Dogs, just like us can feel pain. Sometimes it's obvious when they give a little yelp if the pain is sudden. However, when the pain is gradual and constant, like joint pain, or breathing trouble, the poor creatures never express it in any way that immediately alerts the owner.
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Nonetheless, owners should need to discern certain signs by means of which they can realize that a dog is in pain and take it from there. Dogs will keep the pain to themselves, and only by subtle changes in behavior will you know that they need care.
For example, a hesitancy to jump over a wall or onto furniture where previously they would have bounded up, or a slight slowness in circling or sitting down. Or even restlessness and heavy breathing - all of these can indicate that your dog is in pain and that it's time to pay some attention to what could be wrong.
One has to be sensitive to such changes, as your dog is not aware that you can help them and will not try and show it to you. By early detection, you can ensure that your dog is saved from suffering in silence.