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Ask the Vet: Feeding Insect Control to Pregnant Mares

Posted on: July 08, 2009 by Dr. Lydia Gray

I saw in one of your emails that Equitrol II can be fed to pregnant/lactating mares and foals. I have searched Farnam’s site and can find no information to this effect. I contacted Farnam years ago when I first bred my mares and was told that there was no research on Equitrol at that time to say if it was safe or not. Is the new Equitrol II now safe for pregnant mares, etc…? My vet had no information on this. Thanks, DB

Dear DB,

I’ll do my best to answer your question about Equitrol II but I also want to share what I’ve been able to find out about two other feed-through fly control products: SimpliFly and Solitude.

The current generation of fly control products are all insect growth regulators. Equitrol II and SimpliFly (both made by Farnam) have diflubenzuron as the active ingredient while Solitude (made by Pfizer) contains cyromazine. These are modern insecticides that have no effect on mammals because they interfere with production of a substance called “chitin,” a key component of insect exoskeleton that is not found in animals. Here’s how they break the fly life cycle:

• The active ingredients pass through the horse’s system and are excreted in manure
• Flies lay their eggs in the manure and these eggs hatch to the larval (maggot) stage
• The maturing larvae feed on treated manure but are unable to go through their normal molting process, which causes death

In addition to having effect only on insects, these active ingredients are also species-specific, having effect only on stable and house flies. They are also not directly effective on adult flies, so supplemental fly control measures will need to be used including fly repellent, masks, sheets and leggings for horses; fly traps and premise sprays; and good environmental management to reduce other breeding grounds such as old manure, feed and hay spills, and standing water. But with results apparent as quickly as two weeks after starting (full benefits in four to six weeks), feed-through fly control products will eventually reduce adult populations of flies, decreasing the need for these other measures.

As far as safety goes, I will quote exactly what is on each products’ website so I get it right! SimpliFly is clinically proven and the only equine feed-thru fly control product granted “reduced risk” status by the EPA. You can read more about Simplify here:

Solitude has a well-documented efficacy and safety profile, having been extensively evaluated in horses since 2001 as well as intensely studied in other species for more than 20 years. Regarding your pregnant mare question:

“A reproduction study was conducted using Solitude IGR. There was no significant difference in foaling between the control mares and those treated. Three breeding stallions were fed Solitude IGR throughout the breeding season. All sperm counts, motility and morphology were within normal ranges.”

Finally, they say “we have seen no problems with nursing foals on Solitude IGR. We have customers that have purchased Solitude IGR for their nursing foals.”

Read more about Solitude here:

The bottom line is that Pfizer has spent the time and money to do reproduction studies on their product in pregnant and lactating mares but Farnam hasn’t. That means Equitrol II and SimpliFly could also be safe in these populations of horses but it hasn’t been proven.