Adverse animal health events do not occur randomly. Usually, there are identifiable causal–and possibly preventable–factors that can influence the development of disease. Depending upon whether the risk factors are already known or identified subsequent to a disease event, the management of a disease becomes basically one of either prevention or response.

For new or emerging disease conditions, it is likely that the identification of risk factors will occur only after the disease event of interest is over and a retrospective study has been completed. Consequently, these disease events tend to be prolonged and have a wider distribution among a susceptible population. As examples, mare reproductive loss syndrome (MRLS) and equine encephalitis attributable to West Nile virus initially spread unchecked due to the lack of identifiable or available interventions. Conversely, for those diseases with well-delineated modes of transmission and for which standardized screening methods exist, the disease impact associated with their occurrence theoretically should be less.

From the perspective of the Thoroughbred industry, two factors that contribute to equine communicable disease spread are population densities and the frequent movement of animals for breeding or participation at performance venues. It should be no surprise that during the past three decades, communicable disease events such as contagious equine metritis (CEM) and equine viral arteritis (EVA) occurred among horses residing in Central Kentucky. This geographic area has the highest density of Thoroughbred breeding horses in the world, and the industry is dependent on the free movement of animals for its continued economic well-being. Recently, concerns over the apparent nationwide increase in equine herpesvirus-1 (EHV-1) have prompted industry advocates to propose the development of uniform methods to minimize the risk of this disease to breeding farms.

If a communicable disease is widely distributed and causes–or has the potential to cause–significant economic loss, then the institution of a uniform method of surveillance and diagnosis may be appropriate. This is usually accomplished through mandated regulatory controls or through an industry-wide standard code of practice