Unfortunately our pets are prone to many of the same diseases that we are. Those vet bills can sometimes break you. I have been fortunate enough to have fairly healthy animals (except for Frodo, and he's doing good.) I have had a horse die of cardiac arrest (that one killed me, I so loved him, and we have vet visits regularly for the horses, and of course take the dog and cat) but sometimes they don't see it either.
I've lost several dogs to these and one donkey had them in her mouth. We just had surgery done on one of our labs that had a tumor between her toes. Bad stuff. Always makes me worry when I feel a lump on one of them. 1139 days ago