First, I am glad to hear that Shelby has normal gums, no belly pain and isn't likely to have ingested something harmful. Now those pink gums rule out severe internal bleeds but also tell us that despite the panting we have normal oxygen uptake. So, we'd assume the panting is due to discomfort (possibly from his joints or his gut/throat from the vomiting) instead of respiratory distress.
With that in mind and considering all his signs,we'd be most wary of a severe bacterial or viral gastroenteritis, possible stomach ulcer from the recent Previcox, pancreatitis, parasites/protozoa infections,or general dietary indiscretion. Of course, at his age we can also see GI signs related to organs issues, IBD and cancer (usually GI based). So, we do need to tread with care with him.
Now as long as he can keep water down, we can try some home supportive care to try to settle his stomach. To start, if he hasn’t just vomited (since otherwise we’d need to rest the stomach for a few hours first), you can consider treating with an OTC pet safe antacid like 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). 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. Also 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 he is more settled, you can plan to try small meals of a light/easily digestible diet. Examples you can use are cooked white rice with boiled chicken, boiled white fish, or scrambled eggs, or meat baby food (garlic/onion free only) There are also OTC vet diets (ie Hill’s I/D or Royal Canin’s sensitivity) that can be used 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. As long as improvement is being seen, I usually advise continuing this until the signs are settled, and then weaning slowly back to what you normally feed.
Since dehydration is a risk for vomiting in older dogs, we need to keep an eye on his hydration. To check this and make sure dehydration isn’t an issue, there are a few parameters you can test. Further to checking for gum moisture, you will want to make sure his eyes are not looking sunken and that he doesn’t have a "skin tent" when you lift the skin. To see how to check these, you can find a good video HERE (http://www.ehow.com/video_12232503_dog-dehydrated.html). If you do see any of these signs already, then that would be our cue to have him seen before this becomes an additional issue (especially since its often dehydration that makes them feel unwell).
Overall, a wide range of agents could trigger the signs we are seeing and if he is panting in discomfort then we do need to tread with care. Therefore, we’d want to start supportive care to try to settle his stomach. Though if he cannot keep that or water down at any point, appears dehydrated already, or doesn’t respond to the above within a few hours (since we don't want him so unsettled); then we'd want to get his vet involved. They can assess his hydration, rule out fever, ensure there are no sinister lumps/bumps in his stomach that shouldn't be, or any sinister viruses present. Depending on their findings, his vet can treat with injectable anti-nausea medication, gsatroprotectants, gut safe pain relief, +/- antibiotics to settle his stomach and get him back feeling like himself.
Please take care,
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