Telogen Defluxion Associated with Hypersensitivity Causing Alopecia in a Horse

In 2007, a case of telogen defluxion associated with a non-pruritic hypersensitivity reaction causing alopecia was reported in a 12-year-old Thoroughbred gelding. The horse experienced sudden, extensive, and unseasonal hair loss without an obvious cause, with hair loss occurring in a symmetrical pattern on its face, neck, chest, shoulders, and flanks, while the mane and tail hair remained unaffected. Veterinary examination revealed easily plucked hair, heavy scaling, and hyperaesthetic skin, with hematological tests indicating a stress leukogram. Microscopic examination showed approximately 70% of hair follicles in the telogen phase and a superficial perivascular dermatitis, suggesting a hypersensitivity reaction. After eliminating other potential causes, the horse's condition improved with the withdrawal of a coat conditioning powder from its diet, which had ironically been intended to promote healthy hair growth. The horse's symptoms recurred upon reintroduction of the powder, confirming it as the likely cause of the hypersensitivity reaction and subsequent telogen defluxion. The horse fully recovered four months after the onset of clinical signs, once the powder was permanently removed from its diet.