Instructions
Check for sneezing. Rats naturally sneeze occasionally, but continuous sneezing is a symptom of some kind of upper respiratory infection.
Observe the rat's breathing. If you notice obvious in-and-out motions of the rib cage, this probably means it is having some difficulty breathing.
Listen to your rat's breathing. Put the rat's nose close to your ear and listen. If you hear raspy or wheezy sounds, that's a pretty good indication that your rat has pneumonia or a similar upper respiratory infection.
Observe the rat's eyes. Check for clear or yellowish discharge coming out of the corners. Also check for orange-colored staining around the eyes. (This orange stuff is called "porphyrin" and it appears whenever the rat is under stress, either from disease or from some circumstance. It's not blood; it's just a secretion from glands under the skin. You can clean it off gently with a moist cotton swab.)
Notice if your rat has been eating less or sleeping more. These can be signs that your rat is not feeling well and has an infection.
Take your rat to the vet promptly. If your vet confirms the diagnosis of pneumonia (or a similar upper respiratory infection) she will prescribe an oral antibiotic medication. Even if the vet thinks the infection could be viral, she will still probably prescribe an antibiotic medication in order to prevent a bacterial infection from following the viral infection (which happens very frequently).
How to Know if Your Rat has Pneumonia
Rats are very susceptible to upper respiratory infections. Even if a rat looks healthy when you purchase it, it can still be infected and may come down with pneumonia up to two weeks after you bring it home. Older rats are also more susceptible to upper respiratory diseases and should be watched for symptoms. Knowing the signs and symptoms of pneumonia is part of being a responsible rat owner.