First, I am glad to hear that Pixel doesn't have any of those other signs, but I do share your concern about his hernia. You are right that it is the pressure of vomiting that is forcing loops of gut and mesenteric fat into the hernia. Now as long as it always reduces back in, then we aren't in an emergency situation. BUT if you find the loops of gut get caught in there (where we have strangulation of the gut) and cannot be pushed back into the body cavity, that is an emergency situation. And I'd note that if he has a large enough hernia that we are seeing this, you may want to speak to his vet about having it surgically repaired so there is no risk with that.
Now that aside for the moment, since its not likely directly involved, we do have to consider other signs for his nausea. Common causes we need to consider include bacterial or viral gastroenteritis, pancreatitis, parasites/protozoa infections, organ/metabolic disease, general dietary indiscretions, and ingestion of harmful items (ie toxins, wildlife, plants, non-edible items). Though hopefully given your further history, Pixel hasn't been up to any mischief of that nature.
With this all in mind, since 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 his stomach for a few hours first), then 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), 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 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'd 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). 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 him slowly back to his normal diet.
Since dehydration is a risk for a small breed dog, we need to keep an eye on his hydration. To check this and ensure he’s not becoming dehydrated, there are a few things 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 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 him seen before this becomes an additional issue for him (especially as it is often dehydration that makes them feel unwell).
Overall, there are a wide range of agents could trigger this GI upset we are seeing. Hopefully, he hasn't gotten into anything or drank enough pool water to upset his stomach. Still, since he hasn't any emergency signs, we’d want to start supportive care to settle his stomach. If he cannot keep that or water down at any point, appears dehydrated already, that hernia stops being reducible, 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, rule out fever, make sure there is nothing in his stomach that shouldn't be there or any sinister viruses present. Depending on their findings, his vet can treat him with injectable anti-vomiting medication, fluids, +/- antibiotics to settle his stomach, and get him back feeling like himself.
Please take care,
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