23/06/2025
Insect Bite Hypersensitivity
Brian S. Burks, DVM
Diplomate, ABVP
Board Certified in Equine Practice
Insect hypersensitivity is generally a seasonal dermatitis that may result in secondary trauma. It is also known as sweet itch, summer itch, and no-see-um hypersensitivity. This is a type 1 hypersensitivity to the saliva, venoms, excrement, or other body parts of biting insects such as midges (Culicoides spp.) and black flies. Following a bite to a susceptible horse causes the production of IgE, which in turn causes mast cells to degranulate, releasing histamine and other local mediators. There may also be delayed-type hypersensitivity involving cell-mediated reactions.
Horses with insect bite hypersensitivity have a fundamental difference in the building blocks of their epithelial barrier which:
Hinders the ability to form tight junctions in the skin
Increases development of epidermal inflammation
Elevates presence of eosinophils
Impacts ability to modulate immune cell effector functions
Symptoms are similar to atopic dermatitis in other species, including wheals, edema and skin lesions secondary to the bites. Acute lesions include crusty papules and sometimes hives. Chronic pruritus results in secondary excoriation, ulcers, serous effusions, crusts, thickened skin and patchy alopecia, due to self-trauma. Secondary bacterial infection is common.
Insect hypersensitivity often has regional distribution:
1. Dorsal- involving the face, ears, neck under the mane, withers, and tail head. This is often due to Culicoides spp., but others have also been implicated.
2. Ventral- under the jaw, ventral thorax and abdomen, axillae, and groin due to Culicoides spp., Simmulium spp. and Haematobia irritans. (All gnats).
3. Combination of 1 and 2.
Risk factors include proximity to insect habitat, decaying bedding and manure, and standing water. There is also a genetic component causing impairment of the epithelial barrier.
Diagnosis if often based on clinical signs. In some cases, skin scrapings or biopsy may be necessary, especially if there is no or limited response to initial treatment. These tests are useful to look for ectoparasites, fungus, and bacteria via cytology and cultures.
Treatment includes resolving bacterial infection, control of pruritus, and pest management. In cases where there is secondary infection, antibiotics are necessary to control the folliculitis. Long-acting fly sprays such as Equicare's "Flysect Citronella Spray" and Pyranha's "Wipe N’ Spray" help Frequent bathing helps to remove the crusts and surface irritants.
In addition, horses should be stabled with fans during the predominant feeding times at dawn and dusk. The fans help to keep the gnats away, as they are weak fliers. The addition of flax seed provides essential fatty acids (omega-3 fatty acids) which are anti-inflammatory and necessary for skin health.
Initially, to control the itch, corticosteroid medication may be necessary, either parentally, or orally. Antihistamines have limited efficacy as the histaminic reaction is limited, and the delayed-type hypersensitivity is the major reaction. They may have more benefit as a preventative.
BarItch is a new product to help skin. This scientifically formulated supplement is designed to relieve pruritus (itching) and related symptoms in dogs and horses. It uses a unique blend of natural ingredients, BarItch™ targets multiple biochemical and neurophysiological pathways to provide effective relief beyond traditional antihistamines. The product combines betulinic acid from birch bark extract, Alaska wild blueberry/bilberry concentrate, astaxanthin, resveratrol, and a concentrated extract of Alaska wild rosehips. Additional active ingredients—including Alaska wild yarrow concentrate, quercetin, luteolin, red clover extract, and carotenoid-rich carrot powder—offer anxiolytic and mood-enhancing benefits, further supporting overall well-being. This product has been beneficial to many equines with skin disease, shown by clinical testing.
There are special fly sheets and masks that cover the abdomen and face with a small enough mesh to keep the gnats out. Conventional fly masks and sheets will not keep these pests away from the face.
Spring brings warmer weather, and, unfortunately, insects of all sorts. If you know your horse has this yearly, take steps now to control or eliminate the problem through proper stabling, hygiene, and topical fly spray and SDS Horse Lotion, available at Fox Run Equine Center.
Dr. Brian Burks, Dipl. ABVP is the owner/veterinarian at Fox Run Equine Center, a 24-hour medical-surgical center near Pittsburgh, Pennsylvania. He is board certified by the American Board of Veterinary Practitioners (Equine Practice). This certifies him as an expert in all categories of equine practice. He enjoys the diagnostic and treatment challenges of internal medicine and immunology.
Fox Run Equine Center
www.foxrunrequine.com
724-727-3481
