Now I have to say that I am quite concerned about your lad.
As I am sure you are aware diabetics cannot afford to be off food, but its also a concern in cats if they are off food for days since they can develop secondary liver issues (hepatic lipidosis) too. Therefore, while its good his vet treated the infection, if he has vomited then they need to be looking at more being afoot. Issues we'd really want to rule out with his signs would be a possible gut infection, dietary indiscretion but especially pancreatitis (as it can cause these signs and is something diabetic kitties are high risk for). As well, we'd reallly want them treating him symptomatically with anti-nausea medication (since many cats stop eating to avoid vomiting when nauseous) and appetite stimulants. So, it'd be ideal to ring his vet about these concerns and treatments for Charley.
Until you can speak to them about these, we can at least consider trying an OTC pet safe antacid [ie Pepcid (More Info/Dose @http://www.petplace.com/article/drug-library/library/over-the-counter/famotidine-pepcid)]. 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 Charley cannot keep it down due to nausea that is usually a red flag that we need to bypass his mouth with injectable anti-vomiting medicationfrom 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 boiled chicken, boiled white fish, or meat baby food (garlic/onion free only) There are also OTC vet diets (ie Hill’s I/D or Royal Canin’s sensitivity) that can be used too. The aim of these diets is that they will be better tolerated/absorbed by the compromised gut. Though if he refuses these and isn’t actively vomiting now, we can offer or syringe feed watered down Hill's A/D, Royal Canin Recovery, kitten food, Catsure, or Clinicare Canine/Feline Liquid Diet. All of these are critical care diets that are calorically dense, so a little goes a long way nutrition-wise.
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). And again if he isn't vomiting, we can syringe fluids (which he needs 48ml per kilogram of his weight over every 24 hours).
Overall, a wide range of agents could trigger the signs we are seeing. Given his signs and lack of response to the antibiotic, I suspect the abscess isn't the whole story. So, we'd want to use the above to try to break his fast but also consider speaking to his vet about injectable anti-nausea medication +/- appetite stimulants to get him back to eating so we can treat his diabetes without risk of crashing his blood sugar.
Please take care,
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