Nosema is difficult now since we now have the ceranae strain in addition to the apis strain. Fumagilin B treats the apis strain, but it can make the expression of the ceranae strain even worse and may kill your bees. Some people think feeding tea tree oil helps, but that has not been scientifically supported. We don't have anything effective against the ceranae strain.
The best thing you can do is make sure they are healthy, well fed, mite free bees and they will be able to deal with nosema better. I also have a humidity problem where I live over the winter, so they are ventilated with a top entrance and a quilt board on top to absorb any excess moisture.
