since you asked about terramycin, and it seems that is a possibility, let me give you an option.
Years ago, in a discussion about AFB, a researcher made the comment about a two cycle treatment and changing of the genetics as an option for AFB. Maybe not for full blown complete hive loss type stuff. But for cases caught early.
1) Remove any frame with open signs of AFB
2) Treat for three weeks with dusting as directed with terramyacin
3) Do a fall/spring or a spring/fall treatment.
4) Replace the queen as genetics has much to do with AFB.
I started a yard for AFB a number of years ago when I was inspecting. I did as this researcher had suggested. In the years to follow, from the placement of the first colony in that yard, none has come down with AFB again. And no other treatments have been accomplished.
I actually wanted to play around with resistance with AFB when I started this yard. But now I have no visible AFB with the hives I have placed into this yard.
It also seems that dead out hives, cases where comb is allowed to sit around in stored damp/wet conditions, and hives that are dying from other causes, all seem to allow AFB to get a foothold. I've seen some sac or other dead spring brood (chilled brood) in the spring, and then when the cells are all mush, it seems this is a good place for AFB to outbreak.
You actually may have EFB, SAC, or a case of some larvae that were killed by pesticides. They all involve larvae that turn to mush, rot, and turn nasty.
a "Holt's milk test" is better than 95% accurate. With a milk test and confirmation of ropiness, and visual cues, 99% accuracy can be achieved.
As with all cases of AFB, give a moment to think of how you may of gotten AFB. Maybe you can keep from getting it again.