First, if he is vomiting what looks like water, we do need to tread with care and monitor his fluid intake. This is because dogs too nauseous to keep fluids down tend to be the ones that need injectable treatment. That said, 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 the stomach for a few hours first), you can consider treating with an OTC pet safe antacid [ie 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), 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. Also 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 he is more settled, you can plan to try small meals of a light/easily digestible diet. Examples you can use are cooked white rice with boiled chicken, boiled white fish, or scrambled eggs, or meat baby food (garlic/onion free only). 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 slowly back to what you normally feed.
Since dehydration is a risk here, we need to keep an eye on his hydration. To check this and make sure dehydration isn’t an issue, there are a few parameters 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, you can find a good video HERE (http://www.ehow.com/video_12232503_dog-dehydrated.html). If you do see any of these signs already, then that would be our cue to have him seen before this becomes an additional issue (especially since its often dehydration that makes them feel unwell).
Finally, I wanted to note that if funds are a concern, there is help. First, if you have a VCA or Banfield veterinary hospital near you, then you might consider taking advantage of their free first consult offers. You can find vouchers for this via VCA: (http://www.vcahospitals.com/main/offer) & Banfield : (http://www.banfield.com/landing-pages/coupon). Otherwise, you could consider checking out the Humane Society's database (http://www.humanesociety.org/animals/resources/tips/trouble_affording_pet.html) or ASPCA’s database (https://www.aspca.org/about-us/faq/financial-help-my-vet-bills). Both have a lot of branches nationwide, along with ties to other assistance organizations that can keep down costs and surely will be willing to help.
Overall, a wide range of agents could trigger the signs we are seeing but if the water was contaminated from the other dog of anything else, then that could be our culprit. Therefore, we’d want to start supportive care to try to settle his stomach. Though if he cannot keep that or water down, appears dehydrated already, or doesn’t respond to the above within 12-24 hours; then we'd want to get to use those above options to have him seen. They can assess his hydration, rule out fever, ensure nothing in his stomach that shouldn't be, or any sinister viruses present. Depending on their findings, his vet can treat with injectable anti-vomiting medication +/- antibiotics to settle his stomach and get him back feeling like himself.
Please take care,
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