09/09/2026
With the summer coming in, so are the 🪰s
…interesting look into aspects around fly sprays….
Bodywork, Fly Spray & Neurotoxins and the Horse: What We Should Know
Most horse fly sprays work because the chemicals are neurotoxic to insects—they interfere with the insect nervous system. The interesting part is that insects, horses, and people use many of the same basic nervous-system mechanisms. What differs enormously is the dose, route of exposure, metabolism, and sensitivity.
Many common equine fly sprays contain pyrethrins or synthetic pyrethroids such as permethrin, cypermethrin, or related compounds.
Pyrethrins are insecticidal chemicals derived from chrysanthemum flowers. Pyrethroids are synthetic compounds designed to behave similarly but often remain active longer.
They work primarily by interfering with voltage-gated sodium channels in nerve-cell membranes. Normally these channels open and close very rapidly, allowing a nerve impulse to travel and then stop. Pyrethroids keep the channels open longer than they should.
In an insect, that can produce:
nerve hyperexcitability → tremors and spasms → loss of coordinated movement → paralysis → death.
So describing many fly sprays as neurotoxic insecticides is accurate.
Why doesn’t the same spray normally poison the horse?
The target is not completely unique to insects. Horses and humans also have voltage-gated sodium channels.
The safety margin comes from several differences. Mammals are generally less sensitive to these compounds, have a much larger body mass relative to the amount encountered, and can metabolize and eliminate many of them more efficiently than insects.
That means “toxic to insects” does not mean “harmless to mammals.” It means the dose required to seriously affect a mammal is generally much higher.
A horse can still react to a fly product. Depending on the active ingredient, formulation, concentration, and exposure, problems may include skin irritation, itching or sensitivity, eye or respiratory irritation, and, with significant exposure, neurological signs such as tremors, muscle fasciculations, weakness, incoordination, or seizures.
There is another wrinkle: the active insecticide is not the entire bottle. Fly sprays may also contain solvents, fragrances, preservatives, synergists such as piperonyl butoxide, and other ingredients. A reaction may therefore be caused by the formulation rather than the insecticide itself.
For equine massage therapists and other bodyworkers, there is another consideration: repeated occupational exposure.
A horse owner may spray one or two horses and move on. A bodyworker may spend 45–90 minutes with their hands and forearms moving continuously across a treated coat, then repeat that process on several horses in a day throughout fly season.
That is a very different exposure pattern.
Dermal exposure is especially relevant because pyrethroids can cause temporary tingling, burning, itching, or numbness of the skin. Bodyworkers may also transfer residue from the horse to their face, eyes, or mouth without realizing it, and inhalation becomes another concern if horses are being sprayed in enclosed areas.
This is why it is reasonable to ask that fly spray and other topical products not be applied immediately before a massage whenever possible, particularly over the areas being worked.
And “natural” does not automatically mean harmless.
Some natural fly sprays contain pyrethrins—the same class of insecticidal compounds derived from chrysanthemum flowers. They are natural in origin, but they still work by affecting the insect nervous system.
Other products rely on essential oils such as citronella, eucalyptus, cedarwood, peppermint, clove, geranium, or lemongrass. These do not all act in the same way, and their effectiveness varies, but they are still biologically active substances. Depending on the oil, concentration, and individual sensitivity, they can cause skin irritation, sensitization, eye irritation, or respiratory irritation.
So the more useful distinction is not:
chemical = harmful
natural = harmless
It is:
What is the active ingredient?
How concentrated is it?
How much exposure is occurring?
Through the skin, lungs, or eyes?
And how often?
That matters especially in massage, because bodyworkers are deliberately creating prolonged contact with the horse’s coat and skin. Hands move repeatedly over large areas, friction and warmth increase during the session, and whatever residue is on the coat is also being repeatedly contacted.
None of this means fly spray should be treated as inherently dangerous. The important distinction is between hazard and risk.
A substance may have the ability to affect a nervous system—that is its hazard. The actual risk depends on the amount of exposure, the route of exposure, how frequently it occurs, and how the body handles it.
For equine bodyworkers, that makes simple precautions worthwhile: know what has been applied to the horse, avoid working immediately after heavy application when possible, wash hands and forearms between horses, and pay attention to products that repeatedly cause skin tingling, irritation, headache, or respiratory symptoms.
Fly spray is designed to affect biology. The dose matters—but for people who spend hours every day touching treated horses, so does repeated exposure.
Thoughts?
https://koperequine.com/?s=Bugs
