Hair growth occurs in cycles and is influenced by photoperiod and, to a lesser extent, ambient temperature. Hair growth is regulated by a complex interaction of the hypothalamus and hypophysis of the pituitary gland (in the brain) to regulate multiple hormones, including melatonin, prolactin, thyroid hormone, and cortisol. The exact signals to the equine hair follicles that induce active hair growth (anagen), the transitional stage (catagen), and the resting stage (telogen) are not known. The length of time the hair spe...

Hair growth occurs in cycles and is influenced by photoperiod and, to a lesser extent, ambient temperature. Hair growth is regulated by a complex interaction of the hypothalamus and hypophysis of the pituitary gland (in the brain) to regulate multiple hormones, including melatonin, prolactin, thyroid hormone, and cortisol. The exact signals to the equine hair follicles that induce active hair growth (anagen), the transitional stage (catagen), and the resting stage (telogen) are not known. The length of time the hair spends in each of these stages is dependent on the region of the body, age, breed, and sex. The growth of hair can be modified by a variety of physiologic or pathologic factors.

Hair cycle stages and, thus, hair growth and loss, occur in a mosaic pattern in horses, meaning adjacent hair follicles are in different stages of growth at the same time. Horses in temperate climates generally shed the long coat of winter over several weeks to the shorter haircoat of summer. But occasionally horses will have a less-coordinated transition from a seasonal coat (winter to spring, fall to winter), during which more hairs are shed in a period of time than new hair growth occurs, leading to a very thin coat or, very rarely, to areas of alopecia (baldness). In this condition the skin is normal and a skin biopsy reveals normal hair follicles. Over a period of several weeks the new hair growth "catches up" with the hair loss, and the coat is normal.

Hair loss, either patchy or total hair coat, can also occur following periods of stress (such as pregnancy) or generalized illness in which the growth period of the hair is considerably shortened

and many hair follicles enter the resting stage at one time. Patchy or generalized hair thinning or alopecia can occur four to 12 weeks following the stress.

Hairs shed during the resting stage can be recognized by microscopic examination. A febrile episode, infectious disease, or metabolic problem might also interfere with the growth phase, which can result in abnormalities of the hair follicle and the hair shaft. Hair loss in these instances occurs rapidly--within days of the insult. Microscopic examination of the shed hair shows damage to the hair shaft. In both of these instances, the hair will regrow when the initial insult is resolved.

Alopecia areata is an uncommon condition in which the skin is totally normal and patches of well-circumscribed (limited in space, well- localized) hair loss can also occur. The lesions are most often seen on the horse's face, neck, and trunk. The lesions are not painful or pruritic (itchy), and exposed skin might have more pigment than normal. A skin biopsy can be useful in establishing this diagnosis. In some horses hair might regrow, but it will appear thin and lighter in color. No treatment is currently available.

In any skin condition where changes in the skin itself occur with the hair loss, one must consider infection of the hair follicles (such as with staphylococcal species), dermatophilosis (an infectious exudative dermatitis caused by Dermatophilus congolense), dermatophytes (parasitic fungi such as ringworm), or Demodex mites. If in doubt, a careful examination of the entire horse to rule out systemic conditions and a skin biopsy are most likely to yield a definitive diagnosis.
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