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In hopes of information being comforting...the presence of any number of the cells in the spinal tap is considered abnormal. FIP is always a ruleout because it is such a hard infectious disease to put a finger on...even diagnosis is VERY hard.
You are doing everything right. Seizures in cats can be harder to get a finger on than in dogs. A spinal tap is indicated, and although you await results and prognosis, the only true comfort I can give is that you have done everything right.
It can be hard to treat infections in the nervous system, it is not impossible. There is always the chance that he won't have any progressive problems.
I probably can offer more help to you once they give you results. Please let me know.
Better yet, I will keep him in my prayers. "Children" bring some of the most joys in life...even with the stress.
They should have the results soon, but since you are not seeing a deterioration in his current health, maybe the news is good!