Culling Age

I have a six year old dog that is hard to out hunt, out find and out stay. He had two hunts in a row where he wasn't up to par, so I took him and had blood pulled on him. We did the heart worm and the tick born diseas test and sent the rest to the lab. We also checked him for worms. The only thig that showed up was a low blood count.I put him on blood builder and vitamins and gave him a couple of weeks off. He was better the next hunt but still not up to his usual self. I took him back and pulled blood again and told them to check his thyroids and for bruccelosis. The bruccelosis came back positive. When a good dog starts not performing its time to look at health issues.