discoloration could indicate a peripheral nerve disorder or that coronitis of all four limbs in a horse with weight loss and soft stool was strongly suggestive of multisystemic eosinophilic epitheliotropic disease. Although I still find diagnostic medicine entrancing, there was nothing supernatural about the knowledge and wisdom of those teachers. Their knowledge was derived from seeing and learning about horses with similar conditions, and their wisdom was manifested by their ability to apply those experiences from the population of similar cases to an individual case. In this way, our interpretations of individuals are always population-based.

Like swords, population-based studies are double-edged. On one side, they are the veterinarian's best sources of clinical evidence because their findings may be directly relevant to horses we treat. On the other side, they are derived from observations in a natural setting in which many other factors that can contribute to disease development are beyond the investigators’ control. These uncontrolled factors may lead to systematic error in the design or interpretation of results, known as biases. Much of modern epidemiology is concerned with identifying and accounting for bias. Thus, understanding principles of epidemiology is a useful investment of time to make us better interpreters of clinical evidence and thereby better practitioners. So next time you are doing some reading, consider reviewing a little epidemiology—maybe just before bedtime....