Hello, I am Dr. B, a licensed veterinarian and I would like to help you with your wee one today.
First, I do want to say that if your wee one is vomiting every other day, it is likely her esophagus is getting sore. When cats vomit, it is quite a violent event anyway but if she keep putting that kind of force on her throat and the delicate esophageal tissues are being frequently washed in stomach acid every time she vomits, this will lead to an even more painful situation. And because of this, we do need to address the underlying trigger for her vomiting and perhaps even consider some GI and esophageal supports to soothe her throat (which I will come back to in a moment) and keep her from getting a state where she is so sore she won't eat for us.
Now this isn't an uncommon scenario in cats around her age. We can see older cats start struggle to keep weight on despite a good to great appetite for a range of reasons. Issues that can manifest this way at her age will include conditions like hyperthyroidism, diabetes, liver disease, and kidney troubles. As well, while it isn't nice to think about, we must keep in mind that cancer in cats her age can manifest as weight loss with little other signs (just as it doesn't impinge on the body, only to steal nutrients from her).
In cats with hyperthyroidism, diabetes, cancer and early stage kidney disease we often see them maintain good appetites or even be very hungry (even voracious over food). These conditions can also cause the cats to have increased thirst and/or increased urination. As well, it is not uncommon to see vomiting as a secondary issue with these conditions (either from toxin build up with organ dysfunction, part of the disease process itself like hyperthyroidism or as a secondary problem gaining a foothold since their immune systems are already stretched so thin with the primary disease).
To get to the bottom of which is behind her signs, it would be worth having her checked by her vet (if she is due for a vaccination soon, you could move it up a wee bit early and have her checked out at that time). The vet will be able to have a feel of her and just make sure there are no sinister lumps and bumps to blame for her weight loss. And if you were able to bring in a urine sample at that point, the vet could check it for signs of diabetes (ie. sugar in the urine) as well as check its specific gravity (how concentrated it is) that can tell us if there are problems with her kidneys. Urine samples can often be collected by placing the cat in a carpet-less room with an empty (or with non-absorbable litter) litter box overnight.
If hyperthyroidism is a suspicion after your vet has examined her, then a blood sample may be checked to assess her thyroid hormone to determine how severely affected she may be. As well, if you do have a blood sample checked, you can also have her blood sugar, liver and kidney parameters checked at the same time, giving you a good chance of ruling out the above differentials and determining the cause of your wee one’s abnormal weight loss.
Finally, just to briefly touch on what supportive care can be used too at least limit the damage chronic vomiting will be doing to her throat, I wanted to note that sometimes these cats respond well to antacid therapy (which lowers the GI stomach acid volume to decrease the damage vomiting will do). There are a number of antacids that are available over the counter and pet friendly. I would advise only treating with one, but the two I tend to recommend are Pepcid (LINK) or Zantac (LINK). 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. As well, since she is sounding painful when she vomits, you may want to discuss with her vet about using a gastro-protectant (ie Sucralfate) to coat the inside of her throat. And, of course, once you identify and address the agent behind her signs, we'd hope the vomiting would settle (and therefore that would be another positive for this poor wee sore throat of hers).
Overall, in her case it would be prudent to have her examined and at least have a urine sample (or blood sample) checked. Because if you are able to the diagnose which of the above are to blame sooner rather then later, it will give you the best chance to address it, stabilize her to stop these signs, and hopefully get some weight back on her before she just fades away.
I hope this information is helpful.
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