Alopecia is the partial or complete lack of hairs in areas where they are normally present. Hair loss is a sign, not a disease. Its underlying cause must be determined for the condition to be successfully treated. If an animal has hair loss and is also scratching the area excessively, the itching problem should be investigated first (see Skin Disorders of Horses: Itching (Pruritus) in Horses).
There are many causes of hair loss, which can be congenital (the animal is born with the condition) or acquired. Any disease that can affect hair follicles can cause hair loss. Certain diseases may destroy the hair follicle or shaft or interfere with the growth of hair. Some diseases can cause the animal discomfort leading to self-trauma and loss of hair. Acquired hair loss can be inflammatory or noninflammatory.
Diseases that can directly cause destruction or damage to the hair shaft or follicle include bacterial, fungal, or parasitic infections; skin trauma such as burns; and (rarely) poisonings. Diseases that can directly inhibit or slow hair follicle growth include nutritional deficiencies (particularly protein deficiencies), or hormonal imbalances such as hypothyroidism. Temporary hair loss can occur during pregnancy, lactation, or several weeks after a severe illness or fever. Bald patches can occur during the normal shedding process. These types of hair loss tend to be noninflammatory unless a secondary infection of the skin develops.
Itching or pain is a common cause of acquired inflammatory hair loss. Diseases that commonly cause itching or pain include infections, parasite infestations, and allergies. Friction may cause local hair loss, for example, poorly fitted halters or saddles.
An accurate diagnosis of the cause of hair loss requires a detailed history and physical examination. In the physical examination, your veterinarian will note the pattern and distribution of hair loss. He or she will examine the hairs to determine if they are being shed from the hair follicle or broken off and will also look for signs of secondary skin infections or parasites. Often, this involves taking skin scrapings and combing of the hair coat to collect samples for microscopic examination. Fungal cultures are commonly performed on horses with hair loss, as ringworm (which is caused by a fungus) is one of the most common causes of this condition.
Your veterinarian may order diagnostic laboratory tests. These usually include smears and culture of the skin to check for evidence of bacterial, fungal, or yeast infections. If these tests do not identify or suggest an underlying cause, a skin biopsy may be performed. Skin biopsies are often needed to confirm bacterial and parasitic causes of hair loss or to identify cancerous causes of hair loss. In the rare event your veterinarian suspects an endocrine problem, blood and urine samples may be tested.
Successful treatment depends on the underlying cause and specific diagnosis. Because identifying the cause of a skin condition may take some time, many veterinarians will provide or prescribe medication to relieve any discomfort or itching your horse may be experiencing in connection with the hair loss.
Last full review/revision July 2011 by Karen A. Moriello, DVM, DACVD; Carol S. Foil, DVM, MS, DACVD; John E. Lloyd, BS, PhD; Bertrand J. Losson, DVM, PhD, DEVPC; Wayne Rosenkrantz, DVM, DACVD; Patricia A. Talcott, MS, DVM, PhD, DABVT; Alice Villalobos, DVM, DPNAP; Patricia D. White, DVM, MS, DACVD; Thomas R. Klei, PhD; David Stiller, MS, PhD; Stephen D. White, DVM, DACVD