the symptoms you are describing can be from many diseases:
Feline infectious peritonitis
FSE has been documented in a few domestic cats in Switzerland, Liechtenstein, and Norway and in several big cats (puma, tiger and lion) in zoos in various European countries. It is thought that the disease was contracted by eating food containing animal brain tissue or bone marrow that was raw or insufficiently heated.
Clinical signs consist of behavioral changes (aggression, biting, scratching when stroked, hiding, vacant staring, excessive grooming, easily startled), cerebellar signs (ataxia, hypermetria, intention tremors of head), muscle tremors, pelvic limb ataxia or weakness).
A study was conducted to check for possible FSE in Europe on about a 100 cats with fatal neurological symptoms and the follwoing were found: neoplasia (21.8%), toxic-metabolic encephalopathy (18.2%), granulomatous encephalitis (15.5%), suppurative encephalitis (4.6%), trauma (3.6%), circulatory disorders (3.6%), degeneration (2.7%), nonsuppurative encephalitis (2.7%), and neuromuscular diseases (1.8%).
In your case, unfortunately, unless you are in Europe right now, other diagnostic tests would need to be perform to know for sure what is the problem. If all basic blood work, urine analysis, blood pressure, sinus radiographs, FeLV and FIV negative, an MRI and a SCF tap would be indicated. I know you mention a tight budget, if all else fails, you can discus with your primary care veterinarian to try some steroids, or treat for toxoplasmosis. Due to the progressive nature of the problem, a brain tumor is very high on my list. this will not be able to be cured unless you have surgery, or could be helped with radiotherapy. FIP cases all die, even if you try to treat.
it is sad to say, but you may have to think about putting him down. I am sorry.
Let me know if you have more questions, or need additional information.
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