Horses lose hair for dozens of reasons, ranging from completely normal seasonal shedding to parasites, fungal infections, allergies, hormonal disorders, and nutritional problems. The pattern, location, and timing of the hair loss are the biggest clues to what’s going on. Patchy bald spots tell a very different story than a coat that’s thinning all over or a mane that’s breaking off at the roots.
Normal Seasonal Shedding
Before assuming something is wrong, consider the calendar. Horses grow and shed their coats primarily in response to day length, not temperature. As days get longer in late winter and spring, the eye detects the change in light and sends signals to the brain that suppress melatonin production and increase prolactin. This hormonal shift triggers the winter coat to release. Temperature plays a secondary role, which is why horses sometimes start shedding on a warm February day even though winter isn’t over.
Normal shedding looks uniform. You’ll pull out clumps of loose hair with a curry comb, but the coat underneath appears healthy and full. If you’re seeing bare skin, crusty patches, or hair loss in odd locations like the legs, face, or tail base, that’s not shedding. That’s a problem worth investigating.
Parasites: Mites and Lice
Microscopic mites invade the skin and trigger irritation and an immune reaction that leads to itching, hair loss, and inflammation. Several types of mange affect horses, each with a characteristic location:
- Sarcoptic mange (body mange): Starts as small bumps and blisters on the head, neck, flanks, and abdomen. The horse rubs and chews at its skin, creating sores and bald patches. Over time the skin thickens and crusts spread.
- Psoroptic mange (mane mange): Targets thickly haired areas, including under the forelock and mane, the base of the tail, under the chin, between the hind legs, and in the armpits. Lesions lose hair quickly and develop thick crusts that bleed easily.
- Chorioptic mange (leg mange): Shows up around the feet and fetlocks, most often on the hind legs. You’ll see raised bumps first, then hair loss, crusting, and thickened skin. Draft breeds with heavy feathering are especially prone.
- Demodectic mange: Causes patchy hair loss with scaling or small skin lumps, though it’s less common in horses than the other types.
Lice are another possibility, especially in winter when horses have long coats and are in close quarters. Both biting and sucking lice cause itching and a rough, thinning coat. You can sometimes spot lice or their eggs (nits) by parting the hair near the mane or along the topline.
Ringworm and Rain Rot
Ringworm is one of the most common causes of patchy hair loss in horses, and despite its name, it’s a fungal infection, not a worm. The fungi responsible typically live on rodents and in soil, and they spread easily between horses through direct contact or shared tack, grooming tools, and blankets. The classic sign is circular patches of hair loss, sometimes with redness or a raised edge, most often on the girth and saddle areas. It can spread to other parts of the body if untreated.
Rain rot is a bacterial skin infection that thrives in wet, humid conditions. It creates characteristic clumps of matted hair that peel away and leave raw, pink skin underneath. The back, rump, and shoulders are the usual targets. Both ringworm and rain rot are contagious, so isolating the affected horse and disinfecting shared equipment matters.
Sweet Itch and Insect Allergies
If your horse is losing hair along the mane, tail, rump, saddle area, or the underside of the belly during warmer months, insect bite hypersensitivity is a leading suspect. Known as sweet itch, this condition is an allergic reaction to the saliva of tiny biting midges called Culicoides. The bites themselves cause intense itching, but most of the visible damage actually comes from the horse rubbing, biting, and scratching against fences, trees, and stall walls. Over a season, this self-trauma can leave the mane and tail base nearly bald, with thickened, scabby skin.
Some horses develop similar reactions to other biting insects like black flies, stable flies, or mosquitoes. The pattern of hair loss often points to the culprit, since different insects tend to feed on different parts of the body.
Stress-Related Hair Loss
Horses can experience a form of widespread shedding called telogen effluvium after a major physical or psychological stressor. Events like surgery, pregnancy, illness, a long trailer ride, or even a reaction to a topical product can push a large number of hair follicles into their resting phase all at once. The catch is that the hair doesn’t fall out right away. It typically sheds one to four months after the stressful event, which makes it hard to connect cause and effect.
This type of hair loss looks diffuse rather than patchy. The coat thins broadly instead of leaving distinct bald spots. In one documented case, a 12-year-old Thoroughbred developed extensive hair loss and irritability traced back to a hypersensitivity reaction to a coat conditioning powder applied weeks earlier. If your horse suddenly starts losing hair all over and nothing else explains it, think back a few months to any changes in routine, health events, or new products.
Cushing’s Disease (PPID)
In older horses, hair coat changes are one of the hallmark signs of pituitary pars intermedia dysfunction, commonly called Cushing’s disease. The classic presentation is a long, curly coat that fails to shed normally in spring. This abnormal coat growth is considered so specific to the disease that it’s essentially diagnostic on its own in advanced cases. But earlier in the disease, coat changes can be subtler: delayed shedding, a dull or wavy coat, or uneven hair loss.
Other signs that often accompany Cushing’s include wasted muscle along the topline, a pot-bellied appearance, bulging fat pads above the eyes, and recurring bouts of laminitis. Diagnosis involves measuring blood levels of ACTH, a pituitary hormone. Reference ranges vary by season because healthy horses naturally produce more ACTH in autumn, so your vet will interpret results based on the time of year. Horses with borderline results are often retested in autumn when the test is most sensitive and specific. Cushing’s is manageable with daily medication, and treated horses often return to a more normal shedding cycle.
Selenium Toxicity
If your horse is losing mane and tail hair specifically, and especially if the hair appears to break off rather than fall out at the root, selenium toxicity is worth considering. Horses are very susceptible to chronic selenium poisoning when they graze on soils naturally high in selenium, which occurs in parts of the western and central United States. Certain plants accumulate selenium from the soil, and horses eating those forages over weeks or months develop a condition called chronic selenosis.
The classic signs are cracking and crumbling of the hooves along with loss of mane and tail hair. In one study, researchers analyzed mane and tail samples from affected horses and found a cyclic pattern of selenium concentration that matched the periods when horses entered and left contaminated pastures. If you’ve recently moved your horse to a new property or changed hay sources, and mane or tail hair is thinning or breaking, ask your vet about testing selenium levels.
Photosensitization
Some horses develop painful, sunburn-like skin reactions on white or lightly pigmented areas, leading to crusting, peeling, and hair loss on the face, legs, or body. This is photosensitization, and the most common form in horses is secondary to liver damage. When the liver is compromised, it can’t properly clear a byproduct of chlorophyll digestion from the bloodstream. That compound circulates to the skin and reacts with sunlight, causing inflammation and tissue damage.
A long list of plants can trigger the underlying liver injury, including ragwort (Senecio), fiddleneck (Amsinckia), lantana, and kleingrass. If your horse develops skin lesions only on sun-exposed, light-colored skin, liver function testing is an important next step.
Nutrition and Coat Quality
A dull, thinning coat doesn’t always point to a specific disease. Sometimes it reflects gaps in the diet. Zinc, methionine (an amino acid), and biotin all play roles in skin and hair integrity. Biotin has the most research behind it in horses. Studies have found that 15 to 25 mg per day improves hoof condition, and since hooves and hair are made of similar proteins, coat quality often improves alongside hoof health. One study found that 15 mg per day produced better results than 7.5 mg, and most equine supplements targeting skin and hooves aim for around 20 mg per serving.
Protein deficiency can also contribute to poor coat quality, particularly in hard-working horses, growing youngsters, or lactating mares whose protein needs outpace their intake. A forage analysis and a conversation with an equine nutritionist can reveal whether your horse’s diet is falling short.
How Vets Diagnose the Cause
Your vet will start with the basics: where on the body is the hair loss, is the horse itchy, and how long has it been going on? A skin scraping can identify mites, and a fungal culture can confirm or rule out ringworm. For suspected allergies, the history and seasonal pattern are often enough to guide treatment.
If surface tests don’t provide an answer, or if the hair loss doesn’t respond to standard treatments, a skin biopsy may be recommended. Biopsies are particularly useful for persistent, unexplained lesions, ulcerative or pustular skin disease, and cases where cancer is a concern. Blood work, including ACTH levels and liver enzymes, rounds out the picture when hormonal or metabolic causes are suspected.
The location of the hair loss is your single best clue before the vet arrives. Legs point toward chorioptic mange. Mane and tail suggest sweet itch or selenium issues. Circular patches in the saddle area suggest ringworm. A coat that won’t shed in an older horse raises the flag for Cushing’s. Noting these details and photographing the affected areas over time gives your vet a significant head start.