Q: I can’t think of a pretty way to say this: My dogs eat horse manure. I worry about the potential toxic effects of any oral products I give my horses. Could feed-through fly-control products be harmful to my dogs? I’ve also read that some dogs have very bad reactions to even traces of conventional dewormers found in manure. Is this true? Mary Washburn; Chaplin, Connecticut

A: The feed-through products for fly control generally include one of three primary ingredients: tetrachlorvinphos, diflubenzuron and cyromazine. All have very low toxicity to mammals, especially at the levels used in the feed-through products. Further dilution with other gastrointestinal contents would make poisoning in dogs extremely unlikely should they eat manure from horses ingesting these products.

More worrisome is the potential for harm if dogs consume sufficient amounts of manure after a horse has been dewormed using products containing ivermectin, moxidectin, or other avermectins. One study showed that horse manure contained 0.007 milligrams of moxidectin per gram of manure at 2.5 days after deworming. At that level, dogs ingesting 100 grams (a little over three ounces) of manure would receive sufficient moxidectin to cause mild signs of poisoning. The levels of these compounds in manure peak at about two to three days but are still detectable at low levels after nine days.

Some dogs (especially collies and related breeds) have a genetic alteration that makes them unusually sensitive to avermectins, and it would take less manure to cause a problem. So it’s a good idea to prevent dogs from eating manure from horses that have recently been dewormed with these products.

Sharon Gwaltney-Brant, DVM, PhD
Vice President and Medical Director
ASPCA Animal Poison Control Center