Deborah,
5 months is young for cardiomyopathy, although it still can happen.
Other diseases that can cause coughing in a young ferret include
megaesophagus, infection, lymphosarcoma (the thymus is in front of the heart
and when enlarged can put pressure on the trachea).
If he has heart disease then lasix is indicated.  It can cause dehydration
which will in turn cause lethargy.  Any ferret that does not have normally
functioning kidneys would be more susceptable to dehydration.  If bloodwork
was peformed how were the kidney values?
If it is dehydration causing his symptoms it could lead to kidney damage.  It
would be a good idea to have your vet reevaluate him.
In the treatment of cardiomyopathy the goal is to decrease the symptoms, to
make the patient more comfortable.  If he feels worse on the medication you
may have to have the dossage or medication changed.
 
Dr Weiss
301-299-4142
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[Posted in FML issue 1178]