For many equine practitioners, 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 practitioners for at least three reasons.

First, our 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 or scientists 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 we treat 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 (inflammation of the coronary band) of all four limbs in a horse with weight loss and soft stool was strongly suggestive of multisystemic eosinophilic epitheliotropic disease (The eosinophil is a type of white blood cell that responds to allergic and parasitic stimuli; multisystemic eosinophilic epitheliotropic disease is a rare disorder in horses in which eosiniphils invade and damage several of the body's own organs.) 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