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Differential Diagnoses for Insect Bite Hypersensitivity

October 04, 2022 5 min read

Horse bug bites

Insect bite hypersensitivity is an itchy and uncomfortable allergic reaction to the bites of certain insects. Hypersensitivity to bug bites is characterized by hair loss, itchy bumps, scabs, and dry flaky skin. Unfortunately, there are several other dermatologic conditions that display the exact same symptoms. 

It’s important for veterinarians to be aware of differential diagnoses for insect bite hypersensitivity, as well as some unique markers for hypersensitivity to bug bites, so the horse can get the correct treatment and feel better that much faster. 

Atopic Dermatitis

Atopic dermatitis is similar to hypersensitivity to bug bites in that it is also an allergic reaction. However, this reaction is typically caused by other environmental factors, such as dust, hay, bedding, etc., and not insects. If a veterinarian were to treat atopic dermatitis with the same protocol that they would treat hypersensitivity to bug bites, there would most likely be initial improvement. But without reducing exposure to the irritant, the symptoms would most likely persist without topical management.  

The most common symptoms of atopic dermatitis are urticaria (hives), and pruritus (itching). 

Steroids and antihistamines are the most common treatment methods, which is also the traditional method of treating insect bite hypersensitivity. Also like hypersensitivity to bug bites, the areas most commonly affected are the face, ears, chest, abdomen, and legs.

While atopic dermatitis does not initially include hair loss, this can occur as a secondary skin abnormality due to trauma from scratching the itchy areas of the body. 

Oftentimes atopic dermatitis is an inhalant allergy, meaning that it is a reaction to an irritant inhaled by the horse. Unfortunately, respiratory allergies are usually a life-long struggle that tends to get worse with age, unlike hypersensitivity to bug bites, which is cyclical with the seasons. 

The good news is that with the proper diagnosis, atopic dermatitis can be managed by reducing exposure to the allergen, whether that’s by steaming hay, changing hay types, switching bedding, or otherwise removing the irritant from the horse’s environment.

Food Allergy

Food allergy in horses is rare and difficult to diagnose. Because of its rarity, it could be easily mistaken for insect bite hypersensitivity. As the saying goes, “If you hear hoof beats, think horses, not zebras.” But in this particular situation, veterinarians could be looking at a zebra masquerading as a horse! 

Similar to hypersensitivity to bug bites, food allergies result in hives, itchiness, and self-trauma from scratching, to include hair loss. 

While veterinarians often attempt to diagnose this allergic reaction via allergy testing, this method can be complex and unreliable. It will often come back with a multitude of positives that only serve to confuse, instead of clarify. 

Another method of diagnosing this condition is to start an elimination diet. An elimination diet involves removing the suspected allergen from the horse’s diet for four to eight weeks in order to observe an alleviation of symptoms. In some cases, the horse may be removed from all commercial grain and supplements entirely. 

Most horses will make a full recovery once the allergen is removed from their regular diet.

Skin 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. While skin infections can develop as a secondary issue on top of insect bite hypersensitivity, occasionally symptoms of a skin infection can be mistaken for hypersensitivity to bug bites. 


Common skin infections in horses include equine pastern dermatitis caused by staphylococcus and/or Dermatophilus congolensis. Another type of skin infection would be dermatophilosis, which is also caused by the D. congolensis bacteria. Both of these skin infections result in lesions, hair loss, and scabs. Dermatophilosis typically impacts the horse’s back, while pastern dermatitis usually occurs on the lower legs, particularly the pastern. 

Skin infections and hypersensitivity to bug bites have a few symptoms in common. Hair loss and scabbing can occur with insect bite hypersensitivity, dermatophilosis, and equine pastern dermatitis. One notable symptom that is missing from skin infections is the itching. Horses with hypersensitivity to bug bites will be intensely itchy and suffer from self-trauma due to scratching. This intense itchiness is typically absent with skin infections. 

Secondary Issue: Parasites

Itching and bald patches aren’t the only problems you have to worry about when you’re dealing with hypersensitivity to bug bites. Culicoides midges also transmit larvae from the parasite Onchocerca. This larva works its way deeper into your horse's skin, which causes more itching and crusting. If your parasites are suspected to be present, most veterinarians prescribe a deworming protocol on top of the treatment for insect bite hypersensitivity.

While this isn’t a differential diagnosis for hypersensitivity to bug bites, it can be a complicating factor and may be part of the reason why your horse’s insect bite hypersensitivity is not resolving. 

Unique Symptoms of Hypersensitivity to Bug Bites

Luckily, hypersensitivity to bug bites does have a few telltale signs that make it easier to tell the difference between insect bite hypersensitivity and other differential diagnoses. 

If your patient makes a full recovery every winter, only for symptoms to return in the summer, then you’re most likely dealing with hypersensitivity to bug bites. Symptoms of insect bite hypersensitivity will always increase during peak fly season and will most likely return year after year when the insects return to the horse’s environment. For horses in areas of the world where it becomes intensely cold through the winter months, symptoms may disappear altogether as insects die off. 

As symptoms will only occur while your horse is exposed to the insect that causes the hypersensitivity, there are a few ways you can test to see if insect bite hypersensitivity is the correct diagnosis without waiting for cold weather.

An easy way to test whether an allergic reaction of the skin is related to insects, diet, or environmental allergens is to increase a horse’s fly protection gear. Most horses with a hypersensitivity to bug bites will display reduced symptoms with the addition of a few stable management changes and the addition of an insect bite hypersensitivity-specific fly sheet. Try increasing your patient’s fly protection and wait to see a decrease in symptoms. 

Hypersensitivity to Bug Bites: A Tricky Condition to Manage

Insect bite hypersensitivity is a frustrating condition to manage that can have a real impact on the horse’s quality of life. Without the right diagnosis, this allergic reaction can take a long time to resolve. On top of this, it can be tricky to come to a conclusive diagnosis. 

Luckily, Zarasyl Equine makes it easier to manage hypersensitivity to bug bites. With a non-toxic and non-irritating formula that works with the horse’s own immune system, Zarasyl Equine can work to help control these dermatologic conditions, while you work towards finding the right diagnosis. Whether your patient has atopic dermatitis, infection, food allergy, or hypersensitivity to bug bites, Zarasyl is here to help. 

Click here to learn where to buy Zarasyl Equine. 

Click here to learn more about Insect Bite Hypersensitivity.