I am not trying to advocate for Hygromycin B, but I have looked into it in the past, and I do have a jug of it in case I need to go down that road in the future. It is a wormer, so not *THAT* weird/odd that it is advertised as one. Just because it is technically an antibiotic does not mean it can't be an effective wormer. In fact, it seems like the main reason they developed it to begin with was for worming, and not for its antibiotic aspects. I have not seen Hygromycin B ever mentioned when folks have bacterial problems. Probably because it sucks as an antibiotic or something. Who knows. But it kills worms! Just a weird situation all around I guess. When I researched it, I recall that it is often used in swine worming, but that poultry is also a large market. I believe the original development/application of it was for the pig/swine market.
As you mentioned, I also believe it is often recommended as a constant feed additive, so that does make it a little less ideal for occasional worming if you ask me. That being said, I think I recall reading a thread somewhere on here where someone asked the Rooster Booster folks about occasional use, and Rooster Booster said they would love if you feed it all the time (obviously increasing sales for them), but that you could be just as effective by feeding for only a week or two at a time, with long breaks between. Seems the concept is that if you feed all the time, you are protected from worms all the time. Feeding every 3 months just means you allow 3 months of worms to populate before killing them off. A jug of it is supposed to be mixed with 50 pounds of feed, and if I recall, costs about 20 bucks. So depending on flock size it is not a huge $$$$ investment. I wonder if it will become a black market product being sold at a much higher price once the new year hits.
All very interesting!