Researchers Developing Equine Sweet Itch Test

Your horse is itchy. You find patches of missing hair on his sides and shoulders. There are gaps in his mane, holes in his tail. Sweet itch? It could be. But then again, maybe not. Belgian researchers say the only way to be sure that your horse is affected by sweet itch is to evaluate him using a confirmed diagnostic test for the disease.

Unfortunately, no such test exists…yet.

Currently, sweet itch is diagnosed by ruling out other causes of itching, such as lice or skin diseases, said Liesbet Peeters, PhD candidate and researcher in the Department of Biosystems at Katholieke Universiteit Leuven in Heverlee.

But recent studies have led to the hope that, in the near future, a reliable test for diagnosing insect bite hypersensitivity (IBH), also known as summer eczema or sweet itch, will be available to veterinarians. Preliminary experiments with a prototype of such a test have been encouraging, Peeters said.

In a study on 343 Warmblood horses—about half of which were suspected to have IBH—Peeters found that her prototype test has 92% accuracy. The test appears to be most reliable when performed when the actual symptoms are present, she added.

The test works by evaluating the horse’s immune responses to specific proteins found in the saliva of biting midges. It is the midge saliva that causes the allergic reaction in horses that makes them itch. In other words, affected horses are horses that are allergic to midge saliva. The test, then, is an advanced allergy test which measures the horse’s immunoglobin-E (IgE), an antibody present in the blood, when exposed to the saliva proteins.

Since midge saliva is made up of many different proteins Peeters and colleagues investigated the accuracy of the test when measuring IgE to several key proteins. They found that the highest accuracy came from measuring the IgE of four major proteins combined.

While Peeters’s prototype test could be useful to confirm a suspected case, she said she would like to see an even more sensitive test in the future.

“A good diagnostic test could help to identify IBH positive horses under circumstances in which IBH symptoms do not occur (during winter, when stabled a lot, or in environments with few midges or when the owner uses preventive measures such as blankets to reduce clinical symptoms),” she said.

A reliable test will help prevent misdiagnosis, which could lead to wrong or inefficient treatments and prolonged suffering for the horse, Peeters said. There’s still much research to be made in refining the tests, she said, but several

different ones—including hers—are in development. They could be available commercially within five to 10 years, said Peeters.

“However, it is important to understand that the pathogenesis of IBH is very complex,” she said. “Therefore, a ‘gold standard’ that can be used in every horse in every circumstance is a utopia which will probably never be reached.”

The study, "Evaluation of an IgE ELISA with Culicoides spp. extracts and recombinant salivary antigens for diagnosis of insect bite hypersensitivity in Warmblood horses," was published in the Veterinary Journal

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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