Why Is My Horse Losing Patches of Hair?

Unsightly bald patches on a horse's coat, called alopecia, may give his owner cause for alarm. Horses can lose significant portions of hair due to a variety of internal or external causes, or they simply may be shedding their dense winter coat. A thorough veterinary exam will provide the answers.

Pruritus, or excessive itching, can be caused by a variety of pests. Some horses suffer from a hypersensitivity or allergy to gnat bites, which results in allergic dermatitis. Affected horses will scratch the hair from their bodies to attain relief. The gnats feed in the sensitive areas, including his belly, thighs and the dock of his tail. As the horse ages, his allergy to the gnats will worsen, according to "The Horse." To limit gnat bites, allergic horses can be kept inside during the hours of dawn and dusk, when gnats are most active. When pastured, keep your horse away from ponds or standing water, where gnats breed. Your vet may suggest an insect sprays containing permethrin to prevent some gnat bites. Severely affected horses may require corticosteroid injections to control itching. Though uncommon in the United States, mange mites cause pruritus. Your vet may prescribe ivermectin and a deworming agent. Horse lice prefer the dense long hairs of a horse's winter coat and will cause pruritus, resulting in bald patches. The affected horse will require a louse shampoo; all equipment in contact with the horse will need to be disinfected.

Though they are neither itchy nor painful, benign skin lesions called sarcoids may cause horse owners some alarm when they erupt. Though these growths are noncancerous, the horse will often lose the hair surrounding the sarcoid, which will have a rippled, scaly appearance. For an accurate diagnosis, the sarcoid will require a veterinary biopsy. Treatment measures for sarcoids include cryosurgery, where the sarcoid is frozen, or injection with immune boosting chemicals such as cisplastin, according to "The Horse." Topical treatments prescribed by the vet also may be used. Sarcoids can be passed from horse to horse by means of a virus, so grooming supplies, blankets and tack should be kept separately when working with infected horses.

Rain rot is a bacterial infection caused by the organism dermatophilus congolensus and affects horses living in wet conditions whose skin has been compromised by insect bites or scrapes. Though not itchy, it can be painful. Young horses with immature immune systems are at the highest risk. Rain rot manifests as crusty lesions on the horse's face, muzzle, topline and rump. Diagnosis requires microscopic examination of the lesion tissue. Antimicrobial shampoos can kill the bacteria, but the infection often clears up on its own when the horse is moved to a dry environment. Severely affected horses may require antibiotics. Ringworm, a fungal infection, is most common in young and old horses or those living in crowded conditions, according to the Horse Channel. The infected horse loses hair in small, ring-shaped areas on the girth area, hindquarters, chest and neck. Healthy horses recover from ringworm infections over a three-month period without treatment, and most develop immunity to the fungus. To prevent the infection from spreading, grooming tools and tack should be cleaned with diluted bleach. Exposure to sunlight can limit the length of the infection. Skin scald usually is caused by unclean living conditions and is characterized by patchy hair loss and scabs on the horse's legs. The legs of the affected horse should be washed daily. Once his environment has been thoroughly cleansed, the horse should recover. Some horses require antibiotics. Ask your veterinarian for the best course of treatment.

Occasionally, a horse will lose hair in response to toxins in his diet. A prime culprit is selenium, a necessary nutrient that when ingested in large quantities can cause severe hair loss and hoof deformities. The horse's mane and tail hairs will first thin and then fall out. Concerned owners can have their horse's hair, feed and hay tested for high selenium content. If the animal's diet is to blame for the hair loss, it should be changed to prevent further damage, according to "The Horse."