Insect bite hypersensitivity (IBH)—commonly known as sweet itch—occurs when a horse has an allergic reaction to the bites of Culicoides midges or occasionally the black fly of the genus Simulium. It can cause extreme itching in horses and can be challenging for veterinarians to effectively treat.

Scientists know IBH is caused by specific proteins in the midge’s saliva that provoke an exaggerated immune response, leading to welts and severe itching, says Ralf S. Mueller, Dipl. ACVD, FANZCVSc (Dermatology), Dipl. ECVD, of the University of Munich’s Centre for Clinical Veterinary Medicine, in Germany. Some horses scratch so hard they rub off their body hair, manes, and/or tails, and they can even create skin wounds. This chronic disease can make Culicoides season (generally summer and fall) miserable for affected horses as well as the owners who try to keep them comfortable, he adds.

Diagnosing and Treating Sweet Itch Symptoms

When owners first notice hives and itchiness in their IBH horses, they should seek a veterinary examination to determine the cause of the reaction. Veterinarians can treat horses with anti-inflammatories, such as glucocorticoids, to relieve the itching while they determine a cause. Even so, Mueller says it’s important to know whether the itching is due to IBH or other conditions to develop an effective treatment plan.

Unfortunately, veterinarians can’t treat IBH horses long-term with glucocorticoids because these drugs can lead to significant side effects such as complications for metabolic animals and increased susceptibility to infections, says Nora Langreder, PhD, of the University Hospital Freiburg, in Germany. And despite IBH being an allergic reaction, antihistamines are typically not an effective treating IBH, she says.

Fortunately, new, targeted therapies should be on the market within a few years, says Mueller. But in the meantime, the current gold standard for managing IBH continues to be protecting horses from midges, he says.

How to Prevent IBH in Horses

Scientists know that midges don’t fly well in upward wind, so fans can help prevent them from biting stalled horses, Mueller says. The insects tend to congregate around stagnant water. Keeping water troughs clean and refreshed and IBH horses away from standing water can also help reduce these animals’ risk.

Culicoides are most active at night, so keeping horses indoors with midge-protecting screens from dusk to dawn can also prevent bites, he says. While fly sprays can be effective, they need to be reapplied more frequently in humid and rainy regions, so they might not always be practical, says Mueller. Researchers have shown that citronella and lemon eucalyptus oil do not repel midges.

Fly sheets and fly masks are often useful but in humid climates can trap sweat and other sources of moisture, sometimes leading to secondary skin infections, Mueller adds.

Immunotherapy for IBH and Equine Interleukin-5

Scientists on the forefront of IBH treatment research are focusing on a specific immune-modulating protein called equine interleukin-5 (IL-5), our sources say. IL-5 orders some of the horse’s signaling cells (known as eosinophils) to launch an attack in response to midge saliva proteins. That attack leads to the hives and itching.

Swiss researchers have developed a vaccine that combines the IL-5 antigen with viruslike particles, which causes horses to produce antibodies against IL-5. The vaccine works somewhat like a virus, effectively silencing the horses’ own IL-5 proteins for a period of time and reducing their tendency to command the eosinophils to trigger an inflammatory response, says Antonia Fettelschoss-Gabriel, PhD, a researcher in the University Hospital Zurich Department of Dermatology, in Schlieren, and at Evax Biotech, in Münchwilen, both in Switzerland.

“That subsequently leads to a decrease in IL-5, decrease in eosinophil count, and decrease in clinical signs,” Mueller says. While it does not eliminate clinical signs completely, researchers have shown the vaccine results in at least a 50% improvement in IBH-related problems such as lesions and hair loss within a year of vaccination, Fettelschoss-Gabriel says. By the second year of vaccination, these horses improved by almost 90%. Mueller says the vaccine is currently in the approval process.

Even so, our sources say there are a few potential side effects to silencing IL-5. For example, eosinophils appear to be useful for fighting internal parasites, and they might help other important immune responses as well.

With that in mind, Langreder and her fellow researcher Dorina Schäckermann, PhD candidate at Technische Universität Braunschweig’s Institute for Biochemistry, Biotechnology, and Bioinformation Systems, in Germany, created a short-term, reversible IBH treatment based on a monoclonal immunoglobulin G (IgG) antibody against IL-5.

Together, they engineered a molecule that would avoid activating immune responses in horses with IBH within two or three weeks of treatment. Stopping the treatment stops its effects, meaning owners and veterinarians can decide whenhorses should have a lower immune response. Specifically, that means they could potentially choose to lower horses’ immune responses during the critical periods of IBH affects and then restore them afterwards.

Initial tests showed  the treatment was not only safe but likely effective, the team reported last year. Today, tests are ongoing and “progressing well,” although it’s too early to report on findings, according to Kristine Rossbach, DVM, Team Leader of Product Development, Innovation, and Trends, at Wirtschaftsgenossenschaft deutscher Tierärzte in Garbsen, Germany.

Recombinant Allergens For Equine IBH Treatment

Researchers have also started using recombinant allergens instead of allergen extracts to treat IBH, says Mueller. “Allergen extracts often contain very little of the real immunogenic allergen, and that holds particularly true for IBH, where instead of salivary proteins whole body extracts are injected,” he says. Researchers in Switzerland, Iceland, and Munich have recently explored the use of the actual salivary proteins to desensitize horses’ systems to the allergens exacerbating IBH.

“The first study is now completed with encouraging results, and the second is planned,” Mueller says. “With this approach, a much faster and better response to immunotherapy can be expected.”

In its early stages, producing recombinant allergens used to be a “very time consuming and expensive process,” he adds. “But it’s gotten more practical, faster, and cheaper in the last few months.” Even so, recombinant allergen immunotherapy might not be available commercially for several years, Mueller says.

Reinforcing the Horse’s Skin

IBH horses might benefit from treatments to improve skin strength and integrity, says Mueller. Many IBH horses have basic skin health problems, he explains. Topical moisturizers and essential fatty acids appear to benefit coat health for horses with IBH, although more research is needed.

Researchers showed that 95% of horses sprayed with an herbal combination of camphor, lemongrass, may chang, peppermint, and patchouli had improved skin health compared to a placebo goup. That might be because of their anti-inflammatory, antipruritic, and/or repellent activities, Mueller says. A cream containing omega-3-fatty acids, humectants, and emollients resulted in improvements in IBH horses—although some of the horses had adverse effects, he says.

Take-Home Message

While not a fatal disease, IBH can wreak havoc on horses’ comfort, mental health, and welfare while leading to hair loss, skin lesions, and sometimes secondary infections. Currently, most treatments focus on prevention, but new immunotherapy options are on the horizon—including a vaccine that could lead to significant drops in clinical signs.