After I explicitly told her not to, my "colleague" wormed my horses. With the same wormer I gave them yesterday.
I'm furious! More because I instructed her not to do it. But more importantly will there be any I'll effects on them? We've had two cases of colic at our barn and one of those were fatal so understandably I'm already paranoid, now I'm not only worried but I'm p*ssed.:-x
Nope, like Appyt said most dewormers are safe in way higher doses than normally given.
Ivermectin, up to 60x the reccommended amount; fenbendazole, 100-200x; pyrantel pamoate, 20x. The one you have to watch is moxidectin, 5x a normal dose can do serious damage, but even if your horse was dewormed with this 2x, I wouldn't worry, considering he's probably very healthy.
actually our vets told us that ivermectin should NOT ever be overdosed. but the fenbendazoles and pyrantel pamoates are very safe at double the dose. what was she double dosed with? you should probably call your vet and just verify that it's ok.
While double dosing in general may not be a problem with ivermectin, it's not something you want to just do because there are situation where even at normal doses ivermectin can cause neurological toxicity. Certain plants if eaten can lead to this, any injury to the brain/spinal cord (including trauma, wobblers, EPM, any ecephalytic disease, etc), possibly even gastric ulceration.
So, while it's been tested safe in healthy horses it doesn't mean that you should just give it that way all the time.Because when you get one of those surprises where it does cause problems like your horse having picked a bit of bull nettle, then you've created an even bigger problem by double dosing.
There really is no reason to double dose ivermectin because it doesn't make it kill any more parasites. But if it's done accidentally, the risk of it causing problems is pretty small.
The one thing I can think of is possibly destroying the "good" bacteria in their gut...you may want to let them have some ManaPro OptiZyme or a similar product with live and active bacteria...plain yogurt also works.
My Gunner took the Panacur Power Pac (five day double dose), and then Quest for bots a few weeks later with no side affects.