Environmental and genetic factors can both play a role in disease on a given premises; however, environmental diseases usually affect a greater percentage of the individuals on a premises at one time than genetic disorders. For example, if large numbers of horses suddenly develop signs of tying up at one time, the first step is usually to investigate changing environmental factors like feeding and management. Attention is usually focused on potential genetic disease when a specific breed is affected, or when several related offspring are affected. Genetic disease may also be investigated if most horses on one premises are exposed to an environmental factor, but only a fraction of the horses develop a disease because of their genetic susceptibility.
Some genetic diseases can have a delayed onset of expression or variable penetrance, such as forms of tying up. These diseases are extremely difficult for breeders and veterinarians to recognize as heritable. For example, clinical signs of PSSM may only occur in PSSM horses with certain dietary conditions5 and the heritable basis for a major form of this disorder went unrecognized for over a thousand years.4 In contrast, genetic diseases are more readily recognized in young animals and as such the genetic mutations causing 4 lethal foal diseases (Severe Combined Immunodeficiency (SCID)6, Overo Lethal White Syndrome (OWLS)7, GBED3 and Junctional Epidermolysis Bullosa8) have already been identified.