My dog is lethargic, not eating, and has diarrhea. What should I do?

It sounds like Moose may have some gastro-intestinal upset. You didn't mention vomiting-- If there's been no vomiting then that is good. However, decreased appetite and the loose stools are concerning. When the colon (large intestine) is inflamed, sometimes they feel like they need to poop even though there isn't anything more coming out. As for what may be causing the GI upset, it could be gastroenteritis which is inflammation of the stomach and or intestines-- this could be from eating something out of the ordinary such as getting into the garbage or eating mulch, etc, or it may be secondary to anything from kidney disease to cancer. I don't see Moose's age listed; if he is older, the concern that there is a more serious underlying problem is higher. If he is a younger dog, gastroenteritis is more likely. Since he seems lethargic, I strongly recommend that you take him in to the vet at your first convenience. If the lethargy is getting worse, or if he starts vomiting or having blood in the stool or vomit, seek emergency care.
If you are unable to take him to the vet, you can offer a bland diet (boiled chicken and white rice) for a couple of days and see if that helps. If it is not improving, please get him in for an exam.
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