Vitiligo is a quite unusual skin condition that causes a lot of consternation to the owners but thankfully is not at all distressing to the horse or pony. It presents as small, focal and generally well-circumscribed white spots as a result of pigment loss. The cells (melanocytes) containing the dark pigment, melanin, are lost from these areas leading to white hairs.
It is not painful nor is there any damage to the skin. This can be a way to distinguish it from other conditions that may cause white spots such as wounds and ringworm.
It seems to occur more commonly in certain breeds such as Welsh Ponies and Arabs and may have an hereditary aspect to it. Auto-immune problems have been implicated. However the cause is unknown and therefore treatment is difficult. It has been suggested that vitamin and mineral deficiencies can be a cause although the usual suggestion of copper deficiency is generally incorrect. Ultraviolet light is one possible treatment option derived from humans and Aloe Vera Gel may be massaged into the spots but there is no evidence to support this and generally a multi-vitamin supplement is a low-cost option.
Often the white spots will spontaneously disappear which, in my opinion, suggests there may be a component of stress in the condition and any changes to management or exposure to new horses may be considered.