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Insect Bite Hypersensitivity in Horses

12 min read

insect bite hypersensitivity

Insect bite hypersensitivity is more than just a bug bite. Also known as sweet itch and summer itch, insect bite hypersensitivity (IBH) can seriously impact your horse’s quality of life. Secondary infections may develop and your horse can come to serious harm depending on the severity of the allergic reaction. 

What is Insect Bite Hypersensitivity?

Insect bite hypersensitivity is an allergic reaction to the bite of any one of a wide number of bugs. It’s caused by the body’s inflammatory response to the saliva of several different insects. 

Sweet itch in horses is most commonly caused by an allergic reaction to an antigen in the saliva of the femaleCulicoidesmidge. But it’s not just midges that you need to worry about. Mosquitoes, gnats, no-see-ums, black flies, stable flies, and horn flies can cause insect bite hypersensitivity as well. 

There are two different types of IBH. Type 1 is when an immediate reaction occurs after the bite. Type 4 is characterized by a delayed reaction. Both types have similar symptoms and can affect the chest, shoulders, mane, tail, and midline of the abdomen. 

It can be a very serious condition with potentially debilitating consequences. A swarm of gnats can deliver up to 3,000 bites within an hour, and recovery from that kind of exposure can take three to six weeks. Be extra vigilant if you care for Northern European breeds, such as Welsh ponies, Icelandic horses, and Shire horses, as they may be predisposed to it. 

Insect bite hypersensitivity can be misdiagnosed as several different infections and/or conditions. Differential diagnoses for insect allergies includes food allergy, atopic dermatitis, bacterial skin infections, and fungal skin infections. For example, main differentials for sweet itch of the mane and tail include stable vice and food allergy. On the other hand, horses with atopic dermatitis of the face, ears, legs, and abdomen may be misdiagnosed as suffering from insect hypersensitivity.

Symptoms of Insect Bite Hypersensitivity

There are five main symptoms of summer itch that are easily recognized by equestrians. 


Hives are soft raised lumps on the skin. They may appear like a donut, with a depressed center. If they are especially severe, they will burst open and leak a sticky fluid that needs to be cleaned regularly. Hives accompany your horse’s itching and may be localized or cover the body. 


Symptoms Increase During Fly Season

If your horse’s skin condition calms down over the winter months, only to pick back up in the summer heat, you could be dealing with summer itch. Symptoms will only occur while your horse is exposed to the insect that causes the hypersensitivity. 


Abrasive Skin Damage

One of the biggest issues with insect bite hypersensitivity is secondary infection. These infections occur because of the abrasive skin damage that results from the horse’s constant itching. The more they itch, the worse the damage becomes, creating a never-ending cycle that leaves your horse open to infection. 


Thickened Skin

Also known as Lichenification, thickened skin is caused by chronic itching or rubbing. The irritation results in the growth of additional skin cells and magnifies any imperfections, such as cracks or wrinkles. The result is a bark-like or leathery appearance. 


Severe Reactions

Severe reactions may occur, but only rarely and are limited to black fly hypersensitivity reactions. Black flies are known to have a salivary toxin that can result in respiratory distress, shock, and even death when the horse is bit multiple times.


Behavioral Indicators of Sweet Itch in Horses

Sweet itch in horses can appear as not only physical symptoms, but may have behavioral indicators as well. Your horse could be suffering from insect bite hypersensitivity if you notice: 

Excessive Rubbing

If you watch your horse closely, you may notice him rubbing on everything he can find: fencing, buckets, the sides of his shed, the bars of his stall, even his salt block holder. This behavior is a sure sign your horse is very, very itchy. 

When you notice your horse rubbing, check his living environment for anything he could get caught on. Serious injuries can occur if your horse is wearing a halter and it gets stuck on a fence post or salt block holder.


Hair Loss Near Mane or Tail

Hair loss near the mane or tail is a symptom of sweet itch that is commonly mistaken for a fungus, parasite, or other condition. Horses suffering from sweet itch will start to rub these areas on anything within reach, leaving bald spots and raw skin behind. Unfortunately, this starts a downward spiral of reinfection. As the horse loses its mane and tail hair, the skin is left unprotected and becomes an easy target for more insect bites.


Self-Trauma from Scratching

Self-trauma occurs when a horse uses his teeth or hooves to aggressively bite and scratch at himself. While this biting behavior does “scratch the itch” in the short term, it can lead to bleeding, wounds, and secondary infection if the insect bite hypersensitivity goes unmanaged. If your horse is spending the majority of his time biting at his legs or sides, then you could be looking at a symptom of a bad case of sweet itch.  


Itchy During Grooming

While many horses enjoy grooming, an itchy horse will become a bit obsessed. You may notice your horse pushing against you when currying, moving to ensure that you continue grooming a certain area, and becoming irritated or fussy when you groom other areas of the body. Because sweet itch is often located near the mane, tail, underbelly, and back, notice if your horse shows signs of itchiness when you groom these areas in particular.


Frequent Rolling

Rolling is a commonly known sign of colic, but it can also be a symptom of sweet itch. After all, what better way to scratch an itchy back than rolling?

When you see your horse rolling more often than usual, take the time to evaluate his vital signs carefully. If he isn’t suffering from colic, look closely at his back. Do you notice any lumps, scabs, raw skin, or hair loss? If you see any of these, you could be dealing with a case of summer itch. 


Causes of Insect Bite Hypersensitivity

Many different species of biting insects can cause sweet itch in horses. Sometimes multiple species can cause allergic reactions at the same time. While it is nearly impossible to know for sure, you can take a guess at which insect is causing your horse’s reaction based on your local environment and the location of the reaction. 


Culicoides midges, which are the number one cause of insect bite hypersensitivity, prefer to bite the horse’s mane, tail, and lower belly. You’ll notice them more at sunrise and sunset. Like many insects, such as mosquitoes, these midges rely on standing water for breeding. They also feed on decaying vegetation and manure. You may notice them more on days when the air is still and heavy and there is no breeze. This is because gnats are notoriously weak fliers and can’t compensate for the impacts of heavy wind.

Black flies, on the other hand, are much stronger fliers and need running water, not standing, in order to reproduce. They bite the face, ears, abdomen, groin, medial forelegs, and thighs. These flies are more active during the morning and evening and tend to lie low for the afternoon. 

Stable flies aren’t limited by the time of day. They will bite your horse all day long and prefer to stay under the shady areas of trees. These flies bite the legs and abdomen of your horse and like manure and decaying bedding. A good way to prevent these flies from setting up shop close to your barn is by moving your manure pile a good distance away. 

Horn flies are more common around cow pasture as they prefer the manure from cattle over horse manure. They tend to bite around the focal midline and are active all day long. 

Unlike other flies and midges, mosquitoes tend to bite the lateral aspect of the body, meaning towards the sides and away from the underbelly. They’re at their most active immediately after sunset as they feed at dusk. If your barn is near a watery or swampy environment, you’ll probably have more mosquitoes than usual as they prefer the water. 

Deer flies and horse flies target the sides of the chest, flanks, and proximal legs. Both species are more active during the day and require vegetation and proximity to water to survive. 

You may notice some overlap between the environments and preferred biting location of several of these insects. This is what makes it so difficult to uncover which insect is causing a reaction in your horse. 

Possible Secondary Infections

Insect bite hypersensitivity is a breach in the epidermal barrier, which leaves room for secondary conditions, like bacterial infections and parasites, to move in. Lesions caused by sweet itch in horses are essentially open wounds. The presence of open wounds can attract even more biting insects to the area which further contaminates the injury. 

Bacterial infections are a relatively common secondary problem when it comes to IBH. Some veterinarians will prescribe antimicrobial shampoos or ointments as a preventative or therapeutic treatment. If the problem worsens, systemic oral antibiotics may be prescribed as well. 

Itching and bald patches aren’t the only problems you have to worry about when you’re dealing with sweet itch in horses. Culicoides midges also transmit larvae from the parasiteOnochocerca. This larva works its way deeper into your horse's skin, which causes more itching and crusting. If your veterinarian suspects parasites are present, they may prescribe a deworming protocol on top of the treatment for summer itch.

Did you know that your horse has yeast in their ears? All horses naturally have some amount of yeast present in their ears. If your horse’s ears have been impacted by summer itch, there is a risk that the naturally occurring yeast in the ears will become out of balance. An overgrowth of yeast is characterized by a bad odor and scaly appearance. Typically, veterinarians will prescribe the use of an antifungal ointment in the ears for a few weeks until the environment is rebalanced. 

Stable Management Practices to Prevent Insect Bites

You can offer your horse some relief from the biting insects that cause summer itch by changing your stable management practices

Remove Standing Water

The more standing water you have around your barn, the more gnats you’ll have, as midges use sources of still water to breed. Empty all buckets that are not in use. You can’t empty your horse’s water trough, but you can create artificially moving water with the help of a bubbler. Floating bubblers designed for horse farms cause the water to gently bubble and move. This won’t deter most horses and will encourage gnats to find another place to breed.


Use Fans in Stalls

Most equestrians will hang fans in their horse’s stall to help them stay cool on humid days. These fans can double up as a preventative method for sweet itch in horses.

As we said earlier, Culicoidesmidges are horrible fliers. Fans can quickly become the best friend of a horse with sweet itch. Turning on a fan whenever your horse is in their stall works effectively to deter gnats from landing and biting.


Reduce Mud in Pastures

Mud means moisture, which gnats love. The more mud in your pasture, the more gnats you’ll have, and the more likely you are to find yourself with a herd of horses with summer itch.

One good way to reduce mud is by rotating your pastures so there’s less wear and tear on your footing. Many equestrians will also install gravel by their water troughs to prevent mud from developing when the trough overflows.


Keep Horses Stabled from Dusk to Dawn

Gnats are at their most active from dusk to dawn. This problem has an easy solution: to avoid sweet itch in horses, keep your horse stabled overnight. Stabling them while the dew is on the grass will also help to prevent pastern dermatitis and other moisture-related skin conditions. 


Mow Pastures Short

Tall and overgrown pastures cause several problems. Overgrown pastures are more likely to contain harmful plants and weeds that are toxic to your horse. Tall grasses can harbor disease-causing insects like ticks or hide venomous snakes, especially if you’re in the southern United States. But besides these many reasons to mow your pastures, there is also a link between overgrown pastures and sweet itch in horses, as tall grass or wooded areas attract gnats.

To prevent sweet itch in horses, it’s recommended to mow pastures to between two and four inches in height. 


Create Physical Barriers Between Your Horse & Gnats

If gnats cannot land on your horse’s skin, they cannot bite your horse and cause the allergic reaction that results in sweet itch in horses. The easiest ways to prevent 

Culicoidesmidges from landing on your horse is with the use of fly sheets, fly masks, and fly boots. 

You can purchase sweet itch-specific fly sheets, which will have a belly band, wide tail flap, and neck cover. To increase protection from sweet itch in horses, use a fly mask with both a nose and ears, if your horse will tolerate it. 


A Genetic Predisposition to Sweet Itch in Horses

New discoveries in human science have, for the first time, demonstrated that an altered epithelial barrier plays a role in the pathogenesis of allergies, alongside Th-2 cell involvement.

A recent study hoped to expand on the current understanding of the role of the epithelial barrier and skin immune response in the pathogenesis of this disease in horses, as well as investigate if genetic defects in the epithelial barrier may predispose certain horses to developing insect bite hypersensitivity. 

The epithelial barrier of 10 horses with insect bite hypersensitivity was analyzed, as well as skin samples from nine horses without clinical signs or history of IBH. Samples from horses with insect bite hypersensitivity were collected from both lesional and non-lesional skin. Each sample underwent extensive testing, including RNA sequencing, differential gene expression analysis, gene ontology analysis, and pathway analysis. 

The histopathological evaluation confirmed the diagnosis of insect bite hypersensitivity in all lesional skin samples. Lesional skin samples displayed substantial hyperkeratosis and acanthosis, infiltration with lymphocytes and a strong infiltration with eosinophils in all but one sample, which instead showed strong dermal infiltration with lymphocytes. Non-lesional skin samples from horses with insect bite hypersensitivity also showed infiltration with eosinophils, although to a lower degree than the samples with lesions. 

In short, the lesional skin samples of horses with IBH showed changes in the epithelial barrier and substantial immune signatures. While it is perhaps only mildly surprising that the lesioned skin of horses with insect bite hypersensitivity differs genetically from healthy horses, it’s doubly surprising that the non-lesional skin samples also differed genetically from those of healthy horses. According to the study, “596 genes were differentially expressed. 461 genes were significantly upregulated… and 135 downregulated.”

Among the top 30 of these differentially expressed genes were those involved in epithelial barrier formation and metabolism of epithelial lipids. Other differentially expressed genes included those for glycerolipid metabolism, pruritus, and immune signatures.

Changes in gene expression between the non-lesional skin samples from IBH horses and healthy horses seem to indicate that some horses have genetically different epithelial barriers which may predispose them to developing insect bite hypersensitivity. 

Per this study, it appears that horses with insect bite hypersensitivity have a fundamental difference in the building blocks of their epithelial barrier which: 

  • Hinders the ability to form tight junctions in the skin
  • Increases development of epidermal inflammation
  • Elevates presence of eosinophils
  • Impacts ability to modulate immune cell effector functions 

The Future of Sweet Itch in Horses

Traditionally, the only relief for horses suffering from summer itch comes from treatment of symptoms using glucocorticoids. Allergen-specific immunotherapy is the only way to “cure” an allergy. However, this method, along with antihistamines, has been shown to have limited impact on sweet itch in horses. This could be due to the fact that the studies performed were not using pure, well-defined allergens (in this case, Culicoidesmidges). 

Researchers are working to develop a vaccine for insect bite hypersensitivity that will have both preventative and therapeutic implications. According to one review, “Prophylactic vaccination against IBH using recombinant Culicoidesallergen has been developed in unexposed Icelandic horses and is ready to be tested.”

While this is exciting, clinical studies looking at the vaccine's efficacy for both prevention and treatment have yet to be run. Plus, this study only reviewed the possibility of a vaccine for sweet itch caused by Culicoidesmidges. But Culicoidesmidges are not the only insects that cause IBH. Further questions include whether or not a horse would need to receive multiple injections tailored to the variety of biting insects that can cause sweet itch in horses.

With further development, there may come a day when preventive immunotherapy prior to exposure to insect bite hypersensitivity-causing insects becomes the norm.  

Competition-Safe Management of Sweet Itch in Horses

As new data continues to emerge about the root cause and impact of insect bite hypersensitivity, it’s important to look to the future and towards science-backed management options. The genetic component of IBH means that horses who are prone to this condition will require long-term management. Unfortunately, long-term use of the steroidal products that are traditionally used to manage IBH can have significant side effects, including laminitis and muscle wasting.

Veterinary medicine is currently limited to managing symptoms of summer itch, as there is no way to completely remove the cause and options like vaccination are still under development. Go-to options include steroids, such as systemic Dexamethasone, or Hydrocortisone creams and ointments. All of these are unavailable to competition horses, as they contain banned substances. 

Non-steroidal management is key to managing insect bite hypersensitivity in competition horses and avoiding the harmful side effects of long-term steroid use. Unfortunately, there are few options available on the market.

When you’re out of options for managing insect bite hypersensitivity in a competition horse and the owner is faced with the prospect of giving up their show season, reach for Zarasyl Equine.  Zarasyl Equine is trusted by top riders and veterinarians, including racehorse trainer Roger Charlton:

Here’s what Susan L. White DVM, MS, Dipl. ACVIM has to say about Zarasyl:

“Minor lacerations through the skin, subcutaneous tissue and muscle fascia heal especially well and rapidly with the use of Zarasyl without the occurrence of excessive granulation tissue, even when horses are kept in less than ideal conditions. Zarasyl helps to resolve crusting sores on the inner surface of ears due to insect bites as well as aid resolution of pastern dermatitis.”

Click here for more information on the science behind Zarasyl.