Insect bite hypersensitivity is the most common allergic skin disease in horses worldwide, and Friesians are no exception. In fact, previous research studies and data collected by the KFPS indicate the occurrence of IBH in Friesian horses is as high as 18.2%. The cause of IBH is multifactorial and involves both environmental and genetic factors.
WHAT IS INSECT BITE HYPERSENSITIVITY?
IBH is known to occur widely throughout the world by various names such as sweet itch, summer dermatitis, summer recurrent dermatitis, summer eczema, or equine Culicoides sensitivity. IBH occurs in horses as the result of an allergic reaction to a type of biting insect called Culicoides. Commonly, they are referred to as gnats, midges, sandflies, biting midges, or “no-see-ums”. The average life span for Culicoides is 20 days. Only female Culicoides bite and take blood from their host, which is needed for the maturation of fertilized eggs. Females typically bite at dusk or dawn and tend to gather in dense swarms, usually in the vicinity of standing water or rotting vegetation. Females lay an average of 100 – 200 eggs which take an average of 15 days to develop. They prefer to lay their eggs in areas such as standing water, rotting vegetation, slow-running streams, damp soil, or on manure piles.
Horses with IBH are allergic to the proteins deposited in the saliva of Culicoides when they are bitten. This causes a specific immunoglobulin E (IgE) mediated type I hypersensitivity reaction. Type I indicates the reaction causes an immediate release of histamine and other inflammatory mediators. Histamine is a compound the immune system makes which helps the horse’s body get rid of something that's bothering it – in this case, it’s the allergen to Culicoides. Histamines start a process that quickly moves those allergens out of the horse’s body or off the skin. They can make a horse sneeze, tear up, or itch – whatever it takes to get the job done. They are part of the horse’s defense system.
The symptoms of IBH are often gradual in onset and coincide with warm seasons because Culicoides only live during warm periods (Spring–Fall). Clinical signs of IBH include severe itchiness, dandruff or flakey skin, and dry skin, specifically in the mane and tail, face, ears, chest, stomach, and hips. Itching is often so severe that horses result to scratching, rubbing, and self-mutilation (biting) in an attempt to find relief. This behavior often leads to abrasions and bleeding of the skin, skin thickening, skin crusts, bumps, rashes, scaling of the skin, ulceration of the skin, hair loss, and secondary bacterial skin infections.
If you suspect your horse has IBH, the first and most critical step is to consult your veterinarian. There are many conditions that can be confused for IBH. Once you confirm IBH is indeed the issue, you can work with your veterinarian to formulate a plan to manage IBH. Managing IBH is based on 3 principles:
Controlling the Itch
Treating Secondary Skin Infections
Preventing Further Biting
CONTROLLING THE ITCH
In order to control the itching, you must interrupt the histamine response or reduce the sensitivity to the allergen. Most horses with IBH also have a significant number of other environmental allergies they are contending with, such as pollens, molds, grasses, dust, mold, other insects, etc. Each horse’s immune system has its own threshold for how much total allergen exposure it can manage without having a significant histamine response. For many horses, the exposure to Culicoides and other insects during warmer months puts the horse over that threshold, and they find themselves in a perpetual state of extreme histamine reaction. Owners often incorrectly assume insects are the only allergen involved. While there is no effective allergy immunotherapy for IBH, other environmental allergens contributing to the overall histamine response can often be treated and lowered. By reducing the overall threshold for many of these horses, they have significantly reduced histamine reactions and, thus, significantly less itching and other symptoms.
The first step in this process, as previously mentioned, is to consult your veterinarian and request an intradermal skin test for allergies. Most larger clinics/hospitals or universities with a dermatology department and pharmacy offer this diagnostic service. This test will give you a thorough understanding of the other environmental allergens your horse is having a histamine reaction to. Custom-formulated immunotherapy shots are then created for your horse, which, when given, regularly teaches the horse’s immune system not to react to these common environmental exposures. Often an antihistamine is also prescribed along with the treatment.
Unfortunately, some horses live in geographic locations with extremely high populations of Culicoides, or they have such severe IBH that immunotherapy for other allergens isn’t effective enough to lower their overall threshold and adequately reduce the histamine reaction. However, for most horses, a once-weekly immunotherapy shot, along with an antihistamine, can work wonders. Other options for controlling the histamine reaction-related itching associated with IBH include topically applied sprays, shampoos, or lotions containing anti-inflammatory medications such as hydrocortisone, triamcinolone, or fluocinolone acetonide. In severe cases, your veterinarian may prescribe stronger anti-inflammatory drugs such as prednisolone or dexamethasone.
TREATING SECONDARY SKIN INFECTIONS
The most common secondary issue horses with IBH experience are bacterial skin infections. These infections often appear in the form of skin crusts, bumps, rashes, scaling of the skin, ulceration of the skin, and hair loss. Again, this is where your veterinarian plays a key role in treating IBH. By taking a skin scraping, your veterinarian can determine the type of bacterial skin infection your horse has and prescribe a treatment for it.
Treatment for secondary skin infections often includes medicated shampoo baths with products that contain an antimicrobial ingredient. Occasionally, oral antibiotics are indicated for treatment. While it might seem prudent to apply something you find under your bathroom skin or in your tack room to what appears to be a skin infection, this is really the time to seek your veterinarian’s opinion. Harsh products or the wrong product may only accelerate the bacterial infection, dry out the skin or prolong the infection.
PREVENTING FURTHER BITING
Avoiding exposure to Culicoides is the most important consideration to prevent further biting. Culicoides prefer to breed in standing water, so evaluate your pastures for drainage issues and, if possible, keep your horse in dryer areas away from standing water and manure piles. Avoid pasturing horses in low-lying damp areas where the air is stagnant.
Keep your horse inside the barn during peak biting hours (dusk and dawn). If you have fans in your barn, these can help deter Culicoides who are naturally weak fliers in strong winds. If you cannot keep your horse inside during peak biting hours, consider using a sweet itch blanket, such as a Boett Blanket, which is a special type of fly sheet for horses with IBH. These sheets cover the entire horse from ears to tail minus the legs in material that prevents Culicoides from biting.
Permethrin and neem oil repellent sprays such as Equiderma fly spray can also be effective in helping to repel Culicoides but require consistent daily use and thorough application, particularly in areas that are most often bitten such as the ears, neck, belly, and tail head area. It’s important to note that some insecticides, such as pyrethrins, do not repel Culicoides and will be ineffective, so check your labels first.
Research shows that omega-3 fatty acids, which are abundant in flaxseeds, might help reduce the inflammatory response associated with IBH. Consult your veterinarian for a recommendation on an omega-3 fatty acid supplement.
WHAT TO REMEMBER
IBH is an incredibly frustrating condition for both horses and owners. However, it’s important not to assume IBH is what is causing your horse’s itching. Always work with your veterinarian to get the correct diagnosis and develop a thorough treatment plan for your horse. By controlling the itch, treating secondary skin infections, and preventing further biting, your horse’s histamine reaction will hopefully be reduced, and he will be much happier this summer.
References:
Schurink, A., da Silva, V.H., Velie, B.D., Dibbits, B.W., Crooijmans, R., Franҫois, L., Janssens, S., Stinckens, A., Blott, S.C., Buys, N., Lindgren, G., & Ducro, B.J. (2018). Copy number variations in Friesian horses and genetic risk factors for insect bite hypersensitivity. BMC Genetics, 19.
Schurink, Anouk et al. “Risk factors for insect bite hypersensitivity in Friesian horses and Shetland ponies in The Netherlands.” Veterinary journal (London, England : 1997) vol. 195,3 (2013): 382-4. doi:10.1016/j.tvjl.2012.06.037
Schaffartzik, A et al. “Equine insect bite hypersensitivity: what do we know?.” Veterinary immunology and immunopathology vol. 147,3-4 (2012): 113-26. doi:10.1016/j.vetimm.2012.03.017