I saw a dog for a swelling of the lower jaw. A year before I saw him he had a similar episode of lower-jaw swelling and was treated for an allergic reaction. I examined his teeth and one of his lower canines was discoloured. A radiograph of the tooth revealed it was completely devitalized - dead! I took the tooth out and the dog's swelling will never recur (at least not due to that tooth).
This case is not to toot my horn, but rather to show that any odd swelling, abscess, draining tract (fistula), etc., on or near the dog's/cat's head must be accompanied by a thorough examination, including radiographs, of the patient's mouth and teeth.
This is a dog under general anesthesia getting his/her teeth scaled and polished. Note the mysterious gloved individual signing his/her name in the dog's mouth. This is usually avoided.

Below: the darker areas around the roots of the larger tooth are abscesses. This tooth needs to come out. The crown, or portion of the tooth above the gumline, may be visually normal and yet have roots that radiographically look like shit.
