Suffering and death are part of life, for humans and animals alike. We strive to eliminate unnecessary suffering brought about by cruelty and sin. But there is no such thing as a life without suffering, in spite of what popular culture promises. For human beings, suffering can be redemptive and lead one to a deeper commitment to Christ who suffered and died for us on the cross. For animals, the natural suffering of old age also seems to be a part of God’s plan for them as God’s creatures.
In an affluent culture, some people spend money on medical interventions to prolong pets’ lives in a way that would have been unimaginable a generation ago and that remains unimaginable in places where such sophisticated medical treatment is outside of the reach even of humans. You are under no obligation to provide this kind of treatment for an animal. Do your best to keep your pet comfortable and continue to provide for her basic needs of food, water and shelter. Beyond that, you can let nature take its course with a clear conscience.