By Tim John, DVM

Warning: Some contents may be unsettling.

“I wonder what’s really in that fly spray?”

That question echoed through my mind as I watched my wife and daughter douse their horses with fly spray prior to riding. As a veterinarian, I had a pretty good idea of what they were using, but I decided to do some research.

The answer was a little unsettling. Over the years, the active ingredients in traditional fly sprays have not changed very much. What has changed is what we now know about those active ingredients.

Let’s take a look.

Horse fly spray products can usually be broken down into two main groups:

1. Those that contain pyrethroids as the active ingredient
2. Those that contain essential oils as the active ingredient

Note: Some sprays may also contain a synergist (such as piperonyl butoxide, or PBO for short) to enhance the effectiveness of the product. We’ll get to that a little later.

In this post, we’ll talk about the first group – pyrethroids. Originally, pyrethrum was derived from East African chrysanthemum flowers. It was found to rapidly kill flying insects. However, it does not last very long, and it is quickly broken down in sunlight. This is good for the environment … but it means that multiple applications are needed to maintain effectiveness throughout the day.

Pyrethroids are essentially stabilized forms of natural pyrethrum that were originally developed in the early 1900s. They work by causing an overexcitement of neurons, which eventually leads to death in an animal. Mammals have adequate enzymes to rapidly break down these compounds, but insects do not. Pyrethroids are extremely toxic to beneficial insects, fish and other aquatic organisms, even in extremely low levels. None of that is really new information.

The new information has to do both with efficacy issues (how well something works) and potential long-term health consequences to us humans.

As might be expected, the longer a population of insects is exposed to a chemical, the more likely the bugs will become resistant to it. One study estimated that up to 90% of flies have become immune to permethrin, a type of pyrethroid.

The worst part? Our kids are being exposed to these chemical compounds even though the compounds are not helping as much as we thought. In fact, one study found that permethrin metabolized in 80% of the kids tested. A part of that percentage was likely exposure from the foods we eat, but it does make a person think.

The way most sprays get around the above-mentioned resistance is by adding multiple compounds to one product. Unfortunately, some studies have shown that there are cumulative toxicities with pyrethroid compounds.

The most concerning health issue is endocrine disruption. Endocrine disruption from pyrethriods has been shown to be a particular concern because it blocks estrogen’s hormonal effects. Wow, that got my attention! Here is my teenage daughter starting her journey to becoming a young woman … and we’re increasing her exposure to these products?

As the late-night infomercials say: “But wait – there’s more!”

PBO, the synergist mentioned earlier, is sometimes added to formulations because it works to stop liver enzymes from metabolizing the pyrethroids, thereby increasing their duration of activity in the animal. That animal (the insect) is the target, but we humans are also affected. PBO has other interesting links to our health. For example, it has been shown to cause fetal changes in pregnant women.

And the more we use, the more potential for negative consequences.

Stay tuned for part 2. We’ll take a look at the fly sprays that have essential oils as their active ingredient.