Shedding Haircoat Inappropriately, Losing Coat Out of Season

SUMMARY
Horses normally shed their coat twice a year, in spring and fall, on a schedule set mostly by day length and to a lesser degree by temperature. When a horse sheds heavily at the wrong time of year, sheds in patches, drops its coat weeks after a fever or illness, or seems to be shedding constantly, something has disturbed that schedule. This is different from a horse that fails to shed its winter coat in spring (see the record on that observation). Here the horse is losing coat when it should be holding it. Some causes are harmless: a horse moved from a cold climate to a warm one, a horse suddenly kept under barn lights, an unusually warm stretch in midwinter, or a mare in late pregnancy or early lactation. Others matter. Cushing's disease (PPID) disrupts the shedding cycle and often shows up first as patchy or out-of-season shedding long before the classic long curly coat. A high fever or serious illness a month or two earlier can cause the coat to let go all at once as the hair follicles reset. Poor nutrition, heavy parasite load, and chronic disease weaken the coat. Chronic selenium poisoning from high-selenium forage or over-supplementation causes loss of mane and tail hair and sometimes body hair, along with hoof problems. Skin parasites and infections (lice, rain rot, ringworm) can also look like abnormal shedding, but those usually come with itching, crusts, or clearly defined bare patches, and belong under the hair loss records rather than this one.