If your bees have nosema, by all means treat them with fumidil if you can
afford to.  Before spending your money it is worth checking that they do
actually have it in sufficient severity to make it worth while.  Easily done
with a microscope and a little practice.  It always strikes me as pointless to
treat you bees for a disease they don't have.  If they do have heavy nosema
ask yourself why.  Although probably endemic it tends to be a nuisance only
when the bees are stressed so before feeding fumidil do what you can to
identify and remove whatever may be stressing your bees.
If your bees do have nosema, fumidil, correctly applied, will slow it down a
lot but it won't do anything about the spores already on the comb.  These will
be ingested and activated then the cell cleaning bees get to work as the
colony expands in the spring leading to what used to be called spring
dwindling.
The answer is to move the bees onto foundation or to comb sterilised with 80%
acetic acid as soon as practible in the spring.  Do this and you may not need
the fumidil.
Chris Slade