By Christine Barakat

By the time Charlie, above, was referred to the University of Cambridge Veterinary Hospital, he had lost most of his hair. Photos courtesy of Frances Henson, VetMB, PhD

For the first three years of his life, Charlie, a gray Thoroughbred-Percheron gelding, was normal in every way. Then, just after he turned four, something strange started to happen: He began losing his hair.

First, oval bare patches appeared on his face, but soon the hair loss steadily spread to his neck, shoulders, chest and back. Charlie, kept in a pasture just outside of Kent, England, was otherwise in perfect health, so initially his veterinarian thought his problem was ringworm, a fungal infection that can cause hair loss in a similar pattern. Although laboratory tests failed to confirm the diagnosis, a course of antifungal medication was prescribed for lack of any other ideas.

The hair loss continued, however, spreading to Charlie's hindquarters and legs in small patches, less than one inch in diameter. Given that none of the other three horses who shared Charlie's pasture showed signs of hair loss, it seemed unlikely that the gelding's problem was contagious. The veterinarian suggested antibiotics and a variety of insecticides, hoping that something might work, even if it wasn't clear why. Nothing seemed to have any effect.

Finally, when half of Charlie's body was hairless, and his mane and tail had begun to fall out, his veterinarian referred the case to the University of Cambridge Veterinary Hospital.

"I remember thinking the moment I saw him that this did look like ringworm," says Frances Henson, VetMB, PhD, who oversaw Charlie's care at the clinic. "He was dropping hair in ring-like patterns, which is a distinctive sign of the fungus, but--and this was significant--he wasn't itchy. Ringworm is horribly itchy, and this guy seemed perfectly comfortable. He was rubbing his head, but only because of all the different lotions and tonics they were trying. Once you washed them off, he wasn't itchy at all."

Although hairless, Charlie's skin appeared normal as Henson took tissue scrapings and hair samples for laboratory analysis. Every test for pathogenic bacteria or fungi came back negative.

Unable to find an external culprit, Henson's team began to suspect that Charlie had an autoimmune disorder, in which the immune system mistakenly attacks the body's own healthy cells. In particular, Henson suspected pemphigus foliaceous, which occurs when the immune cells attack the skin, leading to hair loss and painful crusting. Although Charlie didn't have the tenderness or skin disruptions characteristic of pemphigus, it was possible that he had an atypical case.

Henson arranged for Charlie to have two blood tests--an antinuclear antibody titer and a so-called Coombes test--that can help identify immune malfunction. Both were negative. Then Henson had Charlie's blood analyzed for signs of elevated thyroid activity, which can be an indication of autoimmune disease. Again, the readings were normal.

"By this point, we were baffled and went into full investigative mode," says Henson. Clinicians took multiple full-thickness skin biopsies from a variety of locations on Charlie's body. The samples were sent to Mark Stidworthy, PhD, who specializes in the histopathology of skin diseases. A day passed with no word. Then came a most unexpected and unusual diagnosis.

"I remember Mark running down the hall toward me, waving papers over his head and shouting, 'This horse looks just like a dog.' It was all very dramatic, when you consider what normally goes on around the clinic," says Henson.

What Stidworthy had found was that Charlie's skin samples showed all the classic signs of color-dilute follicular dysplasia, a condition that, until that point, had been seen only in dogs.

"Color-dilute follicular dysplasia is a condition of animals that are that wonderful bluey-gray color," explains Henson. "It's rare, but occasionally is seen in weimaraners and whippets that are that distinct color."

Normally, pigment within hair follicles is spread evenly to color the emerging hair. In color-dilute follicular dysplasia, black pigments clump abnormally at the bottom of the follicle, slowing hair growth. Eventually, the hair falls out and is unable to regrow. Diagnosis of the condition in dogs is made through a skin biopsy to reveal the characteristic clumping of black pigment. Stidworthy had often seen the condition in dogs and recognized the problem immediately in Charlie's skin sample.

"Nobody knows the primary pathogenesis--the underlying reason why it happens--of this condition," says Henson. "It is obviously related to this color, although not every animal of this color is affected." Henson says the onset of Charlie's hair loss at age four is similar to what is seen in dogs. "The later onset could possibly be related to the color change that some gray animals can go through as they mature," she says. "But that's just a theory."

Pedigree analysis in dogs has shown that the disorder is a recessive trait, which means that an affected animal inherits a copy of the responsible gene from each parent. "I wouldn't be surprised if the disease in horses follows the same genetic pattern as what we see in dogs," says Henson. "It is so similar in all other respects."