Study: Still No Test to Predict Insect Bite Hypersensitivity in Horses

Also known as summer eczema, sweet-itch, Queensland itch, Kasen or Culicoides hypersensitivity, insect bite hypersensitivity (IBH) causes bleeding, inflamed, and painful skin lesions anywhere on the horse’s body. Being able to predict which horses are at risk to IBH prior to purchase would help owners avoid having to deal with this frustrating disease, yet the latest research proves this remains a far-fetched notion.

“IBH is a chronic, recurrent seasonal dermatitis of horses caused by an allergic reaction to the bite of midges, Culicoides spp., as well as other blood feeding insects like black flies, stable flies, mosquitoes, and horseflies,” explained Laura Petroski-Rose, B.V.M.S, a Kentucky Equine Research veterinarian.

While most horses exposed to Culicoides spp. respond to insect bites without much ado, approximately 3-10% of horses develop an allergic reaction to antigens found in insect saliva. The bodies of these IBH horses overreact to those antigens, producing potent pro-inflammatory molecules such as immunoglobulin E, histamine, and sulphidoleukotrienes (sLT).

In Iceland, Culicoides spp. do not exist; therefore, IBH does not exist. Interestingly, approximately 50% of Icelandic horses moved from Iceland to mainland Europe ultimately develop IBH within two years. This is remarkably higher than the aforementioned prevalence in other horse breeds, and no genetic link to IBH has yet been identified.

Due to the discomfort, frustration, and treatment costs associated with IBH, veterinarians attempted to use an sLT release assay to predict which Icelandic horses were at risk for developing IBH. Such a test would help owners decide whether or not they would be interested in exporting at-risk Icelandic horses to areas home to Culicoides spp.

“The laboratory test, known as a CAST test, is a simple blood test that measures the release of sLT and is currently used to diagnose horses with IBH with much success, shared Petroski-Rose.

Unfortunately, the test was not able to predict which horses were susceptible to the disease.

The researchers further explained, “…a positive CAST result when stimulating with C. nubeculosus in horses recently imported from Iceland can only be used to confirm clinical cases of IBH. The CAST should not be used to identify horses predisposed to IBH before occurrence of first clinical signs, because some horses that remain free of this disease can be transiently positive to the test. This suggests that horses can be sensitized to Culicoides spp. allergens to some degree but that regulatory mechanisms may then control further development of the disease.”

Other research teams are currently working on an “allergy vaccine” that may help IBH horses. In the meantime, using nutritional supplements with natural anti-inflammatory properties such as Kentucky Equine Research’s EO-3 to support the immune system and overall health of the skin and coat.

“EO-3 contains omega-3 fatty acids, which are research proven to help with skin problems, including insect-bite hypersensitivity. This particularly high-quality supplement provides 6,750 mg of EPA and DHA per serving,” Petroski-Rose said.

She added, “Kentucky Equine Research also offers hoof and coat supplements, such as BioBloom PS (BioBloom HF in Australia), which contain nutrients such as biotin, methionine, zinc, and iodine.”

*Torsteinsdottir, S., S. Scheidegger, S. Baselgia, et al. 2018. A prospective study on insect bite hypersensitivity in horses exported from Iceland into Switzerland. Acta Veterinaria Scandinavica. 60(1):69.