Fumagillin-B was the traditional treatment for nosema apis for many years. It is harmless. Treatment applied once in the fall, just before wintering and once in spring to help clear the bees just before brooding. No other time. It was great for my climate, but perhaps not so great for more temperate areas that do not have as distinct cut-offs between seasons and flow.
It is important to know that Fum-B does not work well or at all on nosema cerena. That may be an explanation of the trials referenced which did not show benefits from the treatment. That or the fact that nosema is not usually a problem mid-summer. Nosema kills in the winter and stalls the colony in the spring. There is also a tolerance threshold. Below and the bees are able to manage and can clear themselves in summer. Too high and the bees never get going, they're done for. Just out of curiosity it may be good to check back as to which nosema they were dealing with or if segregated if it was known.
I did not mention Fumagillin-B as it is also no longer manufactured nor available. If you are lucky enough to find some from somebody's stockpile it should be hoarded, used by exception and sparingly, as it is rare to find. If you have it, do not use it until you know which nosema you actually have. For a bad case of nosema apis, use it confidently, it works and will save the hive. Nosema cerena, do not use it, it does not work much if at all.
In absence of Fumagillin, the chase has been on for quite some time for effective alternative treatments. Lots of on going development, trials, elixirs, .... Some pockets of successes, but nothing yet that singularly jumps out as the silver bullet for nosema.
In absence of an off the shelf treatment, we can put our beekeeper hats on and default to best practices around the bee yard. Think of what you would do as a beekeeper to enhance hygiene and prevent cross-contamination as well as what you would do with a hive that has any seriously contagious and infectious disease. Once vacated, the equipment can be fumigated with acetic acid (vinegar) to kill the spores. ( No need to burn it, like AFB ;) )
To try to save a hive and clean-up the bees, think in terms of actions usually taken to clear seriously infected bees out of a hive. For example, move the hive. Then setup new or decontaminated equipment in the old hive location. Salvage the queen and move her into the new setup with a few fresh combs, her best brood frame(s), and a frame worth shake of the youngest nurse bees. Perform a complete shake out of the rest of the hive a distance away. The really sick (nosema bombs) will not go anywhere and will perish at the shake location. The healthy and mildly infected will find their way to the freshened hive setup. Clean and treat the vacated equipment before reusing anywhere.
With some help as mentioned, the infection level is knocked below the threshold and over time with highly nutritious feed and favourable conditions (summer!) the bees manage and usually recover quickly. If the bees do not immediately recover from your efforts then: save the queen and kill all the bees before the infection spreads to others. Fumigate the vacant equipment, reassemble and make ready to start anew from scratch. In otherwords, start a new nuc by drawing resources from healthy hives and install the salvaged queen.
NB: the queen is very rarely infected with nosema, is why I promoted keeping her when ridding of the bees and decontaminating the equipment.
Hope that helps!
PS: No need for hemocytometers and such. Checking for nosema is best kept simple and relatively coarse. There is either: none, a negligible few, some, a bunch, or too many. Need a 400x microscope, view slides, tweezers to pull the abdomen off, a small cup or bag to mash the abdomen(s) in, some clean water, and a end sharpened stick to stir and drop a droplet onto the view slide. It is actually quite easy and quick to do, once you've done a few.
Apologies! I rambled on. I certainly did not intend to get into the weeds on nosema in this thread. Thought it important to get into it for folks not familiar. I mentioned nosema as a very likely contributor to the colony lagging all summer and ultimately the dead-out; I also wanted to put some time in to raise renewed awareness of nosema as a traditional disease that is so often overshadowed and overlooked nowadays amongst the mite-virus hype. Not saying that is what the hive had, just being suggestive as something to look into if there is no prior benchmark done for nosema in the apiary. Note: there does not have to be presence of dysentry for there to be presence of nosema.
