Equine Allergy and the Allergic Threshold

Equine Allergy and the Allergic Threshold

We all know that our horses each have their own individual personality, little traits that make them unique and behave in a particular way. Did you know that the same can be said for their immune system? It’s why some horses will never suffer an allergy, whereas others will flare up only at certain times of the year and why an unlucky few will seem to be itchy a lot of the time!

To understand a little more, it is useful to know the difference between an allergy and allergen:

Allergy – an overreaction by your horse’s immune system resulting in the over production of antibodies and ultimately the occurrence of symptoms

Allergen – a normally harmless substance which causes your horse’s immune system to over produce antibodies

The varying allergic reactions we see between different horses are in part down to something called the ‘allergic threshold’. This is the term used to describe the point at which you start to see the symptoms associated with allergy, for example itching or coughing. How quickly this threshold is reached will vary between horses. For some one single allergen, perhaps midges biting (a type of insect hypersensitivity) or grass pollen (an environmental hypersensitivity) may be enough to push that particular horse over its threshold. For another horse it may be the accumulative effect of multiple allergens that push them above the allergic threshold resulting in the symptoms you see.

One way to explain the idea of the allergic threshold is by imagining your horse as an empty glass, with all the things they are allergic to as different liquids.



Let’s imagine that the red liquid is midges, the yellow liquid is tree pollen and the green liquid is grass pollen.




Imagine you have turned your horse out on a nice breezy, sunny day and the pollen count is high. He seems ok though, not itching and pottering about in the field happy as can be. You have no reason to suspect he has an allergy to any type of pollen…….



The next day the weather is pretty similar, except for one difference; the breeze has gone and the midges are out in full force. When you go to check your horse in the evening, he is itchy and has clearly been bitten. You understandably jump to the conclusion that the midges are the one and only culprit.


In reality, although the midges are in no way blameless, they have only caused the glass to spill over because of the existing pollens already in the glass.

For this reason, it is really important that as many allergens as possible are identified and addressed with the aim of bringing the horse as far under its threshold as possible. This helps to decrease symptoms and, just as importantly, reduce the risk of flare ups.

If you only address the midges, even if you do reduce the itching initially, you leave the glass fairly full. All it could take would be a particularly warm wet evening where lots of midges were out, or for you to run out of fly spray, and the glass would spill. Or even perhaps the farmer who neighbours your paddock decides to cut his crops, and the grass pollens alone increase enough to spill over.

The aim = an empty glass!

In reality this may not always be possible, but once you know which allergens might be the problem, you can implement avoidance strategies to reduce exposure. Keep an eye out for our upcoming blogs on how to manage outdoor and indoor allergens.

Allergy is a lifelong condition often requiring a combination of treatments which may include allergen avoidance, medication and immunotherapy. By understanding your horse’s individual needs you can help take control of the glass and enjoy a happy life with your horse no matter where their threshold lies.