Gastrointestinal system

Gastrointestinal system

Part 1: The mouth

Developmental disorders

Cleft palate (Figs. 1.1–1.3)

Cleft palate is an uncommon abnormality. When seen it most frequently involves the caudal aspect of the soft palate; however the hard palate, lips and external nares may also be affected. The degree to which it is heritable is not well defined. Foals with large clefts may show dramatic nasal regurgitation of milk during nursing. In some cases, particularly those with relatively small clefts or clefts in the soft palate, nasal return of milk becomes obvious only after feeding and may be relatively minor in amount. Small clefts may not always be easily visible or produce significant nasal return of food and, occasionally, some are only detected after some years, when nasal reflux of grass and more solid food material may be present. Consequent rhinitis and nasal discharges may not be immediately identifiable as resulting from a cleft palate. Occasionally the cleft is sufficiently small to produce no detectable evidence and these are sometimes identified incidentally during clinical or post-mortem examinations. Large palatal defects in young foals have profound effects including failure to ingest adequate amounts of colostrum, starvation and inhalation pneumonia.

Parrot mouth (brachygnathia) (Figs. 1.5–1.7)

Parrot mouth is a common congenital abnormality characterized by disparity of the lengths of the mandible and maxilla. The mandible is shorter than the maxilla and there is no occlusal contact between the upper and lower incisors. Males are more commonly affected. In the milder cases, the full extent of the discrepancy may not be obvious at birth, becoming more apparent as the permanent incisors erupt and grow into their normal occlusal positions. The failure of significant occlusion results in an increasing overgrowth of the upper incisors and impingement of the mandibular teeth into the soft tissues of the hard palate. Individuals with lesser degrees of inferior brachygnathia may be less affected but the lingual edges of the lower incisors may become sharp and lacerate the gums and hard palate. More commonly the lower incisors tend to prolong the line of the lower jaw and the labial margins of the upper incisors become long and sharp and may lacerate the lower lip. Simultaneously there is usually a discrepancy in the length of the lower molar arcade and a rostral hook will often develop on the first upper cheek teeth with an equivalent caudal hook on the last lower cheek teeth.

Sow mouth (prognathia) (Figs. 1.8 & 1.9)

Monkey, sow or hog mouth is more rarely encountered. Associated with it are projections (hooks) on the rostral edge of the first lower cheek teeth and the caudal edge of the last upper cheek teeth. Even extensive overgrowth of either superior or inferior brachygnathism, where little or no effective occlusion is present, appears to cause little hindrance to prehension in many cases, with the cosmetic and aesthetic effects being of concern in the early years of life. Effects upon growth and condition are therefore unusual, provided that suitable forage is available. Short grazing however, makes prehension very unrewarding for the horse, and weight loss and poor growth are to be expected. Clearly, once the overgrowth becomes severe, difficulties with prehension are more likely.

Missing and malerupted teeth (Figs. 1.10–1.18)

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


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.

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.

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.

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.

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.

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.

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.

Dental disease

Dental disease is grouped into four basic types:

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.

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.

Feb 27, 2017 | Posted by in EQUINE MEDICINE | Comments Off on Gastrointestinal system
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