Gastrointestinal system

Part 1: The mouth

Developmental disorders

Shear mouth (see also p. 10) arises when there is a discrepancy in the width of the upper and lower jaws. This disorder may be encountered in young horses as a result of developmental differences between the jaws.

Missing and malerupted teeth (Figs. 1.10–1.18)

Defects of the teeth relating either to genesis or eruption are relatively common.

Figure 1.10 Oligodontia. Both central and left lateral incisor missing.

Figure 1.11 Oligodontia (molar). Lateral radiograph of the head of a 6-year-old Arabian gelding with a missing fourth cheek tooth. No dental surgery had ever been performed.

Figure 1.12 Normal lateral (30°) oblique radiograph (of the maxillary arcade). Roots of the maxillary teeth and their relationship to the sinuses.

Figure 1.13 Maleruption (lingual) (permanent corner incisor). Permanent tooth erupted inside the temporary tooth resulting in persistence of the temporary corner incisor.

Figure 1.14 Persistent temporary incisors in a 3-year-old pony.

Figure 1.15 Premolar cap attached to erupting first cheek tooth in a 3-year-old Thoroughbred. The cap was easily dislodged.

Figure 1.16 Oligodontia and persistent temporary dentition. This horse is missing right and left lateral incisors of the upper arcade and has persistent temporary central incisors on the lower arcade.

Figure 1.17 Lateral radiograph of the maxillary arcade of a 3-year-old hunter gelding, showing impaction of the third and fourth cheek teeth leading to maleruption.

Figure 1.18 Extra incisor which was an incidental finding in this horse.

Oligodontia

Oligodontia (absence of teeth) is a developmental disorder when one or more teeth are absent. The incisors are the most commonly affected but the molars may also be affected. In the latter cases the most common missing tooth is the first molar (fourth cheek tooth) which should erupt at around 1 year of age. In the case of the incisors, true oligodontia is reflected in an absence of both temporary and permanent teeth, this being consistent with a complete absence of the dental germ cells. It may be associated with other epidermal defects such as maldevelopment of hair or hooves.

Maleruptions

Although the absence of teeth may be a true developmental defect, most frequently missing and malerupted teeth are due to previous traumatic damage to the dental germ buds or to systemic infections involving these during their maturation. The consequent maleruption of teeth may cause a significant deformity of the incisor dental arcades in particular. Maleruption of the molar teeth may, on occasion, only become apparent in later life, and most often affects the eruption of the fourth upper permanent premolar (third, upper cheek tooth), which is the last cheek tooth to erupt and may be identified at ages of 4 years and over.

Abnormalities of dental eruption may present with maxillary or nasal swelling associated with abnormal positioning or abnormal eruption of the permanent premolar and molar teeth. The third upper cheek tooth, being the last to erupt, is most likely to suffer from defects of eruption with gross deformity of the face over the site. Inability to erupt effectively results in considerable growth pressure within the associated rostral maxillary sinus. A discharging tract or enlarged sinus with a nasal discharge in a young horse should alert the clinician to the possibility of maleruption.

Persistent temporary dentition

Persistent temporary dentition may be accompanied by obvious or, occasionally, mild dental deviations of the permanent tooth, and most often affects the incisor arcade. Retained temporary incisors are usually firmly embedded in the gums and the permanent tooth usually erupts behind it rather than under it and so fails to occlude the blood supply to the temporary tooth or to push it out. They are, however, ordinarily loose and easily removed. Total retention of the temporary molars is much less common. In some cases, the temporary tooth creates a dental cap on the permanent tooth which may be so persistent as to make their identification difficult. However, as the first cheek teeth are most commonly affected, it is usually possible to see the cap on the erupted permanent tooth. Persistent, temporary premolar caps overlying the erupting permanent teeth may occasionally cause oral discomfort and masticatory problems. In the event that these caps rotate there may be associated cheek swelling and in this case the displaced temporary cap will be easily visible. Under such circumstances more significant abnormalities of mastication may be present with quidding (spitting out of partially chewed food material) and reluctance to eat. Most of these caps will resolve spontaneously but some will require removal.

Supernumerary teeth

Supernumerary incisors and molars (polyodontia) are not uncommon and develop as a result of multiple dental stalks from a single germ bud of a permanent tooth. In some cases there may be a complete double row of incisors but more often one or two extra teeth will be present. This polyodontia (dental duplication) may be restricted to one or more teeth, and affects incisors more often than premolar or molar teeth. Due to lack of wear by an opposing tooth, the extra tooth usually becomes elongated and may ultimately cause soft tissue injury to the opposing palate or tongue. Supernumerary molar teeth occasionally occur. Their position in the dental arcade is irregular; but they are frequently found caudal to the third molar tooth (sixth cheek tooth) in jaws which are longer than normal. Less often they are located either lingual or buccal to a normal molar and may in the latter case show obvious facial swelling. They often have a draining sinus onto the side of the face or into the maxillary sinus.

Diagnosis of missing/malerupted teeth

• Oral examination may be diagnostic for the majority of cases of missing or malerupted teeth.

• Radiographs may be required to differentiate supernumary teeth from persistent temporary dentition. A retained deciduous incisor has a more mature root and a shorter reserve crown than those of the adjacent permanent incisors.

• Radiographic examination and comparison with the normal arcade anatomy in cases of maleruptions show a variety of deformities ranging from complete absence of the tooth to an obvious tooth growing in an abnormal direction or position. Sometimes however, definite dental structure cannot be identified in a mass of abnormal tissue.

Treatment

• Regular dental examinations and care to prevent the development of unopposed dental elongations.

• In cases in which the supernumary or malerupted teeth have resulted in secondary complications such as sinusitis, the tooth may need to be removed and the complications treated.

Wolf teeth (Fig. 1.19)

The ‘wolf teeth’ are the vestigial first upper permanent premolar and, while many horses have these, some do not. In some cases their presence is blamed for a number of behavioral problems including head shaking, failure to respond to the bit and bit resentment. A wolf tooth is located just rostral to the first upper cheek tooth and may be in close apposition to this or may be somewhat removed from it. It is believed that the latter state is the more significant with respect to abnormalities.

They should not be confused with the normal canine teeth which occur in many (but not all) male horses and are located in the interdental space of both upper and lower jaw. Incomplete removal of the wolf teeth may result in persistent pain and fragments of enamel or root may be present. More usually, provided that the remains of the tooth are not exposed, there is little pathological effect.

Dentigerous cysts (Figs. 1.20 & 1.21)

Dentigerous cysts, also referred to as ear teeth, aural fistulae or heterotropic polyodontia, are congenital defects characterized by an epithelium-lined cavity containing embryonic teeth. They are most commonly located adjacent to the temporal bone, but can be found in a variety of other areas of the head. Cysts contain a seromucoid fluid and often fistulate. Dentigerous cysts may be recognized at any age, but are most commonly identified in horses less than 3 years of age.

Figure 1.21 Dentigerous cyst. A discrete mass is evident at the base of the ear with a discharging tract.

The cyst-like structures may contain no obvious dental tissue or remnants and may then be described as a conchal cyst. These may be radiographically unconvincing, but consist of a cystic structure with a smooth lining and an associated chronic discharging sinus. Dental remnants may be identifiable in other sites including the maxillary sinuses.

Diagnosis and treatment

• Radiographs can confirm the presence of an aberrant tooth-like structure, which is either firmly attached to the cranium or loosely enclosed in a cystic structure and may in either case be surrounded by a collar of bone forming an apparent alveolus. A contrast fistulogram can be used to delineate the mass and any draining tracts.

• Surgical excision is required but may be difficult in some cases where the ectopic teeth are firmly attached. Prognosis following removal is good.

Non-infectious disorders

Dental tartar (Figs. 1.22 & 1.23)

Dental tartar commonly accumulates on any, or all, of the teeth and is most obvious on the lower canine. It is unusual for this to cause any significant gingival inflammation and/or alveolar infection. Extensive accumulations of tartar on the incisors and canine teeth may be an indication of underlying systemic disease (e.g. renal disease or equine motor neuron disease) but is commonly an incidental observation in healthy (particularly, old) horses.

Figure 1.22 Dental tartar on a canine tooth.Differential diagnosis:

• Incidental finding

• Chronic renal disease

• Equine motor neuron disease.

Figure 1.23 Epulis. Incidental finding in otherwise normal horse.

Chronic gingival inflammation caused by dental calculus or other irritants may give rise to a benign inflammatory hyperplasia (epulis) of the gum. Again, the most obvious site for this is the buccal margin of the canine teeth but it may equally develop at any other site along the tooth–gum margin. It seldom reaches significant size although localized cheek swelling may be detected in severe cases. The subsequent development of neoplastic tissue, usually fibroma or fibrosarcoma but occasionally squamous cell (or undifferentiated) carcinoma, at these sites suggests that the inflammatory reaction may have longer-term significance.

Diagnosis and treatment

• Oral examination. Differentiation from other causes of dental tartar (EMND, see p. 427) or gum swelling.

• No treatment is usually required.

Ossifying alveolar periostitis (Fig. 1.24)

Swellings of the horizontal ramus of the mandible (chronic ossifying alveolar periostitis, pseudo-odontoma) are often encountered in young horses around the time of the eruption of the associated cheek teeth, and are probably due to alveolar periostitis around fluid-filled, active, dental sacs. In a few cases they may be associated with dental overcrowding and horizontally aligned unerupted teeth.

Diagnosis and treatment

• Although the apparent deformity of the mandibular bone may be obvious, radiographs of the area will demonstrate the presence, in young horses, of a normal dental sac, without any evidence of periapical inflammation or infection.

• Usually the extent of the defect improves somewhat with age, but persistence of some thickening and deformity are to be expected. Such swellings are totally benign and, in spite of apparently severe cosmetic changes, they are of little or no clinical significance and are in any case untreatable.

Lampas/inflammation of the hard palate (Fig. 1.25)

Lampas or inflammation/edema of the mucosa of the hard palate was historically regarded as a recognizable clinical disorder when swelling was sufficient to result in the mucosa of the hard palate being below the level of the incisor occlusal margin, warranting treatment. However, edema of the hard palate commonly occurs in young horses and is of no clinical significance. The disorder is occasionally reported to occur in older horses when irritant substances/foods are ingested.

Figure 1.25 Edema of the hard palate (lampas) found incidentally in an otherwise healthy horse.

Diagnosis and treatment

• Diagnosis through oral examination.

• The main treatment is to ensure that diet is acceptable and causes no irritation.

Fractures of the maxilla and mandible (Figs. 1.26–1.30)

Fractures of the premaxilla or mandible are common, especially in young horses. The fractures may involve the whole premaxilla, but quite frequently only one or more of the incisor teeth are distracted and broken back from the alveolus. In the former cases the effect on dental eruption is likely to be minimal but the consequences of the fracture, if left untreated, are likely to be serious with little or no incisor occlusion possible after healing. The latter cases have more serious effects on eruption and less long-term serious effects on occlusion, although an individual tooth may be severely displaced or even fall out.

Figure 1.26 (A) Fracture of the premaxilla with obvious displacement of the incisors. (B) Placement of a cerclage wire for repair.

Figure 1.27 Fracture of the mandible in a young foal again with obvious displacement of the erupted incisor.

Figure 1.28 Radiograph of the head showing the displacement of the incisors that could be seen grossly (same foal as Fig. 1.27).

Figure 1.29 Lateral radiograph of the head of a young foal showing bilateral displaced fracture (arrow) of the mandible with bone fragments (circle).

Figure 1.30 Dorsoventral radiograph of the head clearly showing displacement of the fracture (arrows) and bone fragmentation (same foal as Fig. 1.26).

Diagnosis and treatment

• Usually obvious but radiographs are often required to determine the extent of the fracture and relationship to adjoining structures.

• Treatment is normally surgical; the method chosen is determined by the type of fracture involved. Antibiotic treatment is usually required as the mouth has a high bacterial load and there is usually contamination of the fracture site. In some cases of maxillary fractures secondary infections of the sinuses may occur and require separate treatment.

Dental disease

Dental disease is grouped into four basic types:

1) Abnormal wear patterns

2) Periodontal disease

3) Dental caries

4) Infection of the dental pulp.

All of these are interrelated, and horses with one type will also have, to varying degrees, the other types of disease.

Abnormal wear patterns (Figs. 1.31–1.38)

The horse is anisognathic, meaning that the bottom jaw is narrower (by about 25%) than the top jaw. The molar tables are sloped at a 10–15° angle. Lateral excursion of the jaw during mastication favors occlusal wear of the buccal aspect of the lower arcades and lingual aspects of the upper molar arcades.

Figure 1.31 Computed tomographic image of a section of equine skull at the level of the first molars. Dorsal is at the top. Note that because the upper molars are offset laterally from the lower molars, the molar tables are sloped at a 10-15 degree angle from dorsal lingual to buccal ventral. Reproduced from Smith BP. *Large Animal Internal Medicine*, 4th edn. (2009), with permission from Elsevier.

Figure 1.32 Wave mouth: middle-aged mare with long-term weight loss and slow mastication. Noted to have temporomandibular joint disease. This was assumed to be responsible for the abnormal wear pattern but the rostral hook was a longer-term cornformational problem.

Figure 1.33 Step mouth: teenaged pony presented with a long history of quidding and salivation. The pony also had diarrhea with prominent long fiber fecal content. Euthanized because of the severity of the dental disease and the chronic sinusitis from alveolar infections extending to the caudal maxillary sinuses of both sides.

Figure 1.34 Shear mouth: 10-year-old Shire horse gelding with a long-term history of slow eating and weight loss. On clinical examination it was clear that he had restrictive mandibular movement: no lateral movement was possible. The horse had severe temporomandibular joint arthritis and it was likely that this had resulted in painful lateral movement over some years. The overgrowth was several cm in length over upper and lower cheek teeth.

Figure 1.35 Overgrown cheek teeth. This skull was obtained from an equine slaughter house and was prepared because of the severe dental abnormalities.

Figure 1.36 Mild (early) crib biter. Prominent wear on the rostral margin of the upper central incisors.

Figure 1.37 Severe crib biter with excessive wear of the incisors.

Figure 1.38 Wall raker. This horse developed an abnormal habit of grinding his teeth on the concrete edge of the stable wall resulting in an abnormal wear pattern.

The extent of lateral excursion of the mandible during normal mastication is affected by the length of stem or roughage in the horse’s diet. Horses on pasture or hay have a wide area of mandibular excursion, whereas horses eating large amounts of concentrates have a more limited range of excursion with incomplete wear of the molar surface, predisposing the arcades to development of sharp edges or more pathological wear patterns.

Almost every normal horse, at some stage of its life, develops sharp enamel points along the lingual edges of the lower arcades and the buccal edges of the upper arcades. These enamel edges may result in buccal (or lingual, where the edges are sharp on the lower teeth) erosions and ulceration, salivation and even a reluctance to eat.

Abnormal wear patterns of the incisor and molar teeth may also be due to traumatic displacement or pathological softening of enamel. Painful or physical reasons for alteration of normal occlusal movements including oral ulceration, dental pain and/or abscesses, temporomandibular arthropathy, fractures of the mandible or soft tissue lesions, result in corresponding variations of wear pattern. Disorders inducing significant changes in the physical shape of the teeth have to be longstanding given the normal rate of wear of the cheek teeth being only approximately 3–5 mm per year.

Very old horses commonly have smooth occlusal surfaces even when they have no history of abrasive diets. In some cases the table of the molar teeth is completely smooth and concave and has almost no enamel ridges. This severely limits the effective mastication of fibrous food and has debilitating effects.

• Wave mouth. Localized differences in the density of the occlusal surfaces of the molar teeth may also have marked effects upon the occlusal efficiency and the development of abnormal wear patterns. Alternate hard and soft areas in the structure of the cheek teeth or, more commonly, stereotyped chewing behavior, may result in the development of wave mouth in which either a series of waves develops on the occlusal surface or individual teeth wear faster or slower than their neighbors giving a much coarser irregularity of occlusal surfaces.

• Step mouth. In other cases, there may be one (or more) tooth which, often for inexplicable reasons, wears excessively. This results in gross variations in the height of the teeth (step mouth) and necessarily limits the occlusal efficiency. The full extent of the condition may only be apparent from lateral radiographs when gross variations in height and pyramid deformities of the crowns of the molar teeth may be present.

• Shear/scissor mouth (see p. 3). While the loss of lateral grinding movement of the molars, for any reason, will initially induce sharp buccal margins on the upper teeth, this may progress into a severe and debilitating parvinathism (shear/scissor mouth), in which lateral movement, which is essential for normal chewing, is prevented. These unfortunate horses are often noted to have an abnormally narrow lower jaw but, while under these circumstances it is regarded as a developmental deformity, the same dental deformity may develop as a consequence of a sensitive molar tooth (or teeth) in the opposite arcade or from pain associated with the temporomandibular joints. This results in the upper molars becoming bevelled from the inside outwards with the lower molars worn in the opposite fashion. This deformity prevents further lateral movement of the teeth and seriously interferes with mastication. Under these circumstances extensive overgrowth or bizarre dental deformities may be encountered. This may itself induce secondary temporomandibular arthropathy as the animal attempts to chew with the sides of the molar teeth. It is, in most cases, impossible to identify whether the occlusal problem arose first and caused secondary joint degeneration, or whether the occlusal deformity is the result of abnormal jaw movement created by joint inflammation and degeneration. It represents one of the most serious deformities of the horse’s mouth and carries a poor prognosis.

• Sand mouth. In areas where horses have, of necessity, to eat and chew large amounts of sharp sand or other abrasive substances the occlusal surface of the cheek teeth may become completely smooth and therefore ineffective as grinders of food. Failure to masticate efficiently results initially in difficulty with swallowing and slow eating. Weight loss, as a result of ineffective digestion, is commonly present. Under similar circumstances, because these horses are usually grazing very short pasture or having to find food in soil or sand the incisors may become severely worn down.

• Overgrown cheek teeth. Overgrown cheek teeth may arise from the absence of the opposite occlusal tooth. Such defects may follow either from normal old-age shedding, or from failure of normal eruption or, more often, from surgical extraction of one or more of the cheek teeth. Molar teeth with no occlusal pressure are likely to grow faster than normal teeth and in addition have no occlusal abrasion. The resultant loss of normal control of dental growth creates abnormal wear patterns which are usually visible as gross overgrowth. Pyramidal peaks on the tooth opposite the gap are common where the gap created by a missing cheek tooth is narrowed by angulation of the adjacent teeth but leaving a relatively small area of non-occlusion.

Normal shedding of molar teeth usually begins when the horse is over 25–30 years of age and the first molar tooth, being the oldest, would be expected to be lost first. Under these circumstances the scope for subsequent overgrowth in the opposite occlusal teeth is minimal and dental hooks, overgrowths or pyramids are usually of marginal significance.

The loss of any of the cheek teeth, except the first and last leaves a gap in the dental arcade, which may narrow significantly (and sometimes completely) with time as the adjacent teeth angle inwards. Although these defects may be less in size than the more dramatic overgrowth encountered in either the first or the last tooth the secondary consequences may be more significant and arise more quickly. Thus, limited overgrowth may result in gross discomfort and inability to chew effectively within months of the onset. Alternatively, overgrowth may continue unabated for years before any clinical effects may become apparent. Overgrowth impinges upon the opposing gingiva causing ulceration, necrosis and possibly infection of the ulcerated area.

• Excessive incisor wear. While severe incisor wear is sometimes encountered where grazing is short and large amounts of sand or other abrasive substances are ingested, the wear pattern of the incisors, in particular, may be influenced by behavioral factors. Crib-biting is a common vice (neurosis) developed by horses showing a characteristic wear pattern on the rostral margin of the upper incisor teeth which is variable in extent according to the severity and duration of the vice, and to some extent upon the structural character of the teeth. The earliest indications of the vice may be gained from close examination of the rostral margin of the upper central incisors where a worn edge will be detected. The persistence and severity of the effort involved in cribbing is often enough, even in young horses, to cause severe wear of the central incisors, often down to gingival level.

Habitual grinding of the teeth on metal rails or concrete walls results in wear patterns involving, usually, the corner and lateral incisors. The pattern is usually such that it is hard to visualize any normal behavior pattern which could produce them. Usually only one side is affected.

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