My friend and mentor, Gail Pope, of BrightHaven (brighthaven.org) has a saying: "Living well through the last breath". And indeed, our animals can! Gail, founder of BrightHaven, has been doing animal hospice for over thirty years, and has seen almost all of the animals in her care through dignified, pain-free natural deaths. Euthanasia has a place, but is not necessary in most cases.
I have done the same, though not for nearly as long. As with humans, the animals almost always shut down, go into a coma state, and pass easily.
Introducing Animal Hospice Group! (www.animalhospicegroup.com) TAKE A LOOK!! Gail has teamed up with three other great teachers to create a course in end of life care: palliative care and hospice! It is an extensive series on on-line lectures, with reading material and discussion groups. Just wonderful!! And it fills such a huge void in animal care! Tell your friends! Of course I am taking the classes: they're amazing! And a great deal of the subject matter concerns ways in which to assuage vets and human caretakers' fear of death.
Death is not painful. Some conditions can be painful, as with humans, but not death itself.
At the end of the course, students become certified to guide folks through end of life care of their animals. I have guided two friends when they chose a natural death for their animals, a cat and a dog, and they were grateful and happy to be with them until their natural end.
Again, TAKE A LOOK: animalhospicegroup.com. You'll be glad you did!!