By Tim John, DVM

When it comes to protecting your horse from pests, what’s the difference between an insecticide and a repellent … and why should you care?

The difference, as the names suggest, is that one kills and one repels. Does this really matter when you’re trying to combat the swarms we usually encounter?

The answer is maybe.

It depends on what type of bugs you are trying to stop and how the bugs interact with your horse. The two most important bugs we may want to repel (instead of just killing) are:

1. midges (no-see-ums) and
2. mosquitoes.

Midges, in susceptible horses, may lead to problems with “sweet itch” – a hypersensitivity reaction to the bite itself. Mosquitoes can transmit West Nile virus and other nasty neurological problems.

In both of these cases, it is much better that these pests do not come near us if possible, so you should reach for an insect repellent.

Most repellents act as vapor barriers. That is, they slowly (or too quickly) evaporate to form a shield that stops the bugs from coming near. DEET is the “classic” repellent, but its use in horses is not recommended.

Interestingly, most pyrethrins (neurotoxins that attack the nervous systems of insects, commonly used in sprays) are good insecticides but poor repellents. EXTOXNET, a university-supported toxicology network, states that permethrin (a common chemical used in insecticides and repellents) has only a “slight repellent effect against insects.”

So what happens most of the time with these products is that the bugs are actually landing on the horse, contacting the spray on the horse before the active ingredients in the sprays achieve their maximal effect … and not doing a very good job of protecting your horse from direct contact with the midges and mosquitoes.

So the next time you’re reaching for insect spray, ask yourself: “What am I trying to accomplish? Are these sprays the right ones for the job?”

By Tim John, DVM

After reading over your horse’s fly spray bottle, you think:

Cool! This fly spray has sunscreen in it, too! Bonus.

Sometimes, though, it pays to step back and look at the bigger picture. What is the most likely reason that sunscreens were added to these spray formulations? Cancer protection is one of the reasons, yes. But the main reason is that the active ingredients in most fly sprays are destroyed by UV light. Pyrethroids (see What’s in My Horse’s Fly Spray, Part 1) can be very sensitive to UV degradation, so companies added in sunscreens as a way of protecting the effectiveness of the fly spray formula.

So what’s the big deal?

Image credit: Ellen Sinding,
“Summer sun” (via Flickr)

As we know about most things, there is always a tradeoff. Sunscreens protect the skin by penetrating deeper layers to block the damage caused by UV rays. However, they also help other compounds in the same formulation to penetrate to a greater extent.

That’s right: Research has shown that sunscreens can enhance the absorption of pesticides into an animal’s body. I’m not really sure that I want that.

In fact, our friends to the north (Canada) are so unsure of this combination that they do not allow human products with sunscreen and DEET in the same formulation. Kinda makes you wonder. Here in the U.S., the EPA and FDA asked for comments in 2007. We’re still waiting for their decision.

In the meantime, now you have something else to ponder in our never-ending battle against our flying nemeses.

By Tim John, DVM

Continued from What’s in My Horse’s Fly Spray, Part 1

In the previous post, we discussed fly sprays that contain pyrethroids as the active ingredient. Now, let’s look at those sprays with essential oils as the active ingredient.

As discussed, horse fly sprays can be broken down into two broad categories: those with pyrethroids and those with essential oils. This differentiation corresponds to how the EPA looks at fly sprays or, as the EPA likes to call them, insect repellent products.

In the United States, the EPA divides products into two types: registered and unregistered.

Pyrethroids all fall into the registered category. To become registered, products must submit to and undergo a long process to show efficacy and safety. When they pass, there are explicit instructions on how the product is to be used.

Unregistered products fall into a bit of a loophole. They contain ingredients from an approved list of minimal-risk pesticides and are for the most part essential oils. This list was developed because the EPA feels that these products are generally safe.

Essential oils are nature’s own defense mechanism. They are produced by plants to guard against would-be predators, in order to stop attacks and repel invaders.

These volatile oils are highly effective and literally save the plant’s life. So it makes sense to utilize nature’s best weapons for ourselves, right?

Unfortunately, it is not always so easy to borrow these ideas without running into potential problems. In the case of fly sprays, those issues include:

• efficacy,
• duration of effect, and
• safety.

Essential oils are by their nature very volatile. When they are extracted from plants, the remaining portion tends to evaporate very quickly at animal body temperatures.

Probably the best-known essential oil is Citronella. Undiluted, it may repel flying insects for two hours.

Most common fly spray products contain between 5% to 15% concentrations, and some studies suggest they may only last 20 to 30 minutes. While higher concentrations of these oils work best, they are also the most likely to cause irritation to the eyes or skin. For example, peppermint oil is a known contact skin sensitizer in humans.

As it is expensive to run good scientific studies on these compounds, there is very little hard data available in regards to efficacy or duration of action for essential oil compounds. However, most herbalists agree that mixing different oils together can enhance their effect while minimizing the potential for an adverse reaction.

Most horse owners find that the combination of questionable efficacy and the need to frequently reapply these sprays leads to a very disappointing overall experience when using these products. If they are going to be successful, we need to keep in mind their limitations. Fly sprays with essential oils as the active ingredients are probably best considered when there is light insect activity and no increased exertion by the horse (activity = heat = increased evaporation of product from the horse).

So what are we supposed to do?

Certainly you want to protect your equine friend from pests, and there are many (many!) options out there for you to consider. I believe that knowledge is power in this case. Be a label reader. Be aware of what you’re using on your horse, and in turn yourself and potentially your kids. Ask trusted veterinary professionals and horse friends for input on products. Then do your own research so you can find a product that not only works well for your horse, but is also safe for you, your family, your equine friend and the environment.