Parrot mouth is a common equine dental condition typically identified at birth or shortly after. It is characterized by a pronounced overbite where the horse’s upper front teeth protrude beyond the lower row of teeth, causing the upper lip to overlap the lower one.

Dental misalignment in cases of parrot mouth is due to anatomical abnormalities in one or both jaws of the affected horse’s skull. The underlying causes of this condition are currently not fully understood.

While some degree of dental misalignment is normal and often harmless, severe cases of parrot mouth can impede a horse’s ability to bite and chew, leading to issues such as weight loss, injury to the roof of the mouth, and digestive problems.

Understanding the anatomy of a horse’s mouth and recognizing the signs and severity of parrot mouth can help horse owners ensure their horse receives timely diagnosis and treatment. Early intervention, including dental modification and surgical procedures, can significantly improve the quality of life for affected horses.

Parrot Mouth in Horses

Parrot mouth is a condition in horses where the upper front teeth protrude out further than lower front teeth. [1][2][3][4] The upper lip overlaps the lower one, which can make the profile of the horse resemble a parrot’s beak. [5][6][7]

This condition is also known as: [3][5][6][7][9][10][11]

  • Rostral malocclusion
  • Brachygnathism
  • Brachygnathia inferior
  • Prognathia superior
  • Retrognathism
  • Overshot jaw

As with all skeletal structures, there is variance in presentation of jaw alignment between individual horses. In horses with ideal alignment, the upper front teeth and the lower front teeth meet evenly when the jaws are closed and the head is in a neutral position. [6]

Parrot mouth is the result of abnormalities in jaw bone morphology. Specifically, parrot mouth is due to one or a combination of the following morphological presentations: [1][5][8][9]

  • Upper jaw bone is abnormally long
  • Lower jaw bone is abnormally short

Many horses have some degree of misalignment of their front teeth without significant impact on quality of life. Temporary changes in dental alignment related to head position are also normal. [6]

Parrot mouth requires veterinary attention if misalignment of the jaws leads to difficulty eating or tissue damage in or around the mouth due to overgrown teeth. [3]

Overbite vs. Overjet

Parrot mouth is typically used as a general term that encompasses two abnormalities in the position of the horse’s upper and lower jaws: overbite and overjet. [12]

  • Overbite: In horses with overbite (also known as an overshot jaw), there is vertical misalignment of the front teeth. The upper front teeth overlap and sit in front of the lower front teeth when the mouth is closed. [1]
  • Overjet: In horses with overjet, there is horizontal misalignment of the front teeth. The upper teeth protrude further forward than the lower front teeth when the mouth is closed, leaving a space between the upper and lower front teeth. [1] Some horses with an overjet develop an overbite as well due to wear on specific teeth and overgrowth of others. [3][5]

While parrot beak is often used to describe both overbite and overjet, some sources reserve the term only for overjet and specify overbite as a different condition. [5]

Equine Mouth Anatomy

The horse’s mouth anatomy has evolved for efficient grazing and breaking down fibrous plant material.

To support this feeding behavior, the teeth perform two primary functions: biting, which involves cutting through material with the front teeth, and chewing, which involves grinding the material into smaller particles using the back teeth. [10]

Changes to the structures of the horse’s teeth and jaws resulting from parrot mouth can impact these functions. Horse owners benefit from understanding the specifics of equine tooth anatomy and function to better recognize when misalignment warrants further investigation.

Equine Dentition

Adult horses have hypsodont teeth. These teeth are common in mammals that wear down the enamel by eating abrasive material, such as coarse forage. [10]

In hypsodonts, the tooth enamel extends well past the gumline and continually erupts throughout life as the grinding surface wears away. Due to this continual tooth eruption, domestic horses require routine dental floating to keep their teeth balanced so they can adequately chew forage. [10]

Proper chewing is an essential first step in equine digestion, helping to ensure feed particles are small enough for the digestive tract to handle.

Incisors

Incisors are flat, spade-like teeth at the front of the mouth that are used for biting. [10] In cases of normal alignment, the upper and lower incisors meet evenly to allow effective biting.

In a horse with parrot mouth, the upper incisors are further forward than the lower incisors. This means the horse cannot bite as effectively. In severe cases, the ability to bite is significantly impaired. [9]

If the lower incisors get trapped on the inside edge of the upper teeth, growth of the teeth and movement of the jaw can become limited. [1][3] This can impact the horse’s ability to chew. [3]

In some cases, the line of the upper incisors becomes curved due to uneven wear. In these cases, the row of teeth are described as having a “smile” or a “frown”. [3]

  • Smile: describes cases where the curve is lower in the middle of the incisors
  • Frown: describes cases where the curve is higher in the middle of the incisors

Molars and Premolars

Molars and premolars are the flat, stump-shaped teeth that run alongside the horse’s cheeks. Also referred to as cheek teeth, the molars and premolars are are used for chewing. [10]

In horses with ideal dental alignment, the cheek teeth grind against their opposite on the other jaw. Since horse’s teeth continue to grow over the course of the lifetime, the grinding action helps keep the teeth at the correct height so they can still chew effectively. [1][3]

If the horse’s teeth do not line up properly, the unopposed teeth will keep growing, leading to an uneven bite plate, which can interfere with the ability to chew. [1][3]

In horses with parrot mouth, the upper molars and premolars are positioned further forward relative to the lower ones. This causes uneven growth in the molars and premolars, which can progress to protruding teeth in the back of the mouth. These protrusions are called hooks and they can cut the horse’s mouth and make chewing less effective. [1]

Uneven wear on the molars can also grind down one side of a tooth until the overall shape is sloped. This is referred to as a ramp, and can also impede effective chewing. [1]

Additionally, the cheek teeth are composed of several different materials that wear away at different rates. This ensures that the cheek teeth have ridges to help with the grinding of food. [10]

Parrot mouth is associated with exaggerated ridges on the grinding surfaces of the cheek teeth that can have a negative effect on the smooth movement of the teeth against each other. [1]

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Causes

Parrot mouth is a common condition in horses that is either present at birth or develops within the first few months of life. [4][13]

The exact cause of parrot mouth has yet to be confirmed. It may be genetic, acquired, or developmental. [6]

Since parrot mouth is linked directly to the horse’s jaw and skull morphology, it is likely there is a genetic component. The heritability of the condition is still under investigation, but horses with misaligned jaws are more likely to pass on some degree of misalignment to their offspring. [1][13]

Injury to the joint between the lower jaw and the temporal bone is another possible cause of parrot mouth. [13] Imbalanced or inadequate nutrition, especially early in life when bone development is rapid and profuse, is another possible cause. [13]

Severity

A slight difference in length between the upper and lower jaws is normal in horses. However, if the difference is significant enough that the incisors do not come together to bite, or that the grinding surfaces of the molars cannot properly grind against teeth on the opposite jaw, it is considered a treatable condition. [1][6][14]

Factors that influence the severity of parrot mount include: [1][2][3][13]

  • The degree of misalignment between the upper and lower incisors
  • The impact misalignment has on the horse’s ability to bite and eat
  • Whether the horse develops dysmastication (difficulty chewing)
  • Whether secondary dental or digestive difficulties develop
  • Whether the horse develops issues with dropping feed (quidding)

Complications

Secondary complications of parrot mouth may include: [1][2][3]

  • Uneven tooth wear
  • Damage to the soft tissues of the mouth
  • Digestive problems due to difficulty eating or chewing

Chronic dental issues such as the development of hooks and ramps on the upper cheek teeth are also associated with parrot mouth. [5][7][11]

The pulp in unopposed teeth is often close to the tooth’s surface, which means horses with pronounced parrot mouth often require extra care during dentistry. [5]

Symptoms

The characteristic symptom of parrot mouth is protrusion of the upper jaw past the lower jaw. [13]

Foals with parrot mouth typically do not have trouble nursing, but may have difficulty biting and chewing solid food once they have weaned. [13]
Other symptoms include: [4]

  • Injury to the hard palate
  • Overgrown teeth
  • Weight loss
  • Difficulty with the bridle or bit

Diagnosis and Treatment

Diagnosis of this condition is self-evident based on observation of the upper jaw protruding further forward than the lower jaw. [3][15] Some horses with parrot mouth are born with other abnormalities in their muscles or bones that may require further veterinary investigation. [13]

There is some debate about how much intervention parrot mouth requires. Some veterinarians assert mild cases of parrot mouth can resolve on their own. [5] Others state that treatment is necessary to ensure the horse’s ability to eat is not compromised in the future. [4][13]

Treatment for parrot mouth aims to reshape the teeth or slow the growth of the upper jaw. Treatments include: [1][2][3][4][6][8][9][11][13][14][16]

  • Cephalometry: measurement and analysis of the bones of the skull and face
  • Dental floating: modification of surfaces of the teeth. In cases of parrot mouth dental floatation is performed in stages to avoid injuring the pulp, which is presumed to be close to the tooth surface
  • Orthopedic surgery: to change the shape of the jaws
  • Orthodontia: may include applying tension band wire (braces) to change the position of the teeth or applying a bite plane to change the motion of the jaws when chewing
  • Buccotomy: surgical incision into the cheek to allow space for protruding teeth

Treating parrot mouth earlier in life provides the best chance of a successful, fully corrective outcome for the growing horse. [7]

Prognosis

The prognosis for a horse with parrot mouth depends on several factors, including: [5]

  • Severity of the condition
  • Timing of intervention
  • Age of the horse at time of intervention
  • Secondary complications

With proper treatment, especially in mild cases and where the horse’s ability to eat properly is restored, the prognosis for a horse with parrot mouth is good. [5]

Prevention

The causes of parrot mouth are not fully understood. Therefore preventative measures have yet to be determined.

Foals should have a full examination of their teeth at birth in order to diagnose and treat parrot mouth while the horse is still growing. [12]

Since there is some evidence that parrot mouth is an inheritable condition, neutering or avoiding breeding of affected horses is recommended. [3][5] Some breed associations disallow surgical correction of parrot mouth for breeding stock and exclude affected horses. [3][4]

Summary

Horses with parrot mouth have upper front teeth that protrude further out than their lower front teeth. This is sometimes a cosmetic concern, but in severe cases can impair the horse’s ability to bite and chew properly leading to dental and digestive issues.

  • Parrot mouth in horses can include both overbite (a vertical misalignment of the front teeth) and overjet (a horizontal misalignment of the front teeth)
  • Impairment of eating comes from misalignment of the front teeth, which impedes biting, or from misalignment of the back teeth, which impedes chewing
  • The severity of parrot mouth depends on the degree of misalignment, its impact on the horse’s ability to bite and chew, and whether secondary complications develop
  • Diagnosis is based on the characteristic shape of the horse’s jaws
  • Treatment aims to reshape the teeth or slow the growth of the upper jaw and includes tooth reshaping, surgery, braces, and bite planes
  • The prognosis for horses with parrot mouth is usually good if treatment is sought early

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References

  1. Griffin, C. The Gold Standard of Dental Care: the juvenile horse. Veterinary Clinics of North America: Equine Practice. 2013.View Summary
  2. Auer, J. A. Craniomaxillofacial Disorders. Equine Surgery. Elsevier. 2012.
  3. Dixon, P. M., & Dacre, I. A Review of Equine Dental Disorders. The Veterinary Journal. 2005. View Summary
  4. Domanska-Kruppa, N. et al. Cephalometric Study of the Overjet Development in Warmblood Foals. Frontiers in Veterinary Science. 2019.
  5. Wilson, G. Common Dental Abnormalities. 30th Bain Fallon Memorial Lectures. Equine Veterinarians Australia, Cairns. 2008.
  6. Johnson, T. J., & Porter, C. M. Common Disorders of Incisor Teeth and Treatment. AAEP. 2006
  7. Easley, J. et al. Equine Orthodontics.AAEP. 2006.
  8. Dixon, P. M. Disorders of Development and Eruption of the Teeth and Developmental Craniofacial Abnormalities. Equine Dentistry. Elsevier. 2011.
  9. Easley, J. Basic Equine Orthodontics. Equine Dentistry. Elsevier. 2005.
  10. Peffers, A., Understanding the Horse’s Teeth and Mouth. The Crowood Press. 2016.
  11. Easley, J. et al. Orthodontic Correction of Overjet/Overbite (‘Parrot Mouth’) in 73 Foals (1999–2013). Equine Veterinary Journal. 2016.
  12. Dixon, P. M. The Evolution of Horses and the Evolution of Equine Dentistry.
  13. Gift, L. J. et al. Brachygnathia in Horses – 20 Cases (1979-1989). JAVMA. 1992. View Summary
  14. Klugh, D. O. Acrylic Bite Plane for Treatment of Malocclusion in a Young Horse. Journal of Veterinary Dentistry. 2004.
  15. Kassem, M. et al. Diagnosis and Surgical Management of Prevalent Dental Affections in Horses of Equestrian Clubs. Alexandria Journal of Veterinary Sciences. 2018.
  16. Verwilghen, D. et al. Mandibular Osteodistraction for Correction of Deep Bite Class II Malocclusion in a Horse. Veterinary Surgery. 2008. View Summary