I have three cats-two males and one female (all siblings)- which I have had since they were a couple of months old. They are all neutered and got along fine, but then after I left for college my mom told me that one of the males and the female became vicious towards the other male basically overnight. They will hiss and snarl and the lone male will run and hide. My mom decided to separate them but the lone male started losing weight (he still eats normally), losing his top coat of fur from his lower region, scratching his neck so hard that he cuts his skin, and has a different odor. We have tried reintroducing them after weeks of isolation but get the same negative response. We have taken the lone male to the vet and he says that there is nothing wrong with his tests and gave him a shot to make the fur grow back but he ended up losing it again. I love them all so much and it hurts to see them turning their backs on their brother when they all lived so peacefully for over 4 years. Does anyone know what could be wrong with the lone male or what could have caused the male and female to ostracize the lone male?