Q. I have a Quarter Horse mare who sweats on the right side of her head mainly when she ships in a trailer or gets nervous, but also when she has been lying down. This problem has been ongoing since we purchased her, and now she has lost most of her hair starting at the center of her face and reaching all along the right side. She rides well but does show some resistance when turning left (I don’t know if this is related or just a problem with what I am doing). Do you have any suggestions of what the problem might be?

Chris Beaune, via e-mail

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A. Veterinarians occasionally see focal or patchy sweating in horses, and it is most common on the neck or shoulders but can also be seen on the head, withers, or girth area. There is a well-recognized condition called Horner’s syndrome, which in horses is characterized by sweating on the affected side of the face. Other clinical signs include upper eyelid drooping and a smaller-than-normal pupil. This syndrome is caused by a loss of sympathetic innervation (nerve supply) due to nerve damage or inflammation in the neck or head. It usually results from a traumatic event involving the neck, which can include an injury, injection of medication, prolonged impingement (i.e., due to neck position while under general anesthesia), infections affecting the neck, guttural pouch, or neurologic system, and cancer.

The observed one-sided sweating and resistance turning left described in your horse could be related to a previous injury that resulted in local nerve damage to the head or neck. It is also possible that the one-sided sweating combined with the hair loss could indicate a primary dermatologic (skin) issue. Your veterinarian should perform a neurologic examination and might also perform a skin biopsy to microscopically evaluate the hair follicles, sweat glands, and overall skin health. Also, radiographs and/or ultrasound evaluation could help identify a mass or space–occupying lesion. Additionally, clinicians at a referral center might consider evaluating neurologic signal transduction using