To head off concerns before they're even voiced: No, I'm not planning on using Ivermectin for flea control, and I am well aware this is a completely off-label use of the product and of toxicity of this chemical if used improperly.
So Rob and I have become acquainted with an old cattleman/cane farmer here in Central Florida -- he's probably in his 70s and he's been doing this his whole life, and it's a living his family has been at for a couple of generations -- and he has a handful of working dogs. Some kind of Southern curs, not sure what exactly, and neither is he ... they look like bulldog/hound mixes, some of them have corkscrew tails and long-ass ears. Weird-looking dogs, but they work the cattle and hunt hogs and so forth. Seem healthy as horses, a couple of them are rather old and still working. And we noticed that even though they're in and out of houses and barns and fields, they don't seem to be crawling with fleas ... in fact, he claims they rarely ever have flea problems even though fleas are so prevalent here and even spoiled housedogs are often plagued with them. So we asked him what kind of flea preventative he uses and he said "Ivomec." I had never heard of anyone using it that way before, so I asked for more info. He said that once a month, he gives each of his dogs one tenth of a cc per 10 pounds of body weight and it worms them and keeps the fleas at bay.
Has anyone ever heard of using Ivermectin for flea control? I know it's used for lice in livestock, but I've never heard of anyone using it for flea control in a dog ... seems kind of extreme. I didn't want to question the practice to this dude, since he's probably been doing this his whole life, so I just nodded and listened. So I figured I'd try to satisfy my curiosity here on the forum.
Again again: Just curiosity, I am not asking because I am thinking of trying this!