Before we fully get into summer I wanted to write a newsletter to address a common seasonal concern that many horse owners are faced with this time of year. The concern I’ll be addressing for this month goes by many names such as sweet itch, Queensland itch, summer eczema, and Insect Bite Hypersensitivity (IBH). These are all broad terms to describe the very common allergy that some horses have to the saliva of Culicoides midges. This newsletter will walk through what’s going on, clinical signs associated with the condition, and how to manage/prevent this condition in your horse. We hope this newsletter will help you to understand what’s going on with your horse and what we can do to manage these horses successfully for the long term.

Insect Bite Hypersensitivity

Insect Bite Hypersensitivity is the most common disease that causes itching in horses. It typically plagues horses during warmer times of the year when flying insects are out, such as spring through fall. In severely sensitive horses or warmer geographical locations disease can unfortunately be seen all year round. The main insect group that causes the reaction are from the Culicoides midges, otherwise known as the gnat or no-see-ums. This group of insects have the most antigenic saliva compared to other flying insects that target horses leading to the increased immune response from the horse. While the midges are the most common offender, other insect species such as stable flies, mosquitoes, black flies, and horn flies can also cause a hypersensitivity. This disease can affect horses of any age, breed, or sex.

It’s important to understand that because this is an allergy, your horse will never be completely cured of the condition and it will always be a problem you’ll have to manage. Also, if your horse displays signs of the allergy one summer, the next year will likely be worse for that horse because signs typically worsen with age.

Clinical Signs Of Insect Bite Hypersensitivity

Some horses may have more mild signs of IBH such as a few hives and crusts, while some horses may itch so much they rub their skin raw till it’s bleeding. The characteristic signs of IBH typically occur on the face, ear, trunk, neck, and tail head, but this can vary depending on the biting pattern of the particular insect. Below are some characteristic signs of IBH to watch for when it comes to your horse.

• Itching
• Hives
• Sores
• Crusting
• Hair loss

How To Manage Insect Bite Hypersensitivity

When it comes to managing IBH there is no one true solution to the problem, meaning we have to approach the issue in as many ways as possible to have the best outcome for the horse. The number one way to manage IBH with your horse is through reducing the number of midges exposed to your horse. If we can do our best to eliminate the source of the problem, the rest of the solutions will work that much better.

• Fly Sprays: A temporary solution to help mitigate the number of flies on your horse is through the use of fly sprays daily on your horse. This is especially true when midges are at their peak during the day being dusk and dawn.
• Fans: While the midges’ small size may allow them to get through the fly nets/sheets, their small size also means they don’t fly well in the wind. So by providing fans in your barn alleyways or stalls they can help to disrupt the fly patterns of the insects trying to land on your horses. When purchasing fans for your barn be sure to select the dust proof fans to prevent barn fires.
• Turn Out Time: Because the midge is more active at dawn and dusk, it’s best to avoid these times when turning your horse out to pasture.
• Corticosteroids: This is a mainstay of therapy when it comes to treating IBH because it is an immune system overreaction at its core. So, in order to calm the clinical signs associated with the disease we have to dampen the immune system, and no drug does this better than corticosteroids. Depending on the severity and your horse’s other underlying conditions, a dose and duration will be selected to maximize control and get the horse off steroids as quickly as possible to avoid adverse effects of long term use.
• Antihistamines: Some clients find success giving antihistamines such as benadryl, cetirizine, or hydroxyzine. However, there is not much research supporting its efficacy for use in horses. If your horse experiences hives as their main response to insect bites then these drugs may be more effective. This is because the hives are caused by a release of histamine from specific cells in the body, and so by giving antihistamines you’re combating the body’s overreaction.
• Over The Counter Products: There are numerous products and supplements on the market with varying success. So, if you decide to try one of these products make sure it’s from a reputable company with some research behind its ingredients along with good quality control in place.
• Fly Sheets/Nets: Fly sheets for your horses out on pasture and fly nets in your barn can help mitigate some fly species from reaching your horse, but because the midges are so small a lot of times they are able to penetrate the net/sheet limiting their effectiveness.
• Apoquel (Oclacitinib): This drug is a janus kinase inhibitor that is typically used in small animal species such as dogs and cats for different allergic conditions. This drug has also been used in horses anecdotally with success, but further research is needed to understand the best dosage to avoid adverse effects. This drug is also expensive for use in the horse because it takes so much of it compared to dogs and cats.

How To Prevent IBH And Long-Term Solutions

• Omega 3 Fatty Acids: One way to lessen the extent of the reactions your IBH horse experiences is through feeding omega 3 fatty acids in their diet. The main benefit seen from feeding this to your horse is through a reduction in inflammation. There are a multitude of products on the market that include omega 3’s in them, but just be sure to select products from reputable brands with good quality control and research behind them.
• Allergen Specific Immunotherapy: Another long term management option for owners is through sensitizing the horse’s immune system to the allergen of the midge by administering very low levels of the protein produced by the insect. This is done through intramuscular injections, and helps to reduce the horse’s overreaction to the insect bite. It’s typically an expensive route to managing the disease, and may not work for all horses so it should be selected on a case by case basis.
• Manage Insect Breeding Sites: Anytime we’re dealing with managing the flying insect population on a property we always have to start with properly getting rid of the manure, wet spots, and decaying feed material as these are all prime breeding sites for these insects. When it comes to manure and waste I recommend taking it as far away from the barn as possible preferably to a dedicated site. At the site I recommend having it in the sun and placing a black tarp over it to help heat it and hopefully kill the larvae in the manure and waste. For wet spots I recommend improving drainage in those spots and eliminating any water line leaks.

Insect Bite Hypersensitivity is a field that is constantly evolving and improving with continued research and more products coming onto the market. The main thing to understand with this disease is that it is life long, time consuming, and can be costly. If you know your horse has a hypersensitivity to insects then its best to be proactive when it comes to treating rather than reactive. So, when the temperatures start to warm be sure to start implementing more aggressive management strategies before the insect presence increases. Below are some additional resources for further reading.

https://ceh.vetmed.ucdavis.edu/health-topics/insect-bite-hypersensitivity

https://www.steinbeckpeninsulaequine.com/post/insect-bite-hypersensitivity-managing-the-itchy-horse

https://pmc.ncbi.nlm.nih.gov/articles/PMC10416928/#sec5-animals-13-02514

https://thehorse.com/188994/dealing-with-insect-bite-hypersensitivity/

Written By: Dr. Gunner Edgell

Rocky River Large Animal Veterinary Clinic is here to serve you and your animals