The key point here is the distinction between reduced activity and lethargy.
If your dog only wants to walk instead of running, go outside for thirty minutes instead of an hour, prefers chewing on a toy instead of chasing one, that's reduced activity. Your dog is still doing what they normally would but get tired sooner or choose to tone things down. This is a gradual change; often so gradual that its significance gets lost.
Make no mistake, such a change is important to investigate, but it is not an emergency.
Lethargy is defined as lack of energy and enthusiasm.
The more acute the onset, the more urgent it is to address it. Sudden or pronounced lethargy is an emergency. A marked lethargy that waxes and wanes can mean an emergency such as your dog bleeding from a splenic tumor. Whether the tumor is benign or malignant, such bleeding can still kill your dog.
These are important distinctions.
Towards the end, Bruin was showing signs of slowing down, reduced activity. He was still excited to go for walks but didn't last as long before getting tired. He would go up and down the stairs but reluctantly so. He still had playful spells, but they were short-lived. He didn't have any major joint issues, but his heart was getting worn out. Doing things, as much as he wanted, was becoming increasingly difficult for him. The day we let him go we took him for his last walk and then made a trip to the vet. The vet almost couldn't find a heart beat at all. But it was a gradual process.
Every time Jasmine didn't come to greet daddy at the door, I knew it was an emergency. When all Cookie wanted to do was to lay down under daddy's desk, I knew it was an emergency.
If a puppy stops being playful, it is an emergency.
While there can be some confusion what the is the difference between lethargy and reduced activity, let the speed of onset guide you.
If your dog is happy, active and playful one day and doesn't want to move the next, it is an emergency.
Do not, however, dismiss a gradual change either. Whether there is pain or slow organ failure, this too should not be ignored.
Symptoms to Watch for in Your Dog: Lethargy
Dog Medical Emergencies Survey
Dog Medical Emergencies Survey Results
Is Unproductive Retching an Emergency?
Is Difficulty Breathing an Emergency?
Is Panting an Emergency?
Is Severe Pain an Emergency?
Is Limping an Emergency?
Is Vomiting Bile in the Morning an Emergency?
Is Profuse Vomiting an Emergency?
Are Convulsions or Seizures an Emergency?
Is Loss of Appetite an Emergency?