Arne wrote

"Have I misunderstood something completely with regard to Terramycin,
how is it American Bee Journal recomends this as a mean for treating AFB ?"

No, you haven't misunderstood. That's what most of us have been using for
several decades. In many areas, beekeeping would be extreemly difficult
without it when using European strains of bees. There have been isolated
reports of resistance, but for most of us who use it, it still works fine.