Another tool in the tool box is the ability to put weight on a dog. Dogs get sick or nursing dogs, fussy eaters or really high energy there are a lot of reasons that dogs lose weight. Here's a few recipes for "fat balls' they work, they are easy to make, local ingredients and you can freeze them!
This happens more often then you'd think. It starts slowly and it's frightening when your little bundle of joy leaps at you... there are a few things you should continue to do at home and implement as needed throughout the dogs life. Here's a good reference with lots of good solid ideas.