This is an advanced form of an infected tooth. This condition is usually caused by a fractured tooth(usually the upper fourth premolar) that has been infected by the oral bacteria and died. The bacteria will gain access to the jaw through the apex (or bottom) of the tooth. The combination of the bacteria and the reaction of the white blood cells will cause bone destruction at the area of root tip. If this is allowed to progress, the infection can travel through the bone of the upper jaw and break out either on the gingiva over the tooth, or on the skin under the eye. This is the only time that endodontic disease is usually noticed by the owner, as most dogs and cats do not show any outward signs of disease.

Treating the endodontic disease along with appropriate antibiotics will usually cure the condition, however if there is significant exudate, draining the abscess may be indicated as well. The endodontic disease can either be treated by root canal therapy or by extraction. Unfortunately, the dental part of this condition is often missed or ignored. The patient will be placed on antibiotics +/- drainage of the abscess. This will result in resolving of the external signs, but almost invariably return. In addition, the infection will still be active, and the patient suffering, even if no external signs are present. The worse sequela to this form of therapy is that the bacteria may be resistant to the antibiotics the second time around. The reason that the infection will return is that the tooth protects the bacteria within it. The immune system and antibiotics cannot get into the tooth (I call it a fortress), and so when the antibiotics are gone, the bacteria will leave the tooth again and start the infection all over.