12/29/2011 12:45 pm ET Updated May 15, 2014

Saying Goodbye to a Good Dog

McDuff, my Cairn terrier, looks more like a pot-bellied pig every day. His swollen abdomen is low-slung and his short legs bow out at the elbows -- symptoms of Cushing's Disease. Recently we had to put up a baby gate to keep him from going upstairs; the last time McDuff tried to follow us up to bed, he slipped and went bumping down to the bottom of the staircase, his front legs useless as toothpicks against the pull of his massive weight.

He's an old man, our McDuff. Fifteen. Whenever he goes outside to relieve himself, he stands in one spot for a good five minutes, squinting a little, then turns right around and heads back inside. At this point, his medication costs half as much as our groceries. I don't know what we'll do when it snows. Shovel a path for him, I guess.

Or not. We have been debating, lately, about how and when to play God with our beloved pet. McDuff isn't in extreme pain, and he still wags his tail when I call his name. That's something, right?

But is it enough for a good dog's life? Or is it time to say goodbye?

I grew up on a farm where we had nearly as many dogs as we had horses. They were rescue dogs, mostly. These included one shepherd mix that loved to chase cars and always smelled of skunk; a feisty Yorkie mix; and an Afghan hound that bit anything gray, including our coats. I moved away from home before any of these dogs died or had to be put down; coming home and finding one less dog under the table was a source of brief sadness but not much more.

This is different. I can't stand the thought of losing McDuff.

As an adult, I've had to put just one dog to sleep. Ben was an American Eskimo mix that we adopted from a shelter. A frothy, white, joyful dog, Ben used to race around us in circles whenever we uttered his mantra: "Go Ben go!"

When my husband and I were married in our back yard (a second marriage that combined our four young children), Ben wore a burgundy bow to match my dress. As we repeated our vows in front of a small gathering of friends and family, Ben wandered up and sat down between our children, so that he would be included in the minister's blessing.

At age thirteen, Ben's heart and liver gave out. Making the decision to put him down was easier because he was in such pain that he cried out in his sleep. Still, the kids and I all wept: it was the first time that I fully realized a dog isn't just a dog, but a carrier of family history.

Saying goodbye to a dog you've had for years means shutting the door on an era. In our case, Ben's death earmarked the years between our wedding and the year our oldest son set off for college. Shortly after Ben's death, we moved out of our big family home and into a smaller one; my memories of Ben therefore carry complex emotions: joy and love and grief and loss, rolled into one white ball of fur.

McDuff started his life with us just as Ben was ending his. I got him in the worst way possible -- on impulse, in a mall pet store -- but for a good reason: I was with my stepdaughter, the youngest in our blended family and the one who always felt left out by our other three children. She was newly aggrieved by the arrival of our fifth and youngest child, who immediately displaced her as the baby in the family. Choosing this dog made her feel, for once, that she was in charge.

As a puppy, McDuff was scarcely bigger than the palm of my hand. Like most terriers, he was stubborn, territorial, and ferociously protective. We put a dog door in our basement so that he could come and go at will. His greatest joy was patrolling our yard and barking at any deer, squirrels, or wild turkeys that dared to infiltrate his space.

McDuff became a member of our family a few weeks after our youngest child was born. He has been through a lot since then: older kids graduating from high school and college, family trips to Canada and Wisconsin, youngest child moving through elementary school and into high school, job layoffs and career successes, the celebration of our fifteenth wedding anniversary. Saying goodbye to him means saying goodbye to boisterous family dinners, birthday parties with balloons and water slides, Christmases with so many presents under the tree that you couldn't walk around it, the death of my grandmother and my father, buying a second house in Canada, and the realization that nothing lasts forever.

Not even a very good dog, who still lifts his head whenever I call his name.