Fly spray is intended to “protect” your horse from flies, but does it pose health risks? Depending on the ingredients, it can be harmful to you and your horse with potential long term adverse side effects. Unsafe ingredients have the potential to enter the bloodstream via skin absorption and/or inhalation. Natural ingredients are not necessarily safe either!
Read on to learn more about how fly spray enters the bloodstream, three common ingredients that may not be safe and some suggested - and safer - options.
Have you ever heard the saying, "If you wouldn’t put it in your mouth, don’t put in on your skin?" This is why…
Chemicals can penetrate the skin by passing directly through cells via permeation, weaving their way between dermal cells, or by entering through appendages like hair follicles or sweat ducts. If a chemical successfully passes through the upper layers of skin, it has the potential to be absorbed by the bloodstream or lymphatic system.
It’s safe to say that fly sprays are most commonly used during warmer weather thereby increasing the ability to enter the bloodstream.
How warmer weather increases dermal assimilation of topical applications for horses and humans:
You and your horse’s normal cooling system works by first dilating (make or become wider, larger, or more open) blood vessels on the skin’s surface to allow more warm blood to flow near the surface of the skin where the heat can be lost to the air.
When blood vessels expand (dilate), pores open in the skin that lead to sweat glands increasing the surface area for dermal penetration. Perspiration then moves onto the skin; heat transfers from the body to the sweat and eventually cools the skin. The average person has 2.6 million sweat glands in their skin and can only imagine the number of sweat glands in a horse!
Note: the horse’s hair coat does not provide foolproof protection from fly spray coming into direct contact with the skin.
If fly spray is inhaled, it enters the bloodstream via the vast vascular blood supply to the lungs.
3 Common Insecticidal Fly Spray Ingredients
The following three ingredients are the most common:
Both Pyrethrin and Pyrethroids are highly toxic to fish and moderately toxic to birds and bees. They - along with Piperonyl butoxide (PBO) - are classified as Restricted Use Pesticides (RUP) by the Environmental Protection Agency (EPA), which means that their use in products is restricted to certified manufacturers.
Safety / Side Effects of Pyrethrins & Permethrin
The safety / side effects listed below are for humans. Despite exhaustive efforts, I could find no information/studies on short or long term effects on horses.
Safety / Side Effects of Piperonyl Butoxide (PBO)
We have covered the 3 most common, potentially harmful ingredients in fly spray. There is a plethora that have not been listed.
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Monique Warren is the owner of Hay Pillow Inc.