The skin condition vitiligo can be unsightly in horses, but it is essentially a cosmetic issue, a just-published scientific review says.
“Therefore, any treatment for this disease should always be discussed with the owners to avoid interventions of dubious efficacy and those with possible adverse effects,” Thierry Olivry and his colleagues wrote in the journal, BMC Veterinary Research.
“However, vitiligo could have a significant and negative impact for the owners of show animals,” the researchers acknowledged.
Vitiligo is an autoimmune disease in which melanocytes in the skin are targeted.
A substantial part of their review into autoimmune diseases affecting skin melanocytes in horses, dogs and cats covered this condition. It has many features that mirror the condition in humans, they said.
In vitiligo, melanocytes are destroyed, resulting in depigmentation.
It is the most common depigmenting condition in humans, affecting 0.5 to 2% of the population. Rates as high as 8.8% have been reported in India.
Signs associated with this destruction are primarily seen in the skin, lips and/or oral cavity.
Vitiligo in humans was first described more than 3500 years ago. In ancient times, vitiligo was confused with leprosy and other depigmenting disorders, and it resulted in discrimination and social stigma that, unfortunately, still persists today.
It was only in the last century that vitiligo has gained more interest among medical researchers.
The first case series of vitiligo in dogs was described in scientific literature in 1971. Scientific reports on vitiligo in horses, cat and dogs are limited.
In the horse, it was first mentioned in the literature in 1931, but it was not until the 1960s that a Dutch veterinarian reported equine vitiligo in three separate papers.
There is no available information that allows an estimate to be made of the global or regional incidence and prevalence of vitiligo in horses, dogs and cats.
However, at Cornell University, it was reported that vitiligo accounted for 0.7% of equine dermatoses examined at the veterinary teaching hospital.
“The prevalence of vitiligo in animals may be much higher than that reported,” the review team said.
“However, because this disease is a primarily cosmetic issue in animals, and it might not motivate owners to seek any veterinary care.”
The breeds of horses affected with vitiligo can be inferred from five reports identified by the authors that included 32 horses.
Among these cases, there were 12 Gelderlands, nine Spanish thoroughbreds, four Arabians, four Belgians, one Oldenburg, one Mecklenburg and a quarter horse.
Other studies did not identify breeds.
The sex of affected horses was available from three reports involving 28 horses, with a female-to-male ratio of two to one.
“Vitiligo is reported to be common in Arabian horses (the so-called “Arabian fading syndrome” and “pinky Arab”) and there are numerous references listing this breed in review papers, equine textbooks and the lay literature,” they said.
However, the published information is limited and may not completely represent how common the condition is among horse breeds.
The age of onset was reported in only seven horses, with a median of 48 months, and a range of 1 to 18 years.
Information on the appearance of the first skin lesions was available for just six horses, all of whom initially developed depigmentation on the head/face region.
The lesion distribution was reported in 11 horses, of which eight (73%) involved the head/face region.
Due to the cosmetic nature of the disease, diagnosis is often made clinically without any need for skin biopsies, they noted.
Turning to treatment, the researchers noted that it can have a devastating psychological impact on the quality of life of affected humans, which is is why treatment of widespread, facial or recalcitrant vitiligo in people can be aggressive.
In horses, treatment and outcome information was available for only 11 horses, across three reports.
It took between one to 12 months for significant repigmentation to occur.
Complete repigmentation occurred in one horse treated with oral nutritional supplements containing, among others, vitamin A, D, B12 and E.
In another report, partial repigmentation occurred after supplementation with high levels of chelated copper. A relapse of depigmentation occurred about five months after the copper supplementation was reduced and then a noticeable clinical improvement occurred when the daily intake of copper was re-increased.
“It is plausible,” they said, “that the depigmentation in this horse was associated with a copper deficiency and not vitiligo.”
Nine horses from one case report had complete repigmentation after 1 year of dietary supplementation with carrots (4–5 kg per animal per day).
The author suspected that the administration of high levels of a thyroprotein-based product might have caused a relative vitamin A deficiency and depigmentation. However, to the authors’ knowledge, there is no evidence that vitamin A deficiency leads to depigmentation in horses but, in humans, several studies have shown that deficiencies of D and B12 vitamins are associated with vitiligo.
“Therefore, the outcome of these nine horses of this report should be interpreted cautiously. A plausible cause could have been an idiosyncratic reaction to high levels of a thyroprotein-based product, or any of its ingredients, as the cessation of this diet led to spontaneous resolution.”
The review team said information on the long-term treatment and outcome of vitiligo in animals was rather sparse, with few published case reports to date.
Given its cosmetic nature in animals, interventions of questionable effectiveness and with possible adverse effects should be avoided.
In show animals, where the condition could have a negative impact on owners, highly potent glucocorticoids could be applied topically (i.e., 0.1% bethamethasone or 0.05% clobetasol). However, it would be practical only for areas with sparse or no hair. It should be used sparingly.
“This recommendation is based on the assumption that the pathogenesis of vitiligo in humans and animals is similar and involves an autoimmune mechanism.”
Giving glucocorticoids orally or by any other internal means is not normally recommended for treatment of vitiligo because the adverse effects outweigh the clinical benefits in animals, they said.
“If topical therapy were to fail, oral supplementation with L-phenylalanine could be attempted for six months, although the clinical remission could be only partial. This supplementation might be tried in addition to the use of topical glucocorticoids.
“It is important to emphasize to the owners that these treatments, if effective, may prevent the progression of depigmentation but they do not guarantee the partial or full repigmentation.”
The authors said there was a need for research to determine, among other things, whether the same genes linked to vitiligo in humans have a role in the disease in cats, dogs and horses.
Phototherapy is a safe and effective treatment in humans. “Therefore, phototherapy treatment, either alone or in combination with other drugs, warrants further clinical trials in animals.”
Olivry was joined in the review by his North Carolina State University colleague Keith Linder; and Heng Tham, from the Virginia-Maryland College of Veterinary Medicine.
Autoimmune diseases affecting skin melanocytes in dogs, cats and horses: vitiligo and the uveodermatological syndrome: a comprehensive review
Heng L. Tham, Keith E. Linder, and Thierry Olivry
BMC Veterinary Research 2019 15:251 https://doi.org/10.1186/s12917-019-2003-9