What abnormal hair growth might mean

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Question:  I have a 5-year-old Miniature stallion. He was clipped in mid-spring due to the early hot weather we experienced. Once the days became hot consistently I thought he would grow in his summer coat. But what has happened is that he gets long coarse black hairs that cover his face and back and part of his belly. No sign of shedding or summer growth after the clip. I’m not sure what to make of this. It’s like he’s still trying to grow a winter coat.

Hannah Ziccarelli
Milford, Delaware

Answer: I see two possibilities. The more likely one is that abnormal hair growth after clipping is normal for this horse. Your horse’s summer coat may well be growing in, it’s just slower than you might expect. Miniature Horses can have rather impressive guard hairs, and these may be more noticeable initially than other hairs. I would suggest simply waiting to see whether your horse grows in a more normal hair coat over time.

More rarely, abnormal hair growth can indicate an endocrine (hormonal) condition. If the normal coat does not grow in, I would ask your veterinarian to obtain baseline bloodwork, including a resting ACTH (adreno-corticotropic hormone) level. An abnormal result on the ACTH test could indicate that your horse has pituitary pars intermedia dysfunction (PPID, also called Cushing’s disease). Although this horse seems too young to have a pituitary dysfunction, it can happen in younger horses. 

Abnormal hair growth is a hallmark of PPID---typically, a long, shaggy winter coat that is slow to shed in spring---but other signs include lethargy, decreased performance, excessive sweating, muscle wasting, low-grade laminitis and the development of fat deposits on the top of the neck, tailhead and around the eyes. Contact your veterinarian if you notice any of these signs.

Your veterinarian might also possibly do a skin biopsy of both the black-haired areas and the non-affected areas for comparison.

Stephen D. White, DVM, DACVD
School of Veterinary Medicine,
University of California Davis, California

Stephen D. White, DVM, DACVD, is a professor of the School of Veterinary Medicine at the University of California–Davis and has been a Diplomate of the American College of Veterinary Dermatology since 1983. Previously, he had held faculty positions at the veterinary schools at Tufts University and at Colorado State University. He led the clinical investigations of the research team that identified a gene mutation causing hereditary equine regional dermal asthenia (HERDA), a debilitating skin disease in horses. He earned his Doctorate of Veterinary Medicine from the University of California–Davis in 1979.

This article first appeared in EQUUS issue #472, January 2017.