Thursday, October 3, 2013


A year ago, a friend walks out to her barn to visit with her horses and give them a routine dose of worm medicine.  As always, her faithful sidekick, a middle aged black lab, is at her side.  It's an ordinary thing and an ordinary day, something she has done dozens, maybe hundreds of times.  One of the horses is not overly pleased about her nasty tasting dose of wormer, which is ordinary too, none of them like it.  One of the horses manages to spit out a big glob of the gooey paste; so many of them are so very good at this.  The stuff ends up all over the place.  No big deal, it's annoying that the darned horse has gone and wasted her medicine, but the stuff is cheap and the horse probably got most of it anyway.

The dog, as dogs are apt to do, helpfully dashes in to clean up the mess.

A few hours later, the helpful dog is drooling uncontrollably, vomiting, staggering, disoriented and finally lapses into a coma.  No one knows what is wrong.  Is he having some kind of seizure?  Did he find rat poison?  He is rushed to the vet, they put him on heavy IV fluids, trying to flush whatever toxin he has found and prevent liver and kidney failure.  Agonizing days go by. 

Will he live? 


Yes, he is awake.

But he is blind.

Will he recover?

No one knows.  No one can know.

More agonizing days go by.

Eventually, some of his sight returns.

Will he get better?

Maybe, no one knows.

More days go by.

He does recover.



He is one of the lucky ones, he might not have.

That afternoon the woman racks her brain, what could have caused this?  The woman's daughter asks, "didn't you worm the horses that day?"


A week ago, another friend on another farm, with another dog, walks out to the barn to visit his horses and give them a routine dose of worm medicine.  As always, his faithful sidekick, a 4 year old English Shepherd, is at his side.....


Ivermectin is one of the most frequently used worm medicines.  It is very effective, cheap, easy to use.  It is used in most livestock including horses, cattle, sheep, goats cats and to treat and prevent heartworm in dogs.  Some dogs have a genetic mutation that makes them have a severe, often fatal reaction to the drug, even in small doses.  Many people think that it is only collies who have this reaction, that is not true.  The mutation can pop up nearly everywhere and can occur in dogs that have suffered a trauma to the head, in any puppy or in an animal who has encountered some other kind of poison.  There is no treatment for it.

In horse size doses, it can be fatal to ALL dogs, whether they have the genetic mutation or not.  

It is Fall and, for many of us, it is time to de-worm our horses.  Nearly all of us will be using ivermectin or one of its related drugs.   Please be careful and keep your faithful sidekicks far away.

Be particulalry careful with any drug in the ivermectin family:

Click here for more information.


  1. not sure if I am remembering rightly, but I think it's also advised to not let dogs eat the post-worming poop, either. That's pretty difficult.

    1. Yes, that is true. The incidence of toxicity is lower, but could still be fatal for a dog with high sensitivity.

    2. I wormed my horses with hostes cupcakes less the filling inserting the quest. Unfortunately they all didn't love the taste. It was valentines day and they were made of white cake in the shape of hearts. My aussie shepherd found the bits of white cake in the snow that I couldn't see and quickly devoured them. That evening as he sat on my lap, he had begun biting at imaginary flies that he saw buzzing around him. There was nothing. He continued doing this with great anxiety and frustration...and could not sleep all night. It was another day till and two more years for us to relax our watch to be reassured he wasn't having seisures and that it was the worming reaction was an isolated incident.

  2. Wow, thanks for the heads -up! Even though we will be selling off our horse, we will be places where other folks have them. Good to know!

  3. Thanks for the warning! How scary.

  4. thank you for passing this along.

  5. I use Ivomec sheep drench, that's interesting about no letting dogs eat the poop either. Mine especially like to look for lamb poop.

  6. Dog recovered but has a new aggressive side and has biten two people to the point of going to the ER. We call it brain farts but we have to be very careful with other people as we can't trust him. I have noticed he is most aggressive when he gets overly warm or down right hot. It's a terrible thing to have happen. Thanks for the reminder.............

    1. This can happen, especially if they have had seizures and extended bouts of unconsciousness. I have a now 11 yr old Heeler who due to a traumatic brain injury was seizing and unconscious for quite a while and had aggression problems afterward. It does eventually subside, but you always have to be extra careful with them.

  7. Kris, what an EXCELLENT reminder for all us!

  8. Thanks for a great reminder, I have known too many dogs fall victim to these drugs. Please also be really aware that it is easy to overdose minis and donkeys with both ivermectin and moxidectin - the effects are similar to those seen in dogs. Always dose carefully according to weight, these are really important drugs and very useful but very dangerous too!

    Great post Kris!

  9. Wow, I had no idea. I mean I knew in big doses but not that some dogs couldn't even tolerate small. My dogs are never near the horses when wormed but this is very interesting and still very good to know. So scary and dangerous!

  10. Whoa. Definitely sharing this on my Facebook page, The Healthier Horse. Thanks, Kris!

  11. This needs sent on to as many as possible.

  12. Smartpak offers "trickytreats" hollow crunchy minimuffins and i no longer battleo wear wormer, even 1100#, 3+ treats, 5 day power pak!

  13. Thank you so much for this information...
    We never have the dog with us when worming but need to pass this on...