There is a feral that lives near my house that I have been feeding for the past 5 years or so. It is ear tipped and from the description I think it once belonged to a colony several blocks away. A bit more than a week ago it started sneezing, then several days ago stopped eating the dry food and I switched to feeding it 2 regular/1 kitten can of food. It is drooling, not grooming well and its mouth looks wrong. Still eating well and doing its normal routine. Third eyelid was out a bit for a couple days on one eye but is now better. It’s a chubby domestic shorthair, doesn’t seem too old.
I am willing to trap it and find a vet to treat it (it is not at all friendly but more scared, not aggressive) but googling sounds like it is likely calcivirus and should resolve shortly on its own? Is that likely /is there anything that should be done? It is clearly not feeling well but it’d be a lot of stress for it if it likely won’t help.