Deafness may be congenital (present at birth) or acquired as a result of infection, trauma, or degeneration of the ear.
Deafness present at birth can be inherited, as is known to occur in horses with white coloring, such as American Paint horses.
Acquired deafness may result from blockage of the external ear canal as occurs in longterm otitis externa (inflammation of the external ear canal), or it may occur after destruction or damage of the middle or inner ear. Other causes include a type of bone disorder known as temporohyoid osteoarthropathy, tumors involving the ear or brain stem, and aging. Deafness in one ear or partial hearing loss is possible in some of these instances.
Animals deaf in one ear may have subtle or no signs of deafness. An electronic test (called brainstem auditory evoked response, or BAER) can be used to assess hearing in horses but is typically only available at referral veterinary hospitals staffed by specialists.