I recently (Feb. 2011) had to have one of my cats (Pretty) put to sleep due to congestive heart failure. I already had three other cats, and adopted another cat (Baby Girl) shortly after that. My oldest cat (Bubba, almost 8 yrs. old, male) who was litter mates with the one I had to put to sleep, has suddenly become very aggressive towards another cat (Daisy, 6 yrs. old, female). I adopted her about three years after the first two. Daisy was previously the "leader", or whatever it's called, of the whole house. Could the loss of the first cat (Pretty) be causing this behavior? I have read several articles that indicate it could be pain that's causing this, but I just had them all in for their 6 month check-up and the vet said all is well.
Also, what can I do to stop this behavior? Daisy has asthma, and it is really taking a toll on her health.