Health Concerns
It is possible that the SW-2 and SW-3 alleles might produce non-viable embryos if homozygous. At present, this cannot be ruled in or out, but mating of two horses that carry SW-2 or -3 is discouraged. Testing for SW-1 has identified individuals who are homozygous for this allele, thus SW-1 probably does not have potential lethal characteristics.
Some horses with the splashed white pattern have been shown to have congenital deafness, though many or most have normal hearing. This type of deafness is probably similar to instances of deafness in white or piebald, blue-eyed examples in other species. In white-coated cats and dogs with deafness, an absence of melanocytes in the inner ear leads to death of the hair cells, which are necessary for perceiving sound. The presence of pigment around the outside of the ears - which most splash horses have - does not prove the presence of that necessary pigment in the inner ear. There must be pigment inside the inner ear to prevent this problem. Domestic horses often cope well with deafness, and deaf horses may go undiagnosed. Some deaf horses are more skittish than normal, while others are distinctly calmer. Deafness in horses can be diagnosed by brainstem auditory evoked potential (BAEP), which is minimally invasive and requires no sedation and minimal restraint. While blue eyes and a white face are often associated with deafness in other species, apron-faced, non-splash horses are not known to be deaf. Nor is the presence of one blue, one normal eye indicative of unilateral deafness. The case horse in Hardland's 2006 case study had one blue eye, while the other was parti-colored, but the horse was bilaterally deaf.
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