Developmental Dental Disorders in Horses

One of the most common aspects people consider when selecting a horse to purchase is the animal's conformation. Are his hocks too straight? Is his back strong? And are his pasterns too long? But some horses that look flawless on the surface could be harboring a developmental dental disorder, some of which could be dangerous and performance-limiting if not treated promptly and properly.

Paddy Dixon, MVB, PhD, MRCVS, professor of equine surgery in the University of Edinburgh's Division of Veterinary Clinical Sciences, lectured on the different types of developmental dental disorders horses can face at the American Association of Equine Practitioners Focus on Dentistry meeting, held Sept. 18-20 in Albuquerque, N.M.

Dixon described some of the common problems horses encounter as their teeth develop, what problems these abnormalities cause, and treatment options.

Overjet and Overbite: "Many horses have some degree of overjet," Dixon explained. An overjet, also referred to as an overshot jaw, occurs when the chewing surfaces of the upper incisors (the front teeth) lie slightly in front of the chewing surfaces of the lower incisors. Most times, this slight misalignment does not cause ill effects to the horse.

"If very marked and left untreated in foals, cases of severe overjet may develop into overbite," he cautioned. This condition, commonly referred to as "parrot mouth," occurs when all of the upper incisors lie in front of the lower incisors, resulting in excess tooth growth on the top row of teeth; this excess tooth growth can sometimes cause the horse to develop a smilelike appearance when the central upper incisors overgrow .

"Overjet and overbite are aesthetically undesirable but surprisingly, these problems rarely cause difficulty in prehension (grasping food)," Dixon noted. If the horse develops these smilelike incisors, veterinarian should evaluate and reduce the excess tooth gradually, ensuring that the underlying pulp is never exposed or overheated during the process. Most horses affected with overjet and overbite also have their top row of cheek teeth slightly in front of the lower cheek teeth, and thus, they develop overgrowths on the front of their first upper cheek tooth and the back of their last lower cheek tooth, as discussed below.

Underjet: Conversely, an underjet, as the name suggests, occurs when the lower incisors protrude further than the upper incisors. Dixon noted this condition is rare in horses and is more commonly seen in donkeys. He added that like overjet, this condition has little clinical significance unless the bottom incisors have no contact with the top incisors. Consultation with a veterinarian to determine what--if any--treatment is needed for a horse with underjet.

Dixon explained that with both underjet and overjet, the main clinical concern is a misalignment of the horse's six cheek teeth (three molars and three premolars).

He continued his discussion of developmental dental abnormalities with cheek teeth disorders.

Rostrally-Positioned Upper Cheek Teeth: It's a mouthful to say, and rostrally positioned (positioned closer to the nose) upper cheek teeth can cause a mouth full of problems for horses. Dixon explained that an imbalance in growth rates between maxillary (cheek bone) and mandibular (lower jaw) bone cause this disorder, which is "nearly always associated with incisor overjet or overbite."

Several problems can arise from the upper cheek teeth being positioned further ahead than the lower ones. "Because the maxillary and mandibular cheek teeth rows are not in full occlusion (bite alignment), localized cheek teeth overgrowth--colloquially termed beaks, hooks, or ramps--develop on the front of the upper cheek teeth," Dixon explained. "These overgrowths may be pushed against the lips and cheeks by the bit or noseband and so cause mucosal ulceration and bitting problems."

Further, Dixon noted that if large, these overgrowths can restrict some cheek and jaw movement when an affected horse raises and lowers his head. "Feeding affected horses fully from the ground rather than from a height may increase normal (jaw) movement and reduce the development of such overgrowths," he suggested.

Dixon recommends these overgrowths be removed (when removal is warranted) in stages rather than all at once, as recent research has shown that removal of more than 4mm of tooth can damage the underlying pulp. With regular expert dental care, all of the above cases can lead fully productive, pain-free lives.

Wry Nose: Wry nose is a condition in which an affected foal's upper jaw and nose are deviated, or turned to one side, Dixon explained. The teeth associated with this condition always have malocclusion (poor alignment), although most foals can still nurse and in most cases are bright and active.

While minor cases have the potential to resolve on their own or cause no clinical problem, more severe cases will require major surgery, which is generally possible until the foal is about a year old, Dixon said.

Hypodontia: This condition--in which one or more of an affected horse's teeth do not develop--has the potential to affect both deciduous (baby) and permanent teeth, although "true" hypodontia most commonly affects permanent teeth, Dixon explained. And while a missing tooth might not seem like a major problem, Dixon explained that its absence can cause long-term damage to the horse's mouth.

"Developmental hypodontia is relatively uncommon in horses, with the absence of equine teeth usually due to traumatic loss, disease, or age-related wear," he explained. The main problem associated with missing teeth is overgrowth of the opposing tooth. In these cases overgrowth can be managed with semiannual dental care to prevent the overgrown teeth causing damage to surrounding structures

Supernumerary Teeth: Not unlike the absence of teeth, the presence of extra teeth can be problematic for affected horses.

"The presence of supernumerary (additional) teeth is relatively uncommon in horses, usually developing in the permanent dentition," said Dixon. He added that supernumerary teeth can be comprised of a single tooth or more than one tooth joined together.

"Equine supernumerary incisors may be more common--or possibly more readily identified--than supernumerary cheek teeth," Dixon noted. Extra incisors often cause overcrowding and displacement of the normal teeth. In some cases he suggested it is possible to remove the additional teeth using sedation and nerve blocks. "If (the supplemental teeth are) interwoven amongst the other incisors, differentiation of supernumerary and normal teeth is difficult and safe extraction may be impossible," he noted. In these cases regular veterinary dental care can help keep the horse's mouth as healthy as possible.

Supernumerary cheek teeth are relatively uncommon in horses and can cause overcrowding and irregular spacing between teeth leading to periodontal disease. Treatment depends on the location of the supernumerary teeth and what problems they cause, he noted. Following careful clinical examination and radiography (X rays), common treatment options include extracting the additional tooth, widening or filling abnormal interdental spaces (that develop painful food packing), and reducing overgrowth.

Dental Dysplasia: "Dysplasia, or abnormal development of teeth, can involve the crown, roots, or all parts of the tooth," Dixon said. "Dysplasias in the gross anatomy include dilacerations (abnormal bending of teeth), double teeth, abnormalities of size, and concrescence (roots of adjacent teeth joined by cementum) of teeth."

Additionally, he noted that teeth can develop either too large (macrodontia) or too small (microdontia). Amelogenesis imperfecta (a tooth development disorder in which teeth are covered with thin, abnormally formed enamel) is another rare dental disorder in horses, Dixon added.

Dental Eruption Abnormalities: "Some cases of 'stepmouth' and 'wave mouth' are caused by mismatched eruption times of opposing permanent cheek teeth, allowing overgrowth of the teeth which erupt first," Dixon explained.

Smaller and miniature breeds have this type of disorder more commonly than other breeds due to a likely mismatch between the size of their teeth and of the support bones (jaws), Dixon noted, and the steplike or wavelike overgrowths can remain--or even increase in size--throughout an affected horse's lifetime.

"Recognizing and removing such overgrowths at an early stage is the key to their management," he noted. Larger overgrowths must not be removed in a single treatment as this can expose or overheat the underlying pulp and cause great pain and even deep infection of the overgrown tooth.

Deciduous ("Milk") Tooth Retention: Dixon noted that both deciduous incisors and cheek teeth can be retained past their normal time of shedding. In some cases, he noted, this will cause the permanent tooth to emerge into a displaced position adjacent to the retained deciduous tooth. The newly erupted tooth is often not fully aligned with its opposite tooth and so develops an overgrowth that can even cause chewing restrictions.

In most cases, Dixon said, these retained deciduous teeth can be readily identified and removed. However, it is sometimes advisable to have a veterinarian take a radiograph prior to extraction to ensure the additional tooth is not of the supernumerary variety.

Cheek Teeth Impactions: Light breeds and miniature horses are also prone to developing cheek teeth vertical impactions (teeth that are too tightly squeezed by their neighbors and cannot freely erupt into the mouth) when permanent teeth are emerging. However, many larger breeds such as Thoroughbreds also develop this problem. They most commonly cause painless swelling on the bottom of the lower jaw (i.e., eruption cysts, or "3-year-old bumps" and "4-year-old bumps") that usually disappear after a year or so, because "as the jaw bones lengthen, the impacted cheek teeth have room to erupt normally," Dixon said.

Dental Displacements: If there is not enough room for the fourth cheek tooth to erupt normally at one year of age or for the fifth cheek tooth to erupt at two years of age, the (overcrowded) erupting tooth might be displaced and cause a potentially lifelong problem. The displaced tooth might then grow toward the cheeks or tongue, causing painful inflammation, but the most significant problem will be the presence of diastemata (space) between the normal teeth row and the displaced tooth. At these sites, food invariably becomes impacted and the horse can develop painful periodontal disease. If the displacement is very severe, a veterinarian might extract the displaced teeth--a procedure most readily performed in older horses, especially when deep periodontal disease is present.

Take-Home Message

Developmental dental abnormalities can cause a host of problems for affected horses. Routine dental examinations of foals and young horses can help identify issues early and allow treatment to begin before the disorders become too serious. Always consult a veterinarian if a horse develops a suspected dental problem.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.

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