Many things can trigger allergic reactions in horses. Dr. Rosanna Marsella, Veterinary Dermatology, University of Florida (Gainesville), says the most common allergy in horses is hypersensitivity to insect bites, particularly tiny gnats (“no-see-ums”) called Culicoides. The resultant itchy condition is called sweet itch or Queensland itch. The horse’s skin reacts to proteins in the gnat’s saliva.
These tiny flies generally bite along the underline, or back, and some species feed around the neck. “This condition is diagnosed by clinical signs (itching/rubbing), in conjunction with a skin test,” says Marsella. “People have tried to create an allergy vaccine to help reduce the hypersensitivity, but success rate is disappointing,” she says.
Dr. Christine Rees (veterinary dermatologist, Hill Country Equine, in Boerne, Texas) says you can try to desensitize the horse using allergy shots for an insect problem, but these work better if the horse also has pollen allergies. “Success rate is fairly low (20 to 30%) if you just use the shots for insect allergies by themselves,” she says.
Insect control is the best strategy, using repellents on horses and fine mesh screen to keep flies out of the barn. “You need an excellent ventilation system,” says Marcella. “If you close up the barn to protect the horses, it may become very hot inside. You need strong fans to provide air circulation, particularly if you have horses with respiratory issues. If you have an open barn, just use a strong fan. The tiny gnats are unable to fly in a breeze. Good fans and repellent will significantly cut down on insect exposure,” she explains.
A variety of products are advertised as repellents. “Most of them are insecticides, not repellents. If a horse has allergies, it’s not enough to kill the insect after it has bitten the horse. You need to prevent the bite. Studies on repellent activity have been done on mosquitoes, not on Culicoides, so we aren’t sure which products are most effective. Many veterinarians recommend products containing permethrin. In order to be a repellent, it has to be at least 2% permethrin.”
It can be a challenge to find something that works. “Despite the fact the label may say it should be effective for a week or two, the reality is that with humidity, high temperatures, horses sweating, etc. it won’t last that long,” says Marsella. A repellent should be applied daily. Preventing the problem by protecting horses early in the fly season is better than having to deal with it after the horses develop the reaction.
“Repellents that give good response include Fly Pel, a veterinary product that requires a prescription. Or, you can substitute TriTec 14 but it has to be applied daily. Florida is probably the most challenging place for insects. There may be other regions where you can get by with less frequent application. Some products are water soluble and readily washed off, while others stick to the hair better. Endure tends to stay on, but collects dirt/dust when the horse sweats or rolls,” she explains. Some people have tried garlic as a repellent, but there is no scientific evidence that it works.
“Topical steroids may decrease itching. Systemic steroids may be necessary to shut down the allergic reaction, but we try to avoid systemic use as much as possible, because of risk of laminitis. We use topicals like Genesis Spray (a triamcinolene spray), or a hydrocortisone leave-on conditioner applied to selected areas of the body to make the horse more comfortable,” says Marsella. Some horses may respond to antihistamines.
It’s also important to treat any secondary infection that may result from insect bites and itching. Depending on severity, treatment may require systemic antibiotics or topical therapy if it’s mild. “Topical antibacterial products like benzoylperoxide shampoo or chorhexidine shampoo may work, but if infection is extensive the horse needs oral antibiotics like trimethoprim sulfa,” she says.