Pyrethrum is an extract of chrysanthemum flowers. It contains two naturally occurring insecticides, pyrethrin I and pyrethrin II. They’re approved for use on crops, as home insecticides, dog flea/tick products and on livestock, like horses.
These chemicals work by disrupting nerve transmission. The EPA is looking at these insecticides and their derivatives called pyrethroids (e.g. permethrin, cypermethrin). The current official toxicity class is ”slight.” The symptoms of acute toxicity include:
• Irritation of eyes and skin.
• Sensitivity to sound or touch, odd facial expression, numbness, paresthesias (odd skin sensations).
• Headache, dizziness, nausea, vomiting, diarrhea.
• With severe exposures, muscle twitching or seizures.
The estimated lethal dose by ingestion for an adult human is 10 to 100 grams. Absorption through the skin is poor. However, these chemicals are easily absorbed across the lungs, and masks are recommended when applying them as dusts or sprays. The EPA is investigating risks to humans exposed to them through things like timed sprayers in barns and when applying them by hand to livestock.
Bottom Line. To put things in perspective, a typical fly spray may contain up to 0.35% pyrethrin/pyrethroid compounds. Even if you could drink a liter of it at one time, the total dose would be 3.5 grams, well below the estimated lethal oral dose for a human.
There will always be individual differences in sensitivity, and heightened sensitivity/allergic reactions can develop over time, especially to respiratory or skin effects. If you notice eye or skin irritation, lung tightness or odd sensations during or within a few hours of using these products, discontinue using them.
Little is known about a horse’s sensitivity level. Any ongoing skin or eye irritation or heavy breathing could be from the insecticide or from something else. If you suspect the fly spray, stop using it for a few days. The problem should resolve if it’s the spray. Overall, though, the risks to both you and your horse should be small.