First, I have to say that I am quite concerned about Zoey.
If she has had both upper and lower GI signs for weeks, this will take a toll on older dogs. Often they get weak, wobbly, and can even collapse from fluid and nutrition loss. Of course, it can be even worse if they had been previously coping with underlying arthritis. So, we need to tread with care if she seems worse; especially if she has had some broad spectrum care already. And I would note that even though she has had antibiotics and the further tests rule out obvious cancers and organ issues; we'd still have to be wary of a resistant bacterial or viral gastroenteritis, pancreatitis, and parasites/protozoa infections.
With this all in mind, since you made no mention of treatment being started to fight her nausea, we can try some home supportive care to try to settle her stomach. To start, if she hasn’t just vomited (since otherwise we’d need to rest her stomach for a few hours first), then you can consider treating her 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), Zantac (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/ranitidine-hcl-zantac), or Tagamet (More Info/Dose Here @ http://www.petplace.com/article/drug-library/library/over-the-counter/cimetidine-hcl-tagamet). Whichever you choose, we’d give this 20 minutes before food to allow absorption. Of course, do double check with your vet if she has a known health issues or is on any medications you haven't mentioned. Though if you give this and she cannot keep it down due to nausea that is usually a red flag that we need to bypass her mouth with injectable anti-vomiting medication from her vet.
Once that has had time to absorb and she is steadier on her stomach, you can consider starting her on a easily digestible diet like cooked white rice with boiled chicken, boiled white fish, cottage cheese, scrambled eggs, or meat baby food (as long as its garlic/onion free). 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 her diarrhea. You can also add fiber (ie canned pumpkin, all bran, etc) to her food to help bulk up her stools quicker. As long as improvement is being seen, I usually advise continuing this until the signs are settled, and then weaning her slowly back to her normal diet.
Since dehydration is a risk with upper and lower GI signs, we need to keep an eye on her hydration. To check that she isn't dehydrated, there are a few things you can test. Further to checking for gum moisture, do make sure she doesn’t have sunken eyes and that she doesn’t have a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a good video HERE (http://www.ehow.com/video_12232503_dog-dehydrated.html). If you are seeing any signs of dehydration already, then that would be our cue to have her seen before this becomes an additional issue for her (especially as it is often dehydration that makes them feel unwell).
Furthermore, you can consider trying a pet safe anti-diarrheal. As I am sure you appreciate, these would not be a cure for infectious issues; but it can still be of benefit. It will reduce diarrhea load, allow the body to absorb more water/nutrients, and soothe the upset gut. In regards ***** ***** options, the one we most commonly use is Kaolin/Kaopectate (More Info/Dose @ http://www.petplace.com/article/drug-library/library/over-the-counter/kaolin-and-pectin-kapectolin-k-p)). This is available OTC at most pharmacies and can be used with the probiotics yiu have. Otherwise, Propectalin, Canikur, Fast Balance, and Protexin Pro-Fiber (all OTC at vets, pet stores, and even Amazon) would be another option.
Finally, once she is more settled and if the gait issues remain, then you may want to add OTC joint supplements (ie glucosamine, chondroitin, Duralactin, Omega3 fish oils) to her care to just help her with that.
Overall, there are a wide range of agents could trigger this GI upset we are seeing. It is good that her vet has ruled out some concerns but it sounds like they have not pinpointed the cause. So, we’d want to start supportive care to settle her stomach but may need to discuss further prescription options with them (ie anti-nausea medication, fluids, gut safe pain relief for her joints), test a stool sample +/- try a different antibiotic to settle this and get her back feeling like herself.
All the best,
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