First, if this all started after an unknown treat, we need to tread with care. We'd hope its not related but it does present a risk. Otherwise and much more commonly we can see nausea signs (vomiting, lip licking/smacking, grass eating, etc) like those Riley is showing related to bacterial or viral gastroenteritis, pancreatitis, parasites/protozoa infections, general dietary indiscretions, and ingestion of harmful items (ie toxins, plants, non-edible items). As well, if there seems to be no effort in the vomiting (where there is no retch/gag), then regurgitation would be a concern and means we'd also have to consider throat based irritation, acid reflux, and even possible partial throat obstructions (less likely). And I'd note that all of these (throat based signs and when vomiting) could cause head shaking but the latter may be more likely to do so since he is likely trying to shake away discomfort.
Now with this all in mind, 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 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 Milk of Magnesia (0.5 tsp every 8 hours). I would note that the last one is a liquid antacid and may be ideal for Riley since it will coat his throat as well as reduce nausea. So, it can allow us to cover more problem areas here. Though 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. And I would 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). 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. 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. Though if he cannot keep it down, we'd need to keep those throat differentials in mind.
Since dehydration is a risk here, 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, a wide range of agents could trigger the GI upset we are seeing. Therefore, in his case, we’d want to start supportive care to settle his stomach. If he cannot keep anything down at any stage, appears dehydrated already, 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, check for throat based issues and inflammation , 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 +/- antibiotics to settle his stomach, and get him back feeling like himself.
Please take care,
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