The illustration shows that both the upper incisors and upper molars overlap their lower jaw counterparts. This is due to a shortened lower jaw which is a deficiency from birth. The molars may in some cases of parrotmouth have matching grinding surfaces.
Where molars are not matched, molar wear is similar to that in horses with severe overbite. Teeth are not worn down in areas where they do not meet opposing teeth. Large points develop on the first upper molars, called 2nd premolars, which can make contact with the lower jaw. Similarly, the last lower molars, called 3rd molars, develop points which may touch the upper jaw if left untreated. The horse suffers considerable pain when trying to chew with teeth which are in contact with the gums and bone structure of opposing jaws. The treatment of reducing extended molars from early age will reduce the amount by which the incisors and molars overlap.
The length of the upper incisors appears to be extended but part of this appearance is due to the front of the upper jaw curving downward. The incisors of horses with a parrotmouth are often laterally misaligned and it is also common to find retained temporary incisors which have not been shed normally. Horses adapt their grazing to cope with the problem of tearing the feed off by grasping it between the lower molars and the palate behind the upper incisors. For this reason horses with this condition are not to be turned out in areas with short grass. If upper incisors are quite long a safe method of reducing them may be used as part of professional treatment in order to improve the ingestion of feed.