I had to put one of my cats to sleep a few years ago. I had adopted Tannie and her sister Blackie from a Petsmart rescue group (I love dogs too but was living in an apartment at the time and worked too many hours - not fair to a puppy.) The rescue group did not know what their history was except that they had been abandoned and perhaps poorly treated - they were still a beautiful black tortoiseshell pair. Tannie wasn't growing as rapidly as Blackie but she seemed fine. One day while home for lunch a few mnths after adopting them, I noticed that she was wobbling on her back legs without explanation. I took her to the vet immediately and he diagnosed FIP (Feline AIDS) as the most likely cause. In 2-3 days she went to complete paralysis and was unable to eat or drink or take medicine. I was heartbroken and I got a second opinion from my uncle, a vet, who ran more in depth tests and determined she was extremely sick and would not recover from her current state. He came to my house to perform the euthanasia (most vets will let you have this option) and he vaccinated Blackie for FIP (cats can be carriers of FIP their whole lives but not develop symptoms). Blackie is fine today and after a couple years I was ready to get her a nice playmate named Manhattan (she found me after I had taken my mom and dad to Manhattan for their 50th birthdays and that is what my dad wanted to name her).
Most vets will only recommend euthanasia when the quality of life of the pet is extremely poor and there are no other options left such as treatment or adoption. But it is usually up to the pet owner.
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Blackie and Manhattan are much healthier and happier with this product (available in white, black and blue). They are very picky about VERY fresh water and I would catch them drinking out of the faucet or in the shower when only day-old water was in their bowl. I purchased this fountain at a target or walmart and the two of them are in heaven. It uses a carbon filter similar to a Brita or Pur.
http://www.petmate.com/Catalog.plx?ID=205
Introducing your pet to the Pet Fountain
Each animal's response to the Pet Fountain will be different depending on age, temperament and breed. Some will be nervous because of the flowing water and the low hum of the pump. We recommend placing the unit unplugged next to the pet's normal water bowl with water in it for at least a week. Then plug in the Pet Fountain. If the pet hesitates to approach the unit, place bits of food or their favorite treats on the rim. This will encourage your pet to approach and drink. As they become used to the Pet Fountain, remove the normal water bowl, leaving only the Pet Fountain to drink from.
http://petmate.com/FreshFlowFAQ.htm