Thank you for your patience,
First, I have to say that I am concerned to hear that you initially saw black stool. The reason is because if dogs are not eating dirt or darkly pigmented foods; this is often a sign of upper GI or stomach based bleeding. If we can rule our sharp foreign bodies, then stomach ulcers or tumors would be on our list of concerns here. And I would note that if we saw black stool after the cortisone specifically, then a stomach ulcer would be even more of a suspicion here. Furthermore, since he has enough chow heritage to darken his gums, remember that you can flip down the lower eyelid to check his mucus membrane color and make sure its not pale.
Otherwise, in regards ***** ***** question for what could induce anorexia, vomiting, loose stool, and secondary lethargy; we do have a range of concerns at his age. On top of those stomach bleed based issues, this also is consistent with bacterial or viral gastroenteritis, pancreatitis, parasites/protozoa infections, or could be secondary to underlying health issues (ie organ dysfunction, metabolic disease, cancer).
With this all in mind, I do want to note some supportive care you can try at this stage that would help allay nausea and counter any ulcer based damage. To start, you can consider treating him with an antacid. Common OTC pet safe options would be Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid) or Zantac (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/ranitidine-hcl-zantac).As well, we could use Milk of Magnesia (0.5 tsp every 8 hours) since this will help with nausea but can also coat the stomach if there is an ulcer present. Whichever you choose, we’d give this 20 minutes before offering food to allow absorption. Of course, do double check with your vet if he has a known health issues or is on any medications you haven't mentioned. Though I would note that if you give this and he cannot keep it down due to nausea that is usually a red flag that we need to bypass his mouth with injectable anti-vomiting medication from his vet.
Once that has had time to absorb and he is steadier on his stomach, you can consider starting him on a light/easily digestible diet. Examples you can use are cooked white rice with boiled chicken, boiled white fish, cottage cheese, or scrambled eggs (made with water and not milk). There are also OTC vet diets that can be used (ie Hill’s I/D or Royal Canin’s sensitivity) too. The aim of these diets is that they will be better tolerated/absorbed by the compromised gut. Therefore, it should get more nutrients in and result in less GI upset and reduce diarrhea load.
That said, if he cannot be tempted, we can try syringe feeding food to break this fast. To syringe feed, we can water down calorie rich diets (ie Hills A/D, Royal Canin Recovery diet, even canned puppy food) or use a liquid diet (ie Clinicare, Dogsure). As well, there are paste supplements (ie Nutrical) that can also be used. And these will all get more in per bite even if we cannot get much in and help stop the weight loss. Of course, any vomiting from this and we’d have to discontinue syringing food.
Overall, a wide range of agents could trigger the anorexia and GI upset we are seeing. That said, if he has passed black stool we do need to tread with great care here since ulcers and bleeding gut tumors are especially a worry. Still if that has subsided, we can try the above just now. Of course, if Loki cannot keep that or water down, appears dehydrated already, or doesn’t respond to the above within 12-24 hours; then we'd want to get his vet involved. They can assess his hydration, check bloods to rule out organ issues, and determine if we have an ulcer or mass present. Depending on their findings, his vet can treat him with injectable anti-vomiting medication, appetite stimulants, +/- alternative antibiotics to settle his stomach, and get him back feeling like himself.
Please take care,
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