Hello & welcome, I am Dr. B, a licensed veterinarian and I would like to help you today.
Now we do need to tread with care here. At her age, these signs can be related to opportunistic infections but can also be precipitated by age related disease. So, while her appetite/thirst decline is likely due to the same nausea that is making her vomit, we'd have to be aware of not just gut infections and pancreatitis; but also systemic issues like kidney or liver disease, metabolic issues, IBD or cancer. And I would note that hacking coughs in dogs can be heart related (not necessarily congenital as we see acquired heart disease cause cough too), we can also see that with vomiting dogs that have inflamed throats (esophagitis) from vomiting.
With this all in mind, given her age and how severely she is affected, it would be ideal to have her seen at this point. A physical exam and a geriatric blood test would help us narrow down which issue is present here, give us an idea of prognosis, and if treatable (like an infection), then injectable anti-nausea medication, appetite stimulants, fluids +/- antibiotics could get her feeling better. Any delay and we can at least try some supportive care for her. In regards ***** ***** you can try to counter her nausea with an OTC pet safe antacid like 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 a liquid one (ie Milk of Magnesia - 0.25tsp every 8 hours) to coat her throat at the same time. 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.
Afterwards, you can consider starting her on an 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). 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 her slowly back to her normal diet.
Since dehydration is a risk with her drinking difficulities, 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).
Overall, there are a wide range of agents could trigger the signs we are seeing at her age. Therefore, we’d be best to have her assessed for the above before this can progress any further or she can become any weaker. Any delay and we can at least start supportive care. If she cannot keep that or water down at any point, appears dehydrated already, or doesn’t respond to the above; then we'd again want to get her vet involved. They can assess her hydration, check bloods, make sure there is no mass in her abdomen that shouldn't be there or any sinister infections present. Depending on their findings, her vet can treat her with injectable anti-nausea medication, appetite stimulants, fluids +/- antibiotics to give her a chance here.
All the best,
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