Sweet itch in horses, also known as summer dermatitis, is an allergic hypersensitivity reaction to biting flies or midges. Midges breed and hatch in stagnant water, and are more abundant around dense vegetation. Although smaller than mosquitoes, they often make their presence known by flying in swarms of black clouds. The seasonal nature of the horse’s skin allergy is correlated with the life cycle of the midges.

While the immune system recognizes and destroys potentially harmful invaders such as viruses, an allergy occurs when an otherwise harmless antigen also becomes an enemy of an overreacting immune system.

One of the most commonly developed allergies in horses is to antigens injected into the skin by biting flies. To obtain a blood meal, biting flies and midges will cut the skin with their mouthparts to create a pool of blood. During the process the flies inject anticoagulants mixed with saliva into the horse’s dermis. The saliva is recognized as foreign by the horse’s immune system. An immune mediated response occurs to rid the body of the “antigens”, in which immune cells surround the antigens and release enzymes in the skin. Over time this immune response can become exaggerated and is then considered an allergic hypersensitivity reaction to the antigens in the fly’s saliva.

The resulting tissue inflammation causes irritation of the nerve endings in the skin and sends itching signals to the brain. Thickening of the skin due to inflammation compromises the skin’s normal barrier function, allowing easier entry of additional allergens through the skin, thereby multiplying the inflammation and itch cycle. The physical damage to the skin from the horse rubbing fixed objects further predisposes the horse to secondary bacterial skin infections.
Sweet itch is primarily manifested by itching of the face, mane, tail and head areas. The lower abdominal and chest can also be involved. Midges typically feed during the day, and prefer biting the head and ears but will also bite any exposed skin. Providing a darkened shed or stall will hinder the midge’s ability to find the host, and utilizing a fan blowing across the horse is an added deterrent. Face masks, fly sheets, or sweet itch rugs offer a physical barrier. A daily or twice daily application of a fly repellant can also be effective. Although these methods are effective they do not address the underlying allergy.

Cortisone, a steroid, is commonly given by mouth or injection to bring relief by reducing skin inflammation and suppressing the immune system. This is usually a quick means of providing itch relief. The downside is that cortisone not only fails to address the underlying allergy, but cortisone, especially in high doses or prolonged usage, not only predisposes the horse to laminitis, but is also detrimental to the kidneys, liver, and other organs that are responsible for secreting natural hormones. Cortisone is a synthetic drug that mimics a hormone naturally produced by the adrenal glands called cortisol. High dose cortisone injections or tablets can negatively affect the production of natural cortisol by the adrenal glands, and can lead to one of a multitude of metabolic disorders.
Other conventional medical treatments include giving antihistamines to help reduce the release of enzymes from the immune cells. Antihistamines have a reduced risk of complications when compared to cortisone administration; however they are also usually less effective in alleviating the symptoms.
Another treatment option is a desensitization protocol by your veterinarian in which, initially, a small amount of the offending antigen is given by injection. Gradually increasing the amount of the antigens injected on a strict schedule may slowly acclimatize the horse to the antigen and reduce symptoms. This treatment method is relatively expensive and requires a long time to reap benefits, if at all.

Many horse owners overlook the fact that healthier skin is more resilient to sweet itch. Because certain nutrients can help improve the skin’s resistance to dermatitis, it’s worthwhile looking into nutritional supplements that can strengthen connective tissue proteins.
Adding a nutritional supplement supporting healthy skin that contains omega fatty acids, phospholipids, minerals, amino acids, and vitamins to your horse’s diet can greatly diminish the effects of allergic skin disease. An excellent option is Farrier’s Formula® Hoof and Coat Supplement for Horses. This concentrated nutrient source supplies nutrients to help strengthen connective tissue proteins, especially collagen, thereby improving the skin’s resistance to dermatitis.

Your veterinarian and farrier are excellent resources for additional information, or contact our EU Sales Rep, Karen Zetsche.

Scott Gravlee, DVM, CNS
J. Frank Gravlee, DVM, MS, CNS
Equine Nutrition Consultants
Life Data Labs, Inc