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Tuesday, March 31, 2015

Horse Manure Toxicity

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:
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. 


  1. Thank you for addressing my question. Very soon after I asked this, Blueberry had an incident where she ingested a bit of dried horse diarrhea and minutes after, became pretty ill (vomiting, panting). Thankfully, after vomiting, she returned to normal. Good advice to keep her away from livestock manure!

  2. Thank you for this story. You make me know more about Horses.

  3. As both a horse and dog owner this is something my dog did for years and I never once considered it being harmful. Thank you so much for the extra information on this. Thankfully it seems as though the dog would have to ingest quite a bit at one time, which is less likely to happen. Still, it is a good cautionary reminder to not let them eat a lot of it.

  4. Thank you for this very useful information... :)

  5. Whenever my dog does grab a quick bite of horse manure, she is immediately banned from doggy kisses for the rest of the day!

  6. My dogs are always eating my horses poo. To date they have never had any effects however, my horses havent needed worming in over 3 years (i have a worm count done twice a year)

  7. Only some dogs have the MDR1 gene. Collies, Border Collies, Australian Shepherds, German Shepherds, Shelties, Old English Sheepdogs - generally herding breeds. Within those breeds, the percentage of dogs who have inherited 2 copies of the gene varies. Aussies top the list at 50%. I had one of those Aussies, a rescue at 6 years old of indeterminate parentage. She had a seizure and died within minutes the day after the horses were wormed. I had moved to the farm well after spring worming and she was previously as healthy as could be (she was with me 3 years). Even though we did not have a necropsy performed, the coincidence is too much to deny. If you have one of the affected breeds, there is a DNA test available through the University of Washington to find out the MDr1 status of your dog. Don't go through the heartbreak and guilt that I did.

  8. I recently lost my border collie to ivermectin poisoning. She did not have the gene sensitive to the drug. What killed her was a constant exposure to the ivermectin in the cow poop which she would often lick. We have over 700 cattle in different pastures. About every month a different group would be brought in for vaccinations and worming. I believe it was the continual exposure to the drug that finally reached a toxic level. My question is what wormer is a safe and effective change for our cattle. ....and safe for our dogs. I want to switch to another cattle wormer. Something that is not ivermectin related. I am having a hard time figuring what to use.

  9. I don't check my email and blog every day, so I can't always reply quickly. If you need a quick answer, this isn't the place to get one.

    Check with your large animal vet about other options. I haven't done any work with that kind of veterinary medicine in 20 years, so I'm not sure what alternatives there might be for cattle and horses.


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