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Tick bites can give your pet such infections as Lyme disease, ehrlichiosis, and Rocky Mountain spotted fever. You need to go back to this vet and insist that more thorough testing is done, to see how this kitten was infected by the tick. I am assuming that this was tick-related, and that the kitten is not suffering from an upper respiratory infection.
Something that can alleviate the "labored breathing" if the nostrils are plugged with mucous, is a product called "Lil Noses Pediatric Drops". Do look into the three illnesses that are mentioned above.