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Hello. Thanks for the question.
I don't think this sounds like FIP. It sounds more like a primary upper respiratory issue. A viral ulcer in the right eye would lead to the discharge, small pupil and squinting. With neurologic disease causing anisocoria (different sized pupils), they are not usually showing any pain in the eyes and they don't have any discharge.
I would stain the eyes to look for ulcers, and then probably start an antibiotic drop and some oral anti-inflammatory meds to help bring the fever down- usually Onsior. The steroid and antibiotic injections should also help get things moving in the right direction. It's just a bit different than how I would typically treat. If the fever resolves and the ocular signs clear up, then we can safely rule out FIP. FIP will usually continue to get worse, despite our futile attempts at treating. It's a terrible disease. I think it's on the differential list, but pretty far down as compared to other upper respiratory or ocular conditions.
I hope this helps, let me know if you have any other questions.