Thanks for reaching out with this beekeeping question. It's good to know that your other colonies are still alive!

Nosema apis can cause dysentery, but we believe that Nosema apis has largely been replaced by Nosema ceranae in the U.S. Nosema ceranae isn't known to cause dysentery. (Note that Nosema was reclassified and is now also called Varimorpha.) So, we usually can't assume that bee poop in hives is caused by Nosema/Varimorpha.

You can send a sample of bees to a lab to check for Nosema/Varimorpha levels if you want. Nosema/Varimorpha is very prevalent, and It's common for us to find it even in some colonies that appear to be thriving. There is a strong seasonality to Nosema/Varimorpha, and we expect to find higher levels in winter and then lower levels in the spring after cleansing flights and the start of nectar flows.

Here are Diagnostic labs and services for beekeepers. Nosema/Varimorpha is a gut microsporidian. The Bee Disease Diagnosis Service : USDA ARS is a free service (you just pay for shipping), and I believe they can check for Nosema/Varimorpha. Here is How to Submit Samples : USDA ARS.

I tend to see bee poop in the winter in hives of colonies that seem otherwise stressed or weak in population. I know you said the colony was strong in population going into winter, but I wonder if it lacked sufficient healthy winter bees or if it was stressed for other reasons.

There is some concern with bee poop and disease transmission, so your management decisions will depend on how you weigh risk of disease vs. replacing equipment. I usually try to scrape bee poop from frames and combs. I discard frames/comb that are covered in lots of poop or where I can't sufficiently scrape it off.

Happy beekeeping!