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Pour-ons are designed to soak into the skin, which puts the medication in the bloodstream. Having the medication in the bloodstream is what kills the parasite -- not simply putting the medication on the skin. In other words, whatever way you can get the medication in the bloodstream is going to work equally as well as a pour-on.
Moreover, pour-ons are usually designed for cattle, which have tougher hides than goats. There are some pour-on medications out there that, when used on goats, cause SERIOUS skin irritations...like, big chunks of hair come out, sometimes taking the hide with it, leaving big weeping, painful, oozing sores. Not great. I've also read reports of neurological issues with some pour-ons, though I haven't been able to personally confirm that.
What I can tell you is that there's one pour-on that I've personally used with success...it's called CyLence. It's a synthesized pyrethrine, and I've actually used it on weeks-old kids without any adverse effects. Not only does it kill lice, but it also helps to control flies.. It's fairly cheap (relatively speaking, anyway) at about $20/pint, and the dosage is 1ml/25lbs. For lice, you'll want to repeat the treatment in about 3wks.
The other option, as you mentioned, is ivermectin. Whether or not the ivermectin horse paste dewormer will take care of lice in goats depends entirely on how much of the ivermectin actually makes it into the bloodstream. And that, in turn, depends on the dosage. Generally, people give an ivermectin 1% injectable as a drench at about 1ml/25-50lbs, and that's usually enough to kill external parasites -- but not always. It seems especially hit or miss with mites, but lice are generally easier to control..
Again, it all comes back to blood levels.. For my money, the best way to ensure that blood concentrations of ivermection reach sufficient levels to kill external parasites is to INJECT the goat with ivermectin. That runs counter to a lot of what you'll read, and if we were talking about deworming a goat here, I'd absolutely recommend giving ivermectin orally...but we're not talking about worms. We're talking external parasites. And, like I said, the best way to get it in the blood is to inject it -- period.
I've personally injected a big, heavy doe (probably 16-170lbs) with a scant 2ml of Ivomec 1% injectable and successfully killed
mites. And, like I said, mites are generally much, much tougher to kill than lice. If she had any lice when I injected her, she didn't shortly afterward -- that much, I can gaurantee.
But...again...if we're strictly talking *lice* here and nothing more, I'd go with CyLence pour-on. Cheap, easy, effective, safe, and there's no injection or drenching to fool with.
Just my $.02.