There are both common and uncommon conditions that can cause white spots in horse ears. You may be able to diagnose the cause and type by observing the spots over time; some spots will disappear with prevention - such as protecting ears from insect bites - and treatment, while others will be present for life. It is always a good idea to check with your veterinarian before treating your horse, especially if he exhibits pain or discomfort. In cases where the spots are transmitted by biting flies or other insects, some horse owners choose to stall their horses during the day, when biting flies persist.
The most common white spots in ears are called aural plaque, reported to afflict as many as 22 per cent of all horses. This condition is caused by the papilloma virus and believed to be transmitted by insects. Aural plaque can manifest in single or multiple spots, either smooth or slightly raised, that can range from 1 to 30mm in diameter. The prevalence in the ear can be significant, covering up to 40 per cent of the inner surface of one or both ears. Many horses are not bothered by aural plaque, but others can exhibit sensitivity, particularly if irritated by insect bites or clipper blades. Aural plaque does not discriminate by breed or gender, although typically does not appear before the first year of life, according to Janet Roark, an equine veterinarian from Austin, Texas. Many people will refer to this as "ear fungus," although it's not actually a fungus. Roark notes that this plaque is more accurately described as a type of wart and generally persists for life.
Ear lesions caused by insect bites are another common ear affliction and are also characterised by crusty, patchy white spots, that can be itchy. You can prevent these by using insecticides and fly masks with ears for the horse. Some horse owners report good results coating the inner ear with petroleum jelly, either alone or mixed with fly repellent. Roark says that these lesions should disappear once the insects are controlled.
Rain rot, or "rain scald," is more likely to occur on the outside of the ear than inside, although it can and does occur in many body areas.This superficial bacterial infection can cause hair loss and crusting. You can control rain rot with a topical treatment such as a disinfectant scrub. If your horse seems prone to rain rot, clean and dry the ears frequently as a preventive measure.
Ringworm is a fungal infection that can also cause hair loss and crusting, creating multiple lesions both inside and outside the ears, as well as other locations on your horse's body. If your horse has had a cut or other trauma in his ear, it is possible - although not likely - that excessive granulation of tissue can occur, resulting in a lesion-like condition known as "proud flesh," but this is very uncommon in ears.