The transmission among ferrets is NOT casual transmission, but CONTACT TRANSMISSION.  Unlike cats which can transmit COVID between cages, ferrets require contact to transmit to each other.  Normal ferrets behavior includes many intimate interactions from mutual tongue licking, to extensive grooming of each other stem to stern, to sleeping tightly wound together.  

This distinction is important.  It makes ferrets less likely to acquire the disease in a home setting, and it gives researchers a level of control that they would not have with a species that can casual contract the illness.  

There too often are reports that incorrectly assume that the ability to contract the illness always means casual transmission, and that is far from the case.  For any species which can be infected it is essential to know whether that transmission is casual as with human and cats, or contact (much harder to get) as with ferrets and dogs, or only in an experimental setting where the disease is imposed within the animal.  

See the extremely precise work in “COVID-19:  Experimental infection of fruit bats, ferrets, pigs, and chickens with SARS-C0V-2 at Friedrich-Loeffler-Institut”, and the more recent U.S. work in which a large group of ferrets housed with infected humans did not contract the illness:

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