If a ferret has a lot of plaque build up and/or infection in the gums there is a lot of bacteria present in the mouth. When he then breathes in he is inhaling this bacteria into his lungs. It can affect the heart/lungs over a long period of time (same as with cats + dogs) so it is advisable if you use a toothpaste rather than wait until there is a plaque build up. If you get an enzyme based toothpaste there is no need to actually brush, just wipe it on their gums or on a paw for them to lick off. It works by breaking down the bacteria in the mouth + prevents plaque build up. Most vets should stock some but the trade names will probably be different depending on where you live [Posted in FML issue 3440]