Fly Spray: 3 Ingredients You Need to Know About

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!
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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.

​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.
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It’s safe to say that fly sprays are most commonly used during warmer weather thereby increasing the ability to enter the bloodstream. 
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​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.​​

The following three ingredients are the most common:

 1. Pyrethrins (natural) are botanical insecticides derived from chrysanthemum flowers most commonly found in Australia and Africa. They work by altering nerve function, which causes paralysis in target insect pests, eventually resulting in death.

 2. Permethrin (included in the class of Pyrethroids) is a synthetic chemical insecticide whose chemical structure is adapted from the chemical structures of pyrethrins and act in a similar manner by altering nerve function, which causes paralysis in target insect pests, eventually resulting in death. Pyrethroids are modified to increase their stability in sunlight. There are more than 1,000 different pyrethroids in use today, though less than a dozen are available in the United States. Permethrin being the most common used in fly spray.

 3. Piperonyl butoxide (PBO) is often included in topical insecticide formulations to enhance insecticidal activity.  It is the most common synergist used in conjunction with Pyrethrin and Permethrin.

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. ​
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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. 

• Inhalation: coughing, wheezing, shortness of breath, runny or stuffy nose, chest pain, or difficulty breathing.
• Skin contact: rash, itching, or blisters.
• Long term effects: disrupts the endocrine system by mimicking the female hormone, estrogen, thus causing excessive estrogen levels in females. In human males, its estrogenizing (feminizing) effects include lowered sperm counts. In both, it can lead to the abnormal growth of breast tissue, leading to development of breasts in males and cancerous breast tissue in both male and females.
• Neurotoxic effects include: tremors, incoordination, elevated body temperature, increased aggressive behavior, and disruption of learning. Laboratory tests suggest that permethrin is more acutely toxic to children than to adults.
• A known carcinogen. There is evidence that pyrethroids harm the thyroid gland. Causes chromosomal damage in hamsters and mice; deformities in amphibians; blood abnormalities in birds. 
• United States EPA Permethrin Fact: “In 1985 a Data Call-In (DCI) for ecological effects data for permethrin was issued. After evaluation of this data, EPA concluded that the current RUP classification be maintained for all products for wide area agricultural uses (except livestock and premises uses) and outdoor wide area non-crop uses because of the possible adverse effects on aquatic organisms from spray drift and runoff.”  ​

​We have covered the 3 most common, potentially harmful ingredients in fly spray. There is a plethora that have not been listed.

• Read all labels and research ingredients (Pyrethroids usually end in an "IN") before purchasing or using any fly spray.
• If it poses a health risk there must be a legal statement such as: Avoid contact with eyes, skin, clothing or inhalation.
• Don't assume that all natural ingredients are safe.