In horses, the most commonly diagnosed oral congenital deformity is parrot mouth (class 2 malocclusion), in which the maxilla is relatively longer than the mandible. 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 and cattle, many anomalies of dental development may result from exposure to teratogenic toxins. However, underlying genetic factors should always be considered.
Dental irregularities accompany systemic fluorosis in both cattle and sheep. In the milder forms of fluorosis, only the dentition may be involved. In extreme fluorosis (eg, 40 ppm in the diet for several years), other skeletal abnormalities may be seen (phalanx fracture). (Also see Fluoride Poisoning Fluoride Poisoning .)
Supernumerary teeth (polyodontia) are seen occasionally. In both horses and cattle, double rows of incisor teeth or extra cheek teeth may be seen. Missing teeth (oligodontia) in a dental arcade is less common and can be developmental or secondary to trauma or previous tooth removal. Treatment is determined on a case-by-case basis and may require extraction or frequent crown reduction of the unopposed teeth.
Also see Congenital and Inherited Anomalies of the Teeth in Animals Congenital and Inherited Anomalies of the Teeth in Animals This photograph of a dog illustrates the most common form of hyperdontia in dogs: supernumerary incisors located on the maxilla. There are 4 incisors per arcade instead of the normal 3. This... read more 
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