I think muzzles should be a LAST resort. I have seen them cause more anxiety-related issues than they solve. However, no one can say when it has reached "last resort" status but the people who are actually living with and dealing with the dogs.

My sister has two dogs that HATE each other and would tear each other to pieces if given the opportunity. They have cost her many, many hundreds of dollars in vet bills, and many sleepless nights. She now keeps them completely seperated at all times. She split her yard in half, and keeps them in seperate rooms in the house. It is a nightmare, but worth it to her, as she gets to keep both dogs. Each individual must make their own decisions, with the dogs' best interests at heart.

BTW ... lovemyshiba ... you have a shiba inu named Kito? I have a jindo named Keito! Small world!