Stefanie emailed me with the following....
I ran across a couple of warnings on line about the dangers of dogs eating horse poop because of the potential of horse wormer being present. This is one of the sites: http://beingstray.com/dogs/
I'd like to know if you've ever experienced anything like this first hand or heard of it? My dog loves horse poop - and given all the hiking we regularly do, we run into it pretty regularly, so it is an on-going training process in getting her to leave it alone. Or maybe I am being overly cautious and should just let her have her dog fun?
I'll admit that this was the first time that I've heard of this particular problem. However, I've always practiced in suburban areas with few livestock around so it's never really been a concern. With that in mind, I read the blog post above with interest and did some digging on my own. And I do think that there is a cause for concern.
Intestinal parasites are common in horses and can really affect their health. Because of this risk horse owners commonly use over-the-counter dewormers as a routine treatment or even in the feed. Most of these dewormers contain ivermectin or moxidectin, as these products are very effective against a broad spectrum of parasites. They are also considered very safe to horses and have been used extensively for decades.
Some of the dewormer can be excreted in the feces, though it's less than they ingest. The problem isn't that the chemicals are inherently toxic. Ivermectin is found in the majority of heartworm preventatives on the market and moxidectin is found in ProHeart (an injectable heartworm preventative for dogs). At appropriate doses these compounds are very safe, even for collie breeds that contain a mutation making them more sensitive to side effects of this category of drugs. However, the horse products are far more concentrated and contain a far higher dose than is used in dogs. Remember, virtually all dogs are going to weigh less than 100lbs (45kg) and horses can easily get over 1000lbs (450kg). It's this higher concentration that is the concern, and can be found in potentially dangerous amounts in horse feces. Side-effects of ivermectin or moxidectin are typically neurological and if treated appropriately most dogs can make it through if the symptoms aren't too severe.
So yes, there can be concern from eating not only horse manure, but the feces of any livestock. Cases have been documented by vets, and the amount of ivermectin found in manure has actually been studied. It's a pretty low concentration, so a dog has to eat a fair amount to become toxic, but it can happen. The half-life of ivermectin in horse and cow feces has been measured at as low as 11 hours and as long as 9 days. That means that it can take up to 9 days for half of the ivermectin to become inactivated, then up to another 9 days for half of the remaining amount, and so on. Realistically you can have measurable ivermectin in the manure for 1-2 weeks.
Stefanie, I think that answers your question! Don't let your dog eat livestock manure.