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If you are concerned that he is becoming dehydrated, you can try an encourage him to drink but offering fresh water or even low-sodium chicken broth. If he isn’t amenable to these, you can syringe feed him pedialyte. A typical maintenance rate for hydration in an animal is 2 millilitres for every kilogram of weight per hour. (2ml/kg x kg x 24hr). This value will give you the total he needs for the day and is a good starting point to give you an idea of his daily requirement. If he vomit when you given pedialyte, I would discontinue this as a therapy. (since we don’t want him vomiting more because of our intervention).
As nausea is possible in his case and cats can go off their food if they are nauseous (even without vomiting),there are also a number of antacids that are available over the counter and pet friendly. I would advise only treating with one, and administering a half hour before offering food. Two I tend to recommend are
Pepcid (http://www.petplace.com/drug-library/famotidine-pepcid/page1.aspx) or
Zantac (http://www.petplace.com/drug-library/ranitidine-hcl-zantac/page1.aspx) .
As long as he is urinating normally, you may intitiate these treatments, but if he isn't perking up or you do not see improvement over the next 24 hours, then I would advise taking him to the vet so that they can rule out possible causes for his being off color. Sometimes we find that cats with tummy bugs or festering infections that won’t settle without veterinary intervention. The vet will be able to give antibiotics and anti-vomiting medications by injection, to get him back on tract as quick as possible.
If you do need a vet and your own is closed, then you can find an emergency vet here, http://www.veccs.org/index.php?option=com_hospitals&nationid=1&Itemid=193
I hope this information is helpful. Please let me know if you have any further questions. If you have no further questions, I would be grateful if you would press the wee green accept. Thank you, Dr. B.