Merck Manual

Please confirm that you are a health care professional

honeypot link
Professional Version

Congenital and Developmental Anomalies of the Mouth and Dentition in Horses


Jack Easley

, DVM, MS, DAVDC (Equine), DABVP (EQ), Easley Equine Dentistry, Shelbyville, KY

Reviewed/Revised Feb 2024

In horses, the most commonly diagnosed oral congenital deformity is "parrot mouth", in which the maxilla is relatively longer than the mandible (brachygnathia). The opposite abnormality, prognathia, is common in Miniature Horses. Depending on severity, these conditions can cause abnormalities in contact and wear of the incisors and the cheek teeth, requiring more frequent floating than in horses with normal mouth conformation.

A unique congenital abnormality of the horse is an "ear tooth". In early embryonic development, the first branchial arch extends as a solid hyaline cartilaginous rod surrounded by a fibrocellular capsule from the temporal region to the midline of the fused mandibular processes. If the first branchial arch fails to close properly in the equine fetus, tooth germ can displace and result in the formation of a dentigerous cyst in the temporal region and a draining tract from the rostral pinna of the ear.

In equids, many anomalies of dental development may result from exposure to teratogenic toxins. However, underlying genetic factors should always be considered.

The presence of supernumerary teeth (polyodontia) occurs occasionally. In horses, double rows of incisor teeth or extra cheek teeth may occur. The absence of some teeth (oligodontia) in a dental arcade is less common and can be developmental or secondary to trauma or previous tooth removal. Treatment is determined case by case and may require extraction or frequent crown reduction of the unopposed teeth.

quiz link

Test your knowledge

Take a Quiz!