What can cause this in pets?
Grief, change of scenery, or sometimes it is a chemical imbalance, needing medication to correct it. Even the weather can adversely affect a normally happy dog. Or your health.
As a pet owner, how do you fight an invisible, inner enemy? The first step is recognizing the problem. You've ruled out all the possible physical causes with help from your veterinarian, now it is time to start looking at the mental causes.
Has your pet recently lost a friend? Perhaps a neighbourhood dog he played with is gone, be it on vacation, or gone over the Rainbow Bridge, but gone nonetheless? Or perhaps his child grew up and moved out?
Losing a playmate, especially an in-home playmate is often a reason for canine depression. We may not notice it very often, but pets do grieve, and in some cases, especially concerning a violent death or even just a disappearance, dogs can grieve to a dangerous point, and it can be very hard to bring them out of it.
Slow deterioration and loss of initiative is a glaring clue that something is bothering your dog. If this is happening to your dog, you need to get right on it and start to make life fun again. Ask your vet about Anti-depressants available as well. Depression may not seem like it, but left too long and it could very well turn into a life-threatening physical condition.
Be sure to have all physical aspects of your dog's health checked out by a veterinarian first. While prescribing Prozac may perk your pet up a bit, it won't help one bit if it's Canine Coronavirus or Distemper that has your pooch in a slump. Those blood tests your vet mentioned could be crucial in determining the problem.
Once you and your vet have determined that depression is the cause, there are a few options. One is medicated therapy. That would be Prozac for dogs, or other veterinarian recommended anti-depressants. An increase in your activity level, or at least your dog's will also likely be called for. In cases of grief, playdates with other dogs or Doggy Daycare would definitely be worth looking into. If you are ready, you may even consider getting another dog.
Introduction to Canine Mental Health
Do Dogs Get Depression?
When Anxiety Attacks
Biting the Hands That Feed You
Obsessive Compulsive Behaviors