Based on Felix's current signs, I would be suspicious to whether the worm was a red herring. Especially if he is still so nauseous after treatment. Now when kitties his age start vomiting there can be a number of culprits. This includes bacterial infection, viruses, parasites, dietary indiscretion, foreign bodies, and toxins. So, we do need to tread with care here.
Now if he cannot even keep water down, then we may need to revisit his vet at this stage for injectable anti-nausea treatment. Otherwise, if you just think he is reluctant to drink because of his GI upset, I would note that you can consider trying to settle his stomach orally at home first.
To start, if he did just vomit, then we'd want to rest his tummy for a few hours to give it a chance to settle. Let him have access to water, but not huge volumes since overdoing it with the water can cause vomiting as well. If he isn't keen to drink at this stage due to nausea, then that is fine (we don't want to force him). We do want to monitor his drinking, since again vomiting after water would mean that we need to bypass his mouth with injectable treatment from his vet.
Otherwise, we can consider starting him on antacid therapy. There are a number of antacids that are available over the counter and pet friendly. I would advise only treating with one, but the ones I tend to recommend are:
*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)
These are usually given 20 minutes before food
(to allow absorption) and of course you want to double check with your vet if your kitty has a pre-existing condition or is on any medications.
Now if you rest his stomach and see no further vomiting at that point, then you can tempt him with a small volume (a tablespoon worth to start) of a light/easily digestible diet. Examples of this would be boiled chicken, boiled white fish, scrambled eggs (made with water and not milk), meat baby food (do avoid the ones with garlic powder in the ingredients) or there are also veterinary prescription diets that can be used (ie Hill’s I/D or Royal Canin’s sensitivity). Now if he can keep the small amount down for, then you can offer a bit more after 30 minutes. And as he keeps it down, he can have a bit more, and so on. By offering a light diet, we are providing an easily digestible protein source for the compromised gut. And offering small volumes will again less the strain on the gut and give us a better chance of getting food down that stays down.
Since vomiting can quickly dehydrate a cat (and dehydration will make them feel worse and complicate their situation), do keep an eye on his water intake and hydration status. To check his hydration status to make sure he is not becoming dehydrated there are a few things we can test. To start, we want to make sure that his eyes are not appearing sunken, that his gums are not tacky instead of wet/moist, not should he have a "skin tent" when you lift the skin. To see how to check these parameters for dehydration, you can find a wee video on this HERE. (http://www.ehow.com/video_12232503_dog-dehydrated.html) They use a big dog but it makes it easier to see and the principle is just the same. If you check these and are already seeing signs of dehydration, then that would be a red flag that you'd want to have him seen by his vet before this becomes an additional issue for him (to avoid it getting to a stage where he has to be hospitalized for IV fluids).
Overall, Felix's signs sound more serious then a tapeworm here. Therefore, we need to tread with care. So, if he can at least keep some water down, you can start the above for him. Of course, if he cannot or you do not see improvement over the next 24-48 hours, or appears dehydrated already, then it'd be prudent to follow up with his vet for reassessment. Depending on their previous treatment, they can use antibiotics and anti-vomiting medications by injection to help settle his stomach and get him back on track as quick as possible.
I hope this information is helpful.
If you need any additional information, do not hesitate to ask!
All the best,
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