Thank you for the additional information on Rosie.
I am sorry to hear that Rosie is feeling unwell. In regards XXXXX XXXXX lack of appetite and turning away from the treats despite seeming to want them, this is not uncommon when their are suffering with severe nausea. And I do suspect that this vomit of bile is significant and related.
Now when dogs start going off their food and showing vomiting there can be a number of culprits. Most common of these includes bacterial infection, viruses, parasites, cancer, organ troubles, pancreatitis, foreign bodies, and toxins (the last 2 hopefully being less likely here at her age).
In all of these cases, since she isn't dehydrated, our first step is to rest her stomach if they are actively vomiting. I would suggest giving her stomach a break for 8-12 hours after the vomit. Let her have access to water, but not huge volumes since overdoing it with the water can cause vomiting as well.
Even if we can settle her stomach with resting, we do have to be concerned about that associated nausea, as that can make getting them to eat difficult. There are a number of antacids that are available over the counter and pet friendly. I would advise only treating with one, but the two I tend to recommend are Pepcid (LINK) or Zantac (LINK) .We tend to give these 30 minutes before offering food to give it time to be absorbed. After you rest her stomach, you can consider trying one of these with her.
If she is more settled by that point, then you can tempt her with a small volume (a tablespoon worth to start) of a light/easily digestible diet. Examples of this would be to give a combination of rice with one of the following: boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), meat baby food (do avoid the ones with garlic powder in the ingredients). As well, there are also veterinary prescription diets that can be used in cases of gastroenteritis (ie Hill’s I/D or Royal Canin’s sensitivity). Now if she can keep the small amount, she can have a bit more after 30 minutes. And as she keeps it down, she can have a bit more and so on. The aim of these light diets are that they are easy on the compromised GI and tend to be better tolerated.
As well, do keep an eye on her water intake and hydration at this stage. Older dogs don't have the body reserves they once had and any dehydration can make them feel poorly. Therefore, it would be ideal to check her hydration at this stage to make sure she isn't dehydrated already and to give you a baseline to compare to if you need to check again. To check this and make sure she is not becoming dehydrated there are a few things we can test at home. One is whether the eyes appear sunken, if the gums are tacky instead of wet/moist, and whether she has a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a wee video on this (LINK) They use a big dog but it makes it easier to see and the principles are exactly the same for any size dog). And if you had been seeing any signs of dehydration already, then that would be a red flag to have her seen urgently by the vet before this gets any further out of control
While an already dehydrated dog will likely need veterinary intervention, if she is adequately hydrated at the moment but you are concerned that she is becoming dehydrated, you can try an encourage her to drink but offering fresh water or even low-sodium chicken broth. You can offer her Pedialyte but do not try to syringe syringe feed it because syringing fluids to a vomiting dog is contraindicated since we don't want to cause further vomiting.
Overall, when our dogs show nausea in the manner, there can be a range of potential causes. Therefore, if she is hydrated, only vomiting very occasionally, and is able to settle, then consider resting her stomach and then try the above. If you initiate these treatments and do not see improvement over the next 12-24 hours (we don't want to let it go too long since she is an older lass) or she shows further vomiting, then it would be prudent to take her to the vet so that they can make sure there is nothing worrisome afoot. The vets will be able to have a feel of her to make sure she has no sinister lumps or bumps or anything that shouldn't be there. Depending on their findings, you may want them to check a blood sample to make sure she is having no organ troubles. As well, the vet will be able to cover her with antibiotics against bacterial gastroenteritis and anti-vomiting medication by injection to help settle her stomach and get her back on track as quick as possible.
I hope this information is helpful.
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All the best,
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