Fly Spray Allergy in Horses

Fly Spray Allergy in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost
Fly Spray Allergy in Horses - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

What is Fly Spray Allergy?

There are all sorts of insects that bother horses and their owners.  Flies are usually a major problem no matter what region you live in.  This leads owners to seek relief for their horse which can often be found in the form of a fly spray.  While this works great for many horses, some can be allergic to it.  There are a large variety of sprays available to owners so your horse may be allergic to one or more of the sprays depending on the ingredients.  

An allergy to a spray is also known as a contact allergy; when the product comes into contact with your horse, his system sets off a type of allergic reaction.  Diagnosis can usually be done by observation of clinical symptoms alone.  Treatment is straightforward and if done correctly, prognosis of recovery is very good.

It is possible your horse is allergic to the fly spray you apply on him.  If you notice he has hives or skin lesions, contact your veterinarian.

Symptoms of Fly Spray Allergy in Horses

Symptoms of this allergy may include:

  • Irritation
  • Profuse sweating
  • Edema 
  • Exfoliation
  • Excessive activity of the sebaceous glands
  • Hives 
  • Itching 
  • Papules/lesions on the area of contact
  • Pain at application site, rare but possible 


The term allergy to fly spray can be used synonymously with a contact allergy.  A contact allergy is typically a delayed sensitivity, but not always.  This means it usually requires weeks to months of repeated exposure to the allergen for the sensitization to develop.  Once there is sensitivity, clinical symptoms typically appear 24 to 48 hours after exposure.  However, there are cases where the horse shows immediate sensitivity to the substance after one or a couple of applications.  The degree of sensitivity varies on the ingredient involved as well as the horse as an individual.



Causes of Fly Spray Allergy in Horses

There are many different fly sprays available over the counter and through your veterinarian.  Each spray has different ingredients and proportions of each ingredient within it.  There are fly sprays that may contain pyrethrins, pyrethroids, fipronil, organophosphates, or N, N-diethyl-meta-toluamide (DEET) concentrations.  Your horse may be allergic to one or multiple of these primary insecticides.  You can try to a natural spray but even then it is possible your horse may be sensitive to the citronella ingredient.



Diagnosis of Fly Spray Allergy in Horses

When diagnosing an allergy to fly spray in your horse, it will be based primarily on clinical signs.  Symptoms of a contact allergy are very similar no matter what the culprit.  Your veterinarian will begin by performing a full physical exam on your horse.  She will make note of all his symptoms and where it is primarily affecting him on his body.  She will also want to collect a verbal history from you to discuss what your horse has come into contact with recently.  This will be important when she is trying to come to a conclusion of what exactly caused his reaction. 

Once you think you have correctly selected the allergen, you will need to retest it for a true diagnosis.  First, you will need to clear up the original reaction by removing the suspected allergen from your horse’s routine.  You will need to avoid using the fly spray until the initial reaction clears up completely.  This may require you to wash the skin thoroughly multiple times to remove any remaining allergen.  If there is no secondary skin infection, the lesions on the affected area should clear up in 7 to 10 days.  Once the lesions have healed, you will now need to rechallenge the suspected allergen source; this means you will need to use the fly spray again.  If it is the spray, you can expect the lesions to reappear and/or worsen within 24 to 48 hours.  

Another way to diagnose a contact allergy is by utilizing a patch test.  You will use a shaved area on the lateral aspect of your horse’s neck and apply suspected allergens to different patches.  This test is beneficial if you are unsure of which substance your horse is allergic to.  You will be able to try multiple substances at once to come to a diagnosis quicker. 

If your horse is not experiencing the typical symptoms of a contact allergy, your veterinarian may want to verify her diagnosis with other diagnostic tests involving the skin.  For example, your veterinarian may want to take a skin scraping sample or perform a skin cytology to check for other possible causes.  These tests can rule out skin issues that produce similar symptoms.  The veterinarian may need to rule out parasitic infections, fungal skin infections, or other likely skin ailments to come to a proper diagnosis.



Treatment of Fly Spray Allergy in Horses

If you notice your horse having a skin reaction like the symptoms listed above, you should immediately bathe your horse with a gentle shampoo and rinse off thoroughly.  This is the best way to remove the applied spray as quickly as possible.  Topical application of calamine lotion or antihistamines can help with any itching; witch hazel can also be utilized to help with itching. 

The skin will need to be treated accordingly with the lesions and symptoms your horse has developed.  If there is a secondary skin infection from constant rubbing or self-mutilation, your horse will need antibiotics.  Your veterinarian may also recommend a topical medication which may come in a liquid, ointment, or spray formula for you to apply directly to the lesions themselves.  Your veterinarian may need to prescribe additional medications or therapies depending on the severity of your horse’s condition.

Avoidance of the allergen is ideal.  It may take some time, but if you are able to narrow down the ingredient of the fly spray you horse is allergic to, it would be much better for your horse.  Then you can shop for a different spray that does not contain the ingredient and therefore can be safely used on your horse.  You can also use other methods of preventing flies from bothering your horse.  There are screens you can put over your windows to decrease fly access points in his area, you can put fans where you keep him to discourage flies from landing on your horse and biting him, you can put on a blanket, and more.



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Recovery of Fly Spray Allergy in Horses

The more severe the allergy is and the region affected will play a part in your horse’s recovery. If the area is small, the symptoms may be quick to resolve; if a large region is affected, recovery may take longer.  You must also take into account if there is a secondary skin infection present.  If so, the recovery process will take longer and need more aggressive treatment.  However, as long as you address the condition of the skin and treat it according to your horse’s veterinarian, prognosis of recovery is good.



Fly Spray Allergy Questions and Advice from Veterinary Professionals

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