I suspect Midnight's white hairs are due to an injury. The vet and farrier agree they look like the shape of a bite. A horse I had was bitten by a rattlesnake; the hair grew in white at the bite site. I thought this article was interesting. Fox Run Equine Center January 12, 2018 Acquired Pigment Disorders of the Horse Brian S. Burks, DVM, Dipl. ABVP Board Certified in Equine Practice How do horses get those darn white spots? Skin color depends upon the amount of melanin, carotene, and oxyhemoglobin contained within the subcutaneous tissue, vessels, dermis, and epidermis. A lack of melanin results in white hair or skin. Leukoderma, or white/depigmented skin, is common in horses. It may be local or multifocal, temporary or permanent, depending upon the cause. Leukoderma is a complication of diseases such as onchocerciasis, pressure sores, papillomas of the ear, ventral midline dermatitis, or freezing and burns. It has been linked to contact with phenolic compounds, rubber bits, and feed buckets. Many rubbers contain compounds which inhibit melanogenesis. Juvenile Arabian leukoderma is the most common form of leukoderma in horses, reported in young Arabian and occasionally in Quarter horses. One- to 2-year-old animals develop leukoderma on eyelids, periocular skin, muzzle, nares, genitalia, anus perineum, and inguinal region. Leukotrichia, or white hair, is common following trauma or inflammation. Ill fitting tack, poor bandaging, and freeze branding all result in white hair. Horses with a dominant allele that causes white skin and hair, but with blue or brown eyes are cremello. In horses carrying the G allele, color is present at birth but increasing number of white hairs develop with age, e.g. Arabians, Andalusians, Lipizzaners, and Percherons. This begins between 2-4 years of age, and by 10 years, the horse is all white. Reticulated leukotrichia- tiger stripes- is an uncommon dermatosis of the horse. Its cause is unknown, though it may be due to vaccination in some horses. There are breed predilections that suggest a genetic component. It is found mainly in quarter horses, thoroughbreds, and standardbreds. It does occasionally occur in other breeds. Clinical signs begin in yearlings with crusty lesions that shed, leaving permanent white marks, but normal underlying skin. Linear crusts develop in a cross-hatched or herringbone pattern on the horse’s back. The crusts are often asymptomatic, but are sometimes painful. When shed, there is temporary alopecia, and the hair grows back as white- -permanently. Diagnosis is based upon history and clinical signs. It is confirmed by histopathology. There is no known effective treatment. Spotted leukotrichia is an uncommon disorder of unknown etiology. Arabians, Andalusians, Shires, and thoroughbreds are overrepresented. Small, white spots, 1-3cm in diameter develop over the sides and rump. There is no skin disruption, but the change is permanent, although some spots may come and undergo spontaneous remission. There is no treatment. Hyperesthetic leukotrichia is a rare dermatosis of the horse. The cause remains elusive, though some cases have been associated with infection or vaccination. This disorder usually occurs in older animals, and is characterized by single or multiple painful crusts along the dorsal midline, from the wither to the tail base. Pain may sometimes precede the crusts, and the horse may react violently when lesions are handled. After several weeks, white spots appear. The disease runs a 1-3-month course, and the crusts and pain disappear. The leukotrichia is usually permanent. Diagnosis is based upon clinical findings and histopathology. There is no known treatment. Sometimes signs are recurrent.