You will need to treat the ferret's environment, also. Change and launder all bedding, but avoid fabric softeners. Treat the area with flea spray and powder and vacuum frequently. You can use cut-up flea collars in your vacuum cleaner bag to help kill flea eggs that are swept inside. Flea collars, however, cause severe irritation on your ferret's skin.
Fleas are nasty parasites.
Aside from causing infection and skin allergies, it causes them to lose sleep, lose their appetite, and become irritable and difficult to handle. Also, it does not take many flea bites to cause anemia from lack of enough blood in the ferret's system. Fleas can also transmit diseases, and if a flea is accidentally ingested when your ferret grooms with his teeth, tapeworms , an intestinal parasite can be a serious problem. You must do all you can and keep doing it, for the sake of your little guys. Fleas are an illness, please help your pets be healthy and happy.