Appetite and the oral cavity

Disturbances of Eating and Drinking

Appetite in animals is generally assessed by reference to food consumption in healthy indi­vidual animals in identical circumstances. Varia­tions in appetite include increase, decrease, or abnormality (depravity). When an animal's appetite is impaired, the cause may be unsuitable food, lack of desire for food or inability to prehend or masticate the food. In all cases of reduced appetite, it is advisable to determine that the food provided is of reasonable quality.

Partial reduction in appetite (inappetence) is associated with many types of gastric disease because of reduced hunger, but all toxaemic and febrile diseases are frequently accompanied by decreased food intake (anophagia). In the more acute forms of these conditions, appetite is com­pletely absent (anorexia). Dietary deficiencies, e.g. cobalt in ruminants and thiamine in carni­vores, are other important causes of inappetence. Hunger sensations are sometimes submerged by others such as fear, excitement or severe pain; this is exemplified by lack of appetite in some dogs when put into strange kennels, or when range cattle or horses are housed.

Increased appetite (hyperorexia) manifest by increased food consumption (polyphagia) is observed in pancreatic deficiency, chronic gas­tritis, certain forms of intestinal parasitism, functional diarrhoea, certain metabolic diseases, such as diabetes mellitus, and as a physiological expression of hunger following a period of star­vation. Because there is either a defect in absorp­tion and/or utilization of food or excessive metabolic demand, an animal may be emaciated in spite of excessive intake of food. Increased consumption of water (polydipsia) occurs when there is excessive loss of body fluid (dehydration) which may be caused by persistent vomiting, diabetes insipidus, diabetes mellitus, chronic interstitial nephritis, and in many cases of catarrhal gastritis. Desire for water in increased amounts is manifested, as a temporary feature only, during the initial stages of many febrile diseases.

A depraved (perverted, unnatural) appetite exists when an animal ingests substances that do not normally form part of the diet of the species concerned. It is manifest by varying degrees of pica or allotriophagia. Abnormal appetite occurs in some nutritional deficiencies, more particu­larly of phosphorus (rickets, aphosphorosis), sodium chloride and less obviously cobalt, as well as when there is inadequate protein, bulk or fibre in the diet. Other causes include chronic peritonitis, chronic gastritis, parasitic enteritis (strongylosis) in horses and certain diseases (rabies, ketosis) which disturb the function of the nervous system. In some animals an unnatural appetite becomes habitual although the cause of its appearance has ceased to operate.

According to the preference exhibited by the affected animal an abnormal appetite can be classified: eating the faeces (seen most frequently in puppies and young horses) is coprophagia; chewing bones (occurs in cattle affected with aphosphorosis) is osteophagia; eating the young, infantophagia, is an important form of canni­balism sometimes observed in farrowing sows; eating earth is allotriophagia. Other types of pica include excessive coat-licking because of salt hunger, wool eating in sheep, eating decomposing animal carcasses, litter eating, etc.

Even when the appetite is normal, the ingestion of food may be rendered difficult, or impos­sible, by reason of disease in one, or other of the associated mouth structures. Depending on whether the abnormality affects the lips, teeth, tongue, oral mucous membrane, soft palate, tonsils, pharynx or oesophagus, there will be difficulty in prehension, mastication or deglu­tition. An abnormal manner of feeding may also result from even mild disturbance of conscious­ness, e.g. in chronic acquired hydrocephalus in the horse. Horses and cattle recovering from chloral hydrate narcosis sometimes exhibit a compulsion to consume almost any fibrous material within reach. In order that the state of the appetite and the feeding behaviour should be correctly determined, it is necessary to under­take prolonged observation of the animal while it is eating and drinking.

Prehension and Mastication

The method of prehending solid food varies according to the species, but in all domestic animals the lips, teeth and tongue are the prin­cipal organs concerned. The sheep, goat and horse, when feeding from a manger, seize the food with the lips and incisor teeth; sheep have a cleft upper lip which permits close grazing. When grazing, the horse draws back the mobile lips and severs the grass with the incisor teeth. The tongue is the main prehensile organ in the ox, being well constructed for this purpose by reason of its length, mobility and rough surface. In the pig the food is taken into the mouth by the lower lip and teeth. The dog and cat take up solid food with the teeth, but they often use the forelimb to hold it. The horse, ox, sheep and pig draw liquid into the mouth by suction. This is accomplished by almost closing the mouth, which is submerged into the fluid, and producing negative pressure by a pumplike movement of the tongue. The dog and cat con­vey fluids to the mouth by using the tongue as a ladle. In very young animals sucking is achieved by creating negative pressure in the mouth by suction with the cheeks and tongue. The manner of food and water prehension is abnormal in painful conditions or malformations (Fig. 95) of the lips, tongue and incisor teeth, e.g. stomatitis, glossitis, gingivitis, penetrating or occlusive foreign bodies, fluorosis, malocclusion and broken, misaligned and deficient incisor teeth. Unilateral or bilateral paralysis of the lips, tongue or muscles of mastication, dislocation of the temporomandibular joint and fracture of the mandible or maxilla also interfere with prehen­sion. Animals with torticollis, caused by dys­function of the vestibular nerve or middle ear disease, have difficulty in eating and drinking because of their inability to orientate the mouth correctly. In these conditions the appetite for food is obviously retained.

Mastication consists of chewing with the teeth, assisted by movements of the tongue and cheeks which keep the food between the teeth. The motive power originates mainly in the mas-seter muscles. With sufficient trituration the food is broken down so that it can be formed into a bolus preparatory to being swallowed. Mastica­tion usually takes place involuntarily although, basically it is a voluntary act under the control of centres in the brain. In all species, mastication consists mainly of grinding the food between the molar teeth. The degree of grinding varies between carnivorous and herbivorous species and the molar teeth are structured to meet the particular requirements. An important concomi­tant of mastication is the production of saliva and its admixture with the triturated food. In herbivores the upper jaw is wider than the lower and there is considerable lateral move­ment of the jaws so that mastication occurs only on one side at a time. In carnivores and omnivores the jaw movements, during mastication, take place in a vertical plane so producing a shearing effect on food.

Mastication is impaired, the movements of the jaw being performed slowly and with ap­parent caution, in painful diseases of the buccal mucous membrane (stomatitis, glossitis, gingi­vitis, etc.), the molar teeth, the muscles of masti­cation (eosinophilic myositis, actinobacillosis, paralysis), the temporomandibular joint (fracture) and the mandible (actinomycosis). In her­bivorous animals, diseases causing pain during mastication are usually characterized by 'quid-ding', i.e. dropping of semi-masticated food from the mouth, and the presence of undigested food particles in the faeces.

Deglutition

Deglutition, or swallowing, is the action of transferring food from the mouth through the pharynx and oesophagus to the stomach. The normal act of swallowing occurs in three stages, the first of which is voluntary; the other two are reflexly controlled and are, therefore, involun­tary. The deglutition centre is composed of nerve cells situated in the floor of the fourth ventricle.

Following mastication and lubrication with saliva, the food is compressed into a bolus which is initially placed in the midline between the tongue and hard palate and then thrown back between the pillars of the fauces to reach the posterior wall of the pharynx. At the commence­ment of the second phase, the soft palate is elevated to close off the posterior nares, and the epiglottis closes off the larynx (see Fig. 90, p. 88). Respiration is momentarily inhibited and, with approximation of the posterior pillars of the fauces, the mouth cavity is shut off. Then the pharyngeal muscles contract so that the bolus is propelled into the oesophagus which simulta­neously dilates. The third phase is concerned with propelling the bolus along the oesophagus. The structures of significance in deglutition in­clude the tongue, floor of the mouth, hard and soft palate, fauces, laryngeal muscles and oeso­phagus. Afferent receptors at these sites transmit impulses along the maxillary division of the trigeminal nerve, the glossophrayngeal nerve, and the superior laryngeal branch of the vagus nerve to the deglutition centre. Efferent impulses are transmitted via the trigeminal, glossopharyngeal, vagus, hypoglossal and spinal accessory nerves.

Disturbances of swallowing are attributable to structural or functional defects in any of the component structures essential for the act and include painful inflammatory swelling of the pharyngeal tissues, oesophageal dilatation and diverticulum, paralysis or cleft state of the soft palate and paralysis or dilatation of the oeso­phagus. In addition physical causes, such as partial or complete obstruction of the pharynx or oesophagus by a foreign body, tumour or enlarged lymph node, may interfere with swal­lowing, or even prevent it altogether.

In the horse, more rarely in the ox, various movements of the lips, tongue or jaws may be seen that have no immediate connection with eating or drinking, and which may be regarded as vices or bad habits. These include smacking the lips, blowing air out of the mouth between the closed lips, playfully licking objects with the tongue, crib-biting and wind-sucking.

In the dog affected with extensive pneumonia or advanced congestive heart failure, severe dysp­noea causes oral breathing with obvious move­ments of the lips.

Oral Cavity

All the structures comprising the mouth can be subjected to clinical examination. In order to examine the labial mucous membrane the upper and lower lips are gently grasped, lifted and turned back. In the larger species of animals, if the interior of the mouth is to be examined only briefly, the free portion of the tongue is grasped inside the mouth by inserting one hand at the interdental space, the hand is then rotated so that the thumb is uppermost (Fig. 96); alternatively the tongue is grasped in the same man­ner and then drawn out of one side of the mouth so that part of the attached portion is pulled between the tables of the opposing molar teeth (Fig. 97), and so prevents the animal from closing the mouth. This, in conjunction with pulling the cheek on the other side outwards with the fingers at the oral commissure, permits visual examination of a part of one side of the oral cavity; in order to complete the examination it is necessary to repeat the manoeuvre by withdraw­ing the tongue from the opposite side of the mouth. In the dog, the upper jaw is grasped with one hand and the lower jaw with the other, the animal's lips being pushed inwards over the crowns of the molar teeth on both sides with the thumb and middle fingers while the forefingers rest against the canine teeth (Fig. 98). If the dog is inclined to bite, the mouth is opened by means of tapes placed round both jaws just behind the canine teeth. One assistant holds the tapes, one in each hand, and by pulling them apart opens the mouth, while another assistant holds both forefeet (Fig. 99). The clinician is thus able to carry out a detailed examination of all parts of the mouth and a substantial area of the pharynx. In a quiet cat, the mouth can be opened by placing the dorsal surface of the little finger against the nape of the neck, grasping the upper jaw between the thumb and forefinger, with the eyes covered by the palm of the hand, and bending the animal's head upwards, using the little finger as a pivot (Figs 100, 101). The lower jaw is then depressed by applying downward pressure with the finger of the free hand. It is essential for an assistant to hold both forefeet. When the deeper parts of the oral cavity require thorough examination, in all but the smaller species, it is advisable to use a gag (Figs 102-105). If the animal resents handling, because of pain­ful conditions of the mouth, or fear, a suitable sedative or narcotic drug should be adminis­tered prior to undertaking the examination. This is made by means of inspection and palpation of selected tissues. In the horse and ox, and in all circumstances when the light is poor, an electric torch is required.

Abnormalities of the buccal mucous membrane include changes in colour of a focal, or more general, distribution such as jaundice, cyanosis and the pallor of anaemia and those associated with alteration in vascularity (hyperaemia, hae­morrhages, etc.) which are clinical features of diseases with an inflammatory or allergic origin.

Inflammation of the oral mucosa may be more severe in one part than in another (stoma­titis, glossitis, gingivitis) and may be caused by infectious, chemical or physical agents. The in­fective agents are bacterial, viral and fungal in character. Bacterial stomatitis is manifested by necrosis and ulceration as in fusospirochaetal infection in the dog caused by Fusiformis fusiformis and Borrelia vincentii; oral necrobacillosis (calf diphtheria) caused by Sphaerophorus necrophorus; ulcerative granuloma of pigs caused by Borrelia suilla which may spread from the lips and cheeks to involve the labial mucosa; actinobacillosis {Actinobacillus lignieresi) in the ox, which initially involves the tongue causing ulceration in many cases, may spread to the lips, and the gums may be involved in actinomycosis.

The oral lesions in viral stomatitis vary in character and may be vesicular, ulcerative or proliferative. In foot-and-mouth disease, vesicu­lar exanthema and vesicular stomatitis the initial stage is vesicular but the lesions become ulcera­tive within a few days. Malignant catarrhal fever, mucosal disease, rinderpest, bluetongue, infectious ulcerative stomatitis and diseases such as contagious pustular dermatitis, sheep pox and ulcerative dermatosis, when the lesions extend from the lips into the oral cavity, are manifested by erosion and secondary ulceration of the oral mucosa. Ulcerative glossitis involving the tip of the tongue is a feature of some cases of feline panleucopenia. Necrotic lesions con­fined to the anterior part of the tongue have been reported in feedlot steers in the USA; no specific cause has been identified. Buccal papillomatosis, seen in the dog and very occasionally in other species, and proliferative stomatitis and papular stomatitis of cattle are associated with pro­liferative lesions.

Vesicular, erosive or ulcerative lesions tend to be modified by secondary bacterial invasion so that they become suppurative and ulcerative, resembling the lesions of bacterial stomatitis. In a small proportion of such cases an extensive cellulitis, involving the facial tissues may develop. In certain diseases, including blacktongue and interstitial nephritis in the dog, stomatitis de­velops as a result of toxaemia and general debility permitting abnormal bacterial activity on the oral mucosa. Ulceration of the gingival mucosa occurs in old dogs and cats, most commonly those with dental calculus.

Mycotic stomatitis, a somewhat rare affection of the oral mucosa, is mainly caused by Monilia spp.; in dogs it is usually caused by Candida albicans which is responsible for diffuse, patchy areas covered by a whitish film. Chemical agents which may cause stomatitis are corrosive acids or alkalis, counter-irritants improperly applied so that the animal licks them off the surface of the body (mercuric iodide, cantharides) or chloral hydrate administered orally in strong concentrations. Toxic irritants contained in plants such as hemlock, buttercup, mustard, spurge, water hemlock and water dropwort may produce somewhat similar effects.

Physical agents that may be associated with the development of stomatitis include sharp awns, especially from certain cereals, thorns or spines on plants, sharp-edged or pointed pieces of bone or other foreign objects, particularly in dogs and cats, maloccluded teeth, frozen or very hot food or water and trauma during the administering of medicine.

Salivation

Saliva is the mixed secretion of the three main paired glands, the parotid, submaxillary and sublingual, together with that of numerous small glands in the oral mucosa. The glands are serous, mucous or mixed in character according to their structure and the nature of the secretion they produce. The cells of the serous glands contain zymogen granules which are thought to be the precursors of enzyme. The parotid gland in most mammals is serous. The submaxillary gland is mixed in ungulates, the dog and the cat; it is serous in rodents. The sublingual gland is serous in domestic animals.

Saliva is produced spontaneously and in response to neural or hormonal stimuli. Neural stimulation, which is of most significance in animals, is through the medium of efferent innervation from both sympathetic and parasympathetic components of the autonomic nervous system. Sympathetic preganglionic fibres origi­nate from the spinal cord in the first few thoracic nerves and pass, via the cervical chain, to the superior cervical ganglion where they form synapses with nerve cells, the fibres of which are distributed to the blood vessels and cells of the glands. Parasympathetic preganglionic fibres traverse the glossopharyngeal and then the trigeminal nerve to the parotid gland; those to the submaxillary and sublingual glands are contained in the facial nerve and reach their destination in the chorda tympani. Afferent impulses originate in the mouth, pharynx and olfactory area and are conveyed via the trigeminal and glosso­pharyngeal nerves to the salivary centres in the medulla.

In normal animals the secretion of saliva is abundant, particularly in ruminant species, in which the parotid salivary gland secretes spon­taneously. Reduction in the quantity of saliva secreted, resulting in a dry condition of the oral mucosa, is seen in very acute febrile conditions, in diseases accompanied by excessive loss of body fluid with severe dehydration (enteritis, nephritis, diabetes insipidus, etc.) and in poison­ing with belladonna alkaloids. Dryness of the mouth occurs in animals that are breathing through the mouth, but here the dryness is caused by excessive evaporation rather than by deficient secretion.

The quantity of saliva produced is increased in painful inflammatory conditions of the mucous membrane of the mouth, tongue, pharynx or oesophagus. Foot-and-mouth disease, mucosal disease, rinderpest and actinobacillosis when the tongue is involved are some of the important causes. Foreign bodies in the mouth, more particularly when they penetrate the soft tissues, because they cause excessive chewing movements and interfere with swallowing, are a common cause of excessive salivation in the dog and cat and occasionally have a similar effect in the ox and horse. In obstruction or paralysis of the oesophagus, particularly in the ox, the flow of saliva from the mouth is an indication of the inability of the animal to swallow, not of exces­sive secretion. Ptyalism is a clinical feature of certain forms of mineral intoxication, e.g. chro­nic mercury and acute lead poisoning in young cattle. In the former it is due to a mild stomatitis and in the latter because the encephalopathy which develops involves the salivary nucleus.

Increased salivation may be revealed by observing that the animal makes frequent swal­lowing movements not associated with feeding (empty swallowing). With experience it is pos­sible to recognize an increased flow of saliva by inspecting the tissues contiguous to the openings of the salivary ducts (Stensen's duct from the parotid gland opens onto the labial mucous membrane adjacent to the third upper molar tooth; Wharton's duct from the submaxillary gland opens, in association with the duct from the sublingual gland, on either side of the fraenum linguae.) Particularly abundant secretion of saliva is easily recognized: the saliva hangs from the mouth in strands and movements of the lower jaw and tongue, when they occur, are accompanied by smacking or snapping sounds (foot-and-mouth disease in cattle or sheep). Pus or blood mixed with saliva indicates purulent or haemorrhagic inflammation or injury, affecting the mucous membrane of the oral cavity or pharynx. Admixture with air causes the saliva to become frothy, as in dyspnoea, epileptiform convulsions, etc.

Owing to its exposed situation on the lateral aspect of the ramus, the duct (Stensen's) of the parotid salivary gland may suffer an injury resulting \n a salivary fistula. The condition is recognized by its anatomical position, and by the fact that the escape of fluid is greatly increased when the animal is eating.

The odour of the mouth in healthy animals is not usually unpleasant, except possibly in cattle when they are fed on a diet containing silage or roots. Unpleasant odours may arise from local or remote local organs, although in many cases they are associated with general debilitation arising from acute or chronic diseases of the ali­mentary tract, or specific diseases causing ulcera­tion of the mucous membrane of the mouth and pharynx. In the horse, gangrene of the lung causes the odour of the mouth and the expired air to be particularly offensive. Calves with necrotic stomatitis have a sour, rather putrid odour. In the dog, acute and advanced chronic nephritis, and deposits of tartar on the teeth, are recognized causes of unpleasant odours.

Tongue

Changes similar to those affecting the other parts of the oral mucous membrane also occur on the surface of the tongue. A covering of white, somewhat viscous, mucoid material, readily scraped off, is found on the dorsal surface of the tongue, more particularly towards the posterior part, when the intake of food has ceased, as in various febrile and alimentary diseases, etc. This 'fur' is formed when, as the result of the intake of food being diminished or absent, desquamation of the lingual epithelium is in abeyance. When the tongue is furred, the mucosa is dry, and there is an unpleasant odour. The tongue may also show changes in colour (e.g. in the dog, severe toxaemia and uraemia may cause the development of a copper-red colour), oedema, injuries, hardening with en­largement and not uncommonly ulceration (Actinobacillus lignieresi infection in the ox), or reduction in mobility, and sometimes size, as a result of paralysis. The underside of the tongue, close to the fraenum linguae, may be affected with a ranula (a salivary or mucous gland retention cyst).

Teeth

Tartar (dental calculus) often occurs on the teeth, more especially in dogs and cats, in the form of a hard yellowish-brown deposit, which usually commences to accumulate near the alveo­lar margin and causes the gingival tissues to recede, thus leading to alveolar periostitis and periodontitis. Heavy deposits of tartar are in­jurious to the labial mucosa as well. Note should be taken of missing, damaged, excessively worn, displaced or defective teeth, dental caries and foreign bodies wedged between the teeth. Teeth may be missing because of very old age, or delayed eruption, which in sheep signifies mineral deficiency. Missing incisor teeth may also arise from trauma or, as in sheep, from grazing on poor, stony soil, or infection of the alveoli with Actinobacillus lignieresi, in which case a variable number will be absent.

Chronic fluorosis in cattle and sheep causes mottling and pitting of the enamel of the incisor teeth, with excessively and unevenly worn molar teeth. These defects only appear in those perma­nent teeth which developed while the animal was on a diet containing excessive amounts of fluorine. In dogs which were affected with dis­temper prior to the eruption of the permanent teeth, it will be observed that the enamel layer is present, to a variable degree, only on the crowns of the teeth, the exposed dentine giving the teeth a dirty, yellowish-brown colour. The deciduous teeth have a bluish, translucent appear­ance in young animals, especially puppies, af­fected with rickets. Transillumination of the mouth will outline the pulp cavity of the teeth in such cases. In adolescent herbivorous animals, retention of some of the deciduous molar teeth on the crowns of the erupting permanent teeth in the lower jaw is a possible cause of impaired mastication.

In a horse that is addicted to crib-biting, the anterior edge of the teeth in both incisor arcades is worn down (Fig. 106). It is particularly important to examine the molar teeth for evidence of uneven wear, because sharp edges on these teeth can make mastication painful, cause ulceration of the labial and lingual mucosae, which may permit bacterial invasion of the deeper tissues, and lead to loss of condition because of impaired appetite and digestion. In the horse, it is very commonly found that both the labial edge of the tables of the upper and the lingual edge of the tables of the lower molar teeth are sharp and pointed. A more severe form of this condition ('shear-mouth', scissors-mouth), possibly of ge­netic origin, occurs in horses and sheep and is caused by undue narrowing of the mandible. Other abnormalities of wear which occur in the molar teeth of the horse include step mouth (step-formed table surfaces), wave mouth (un­dulating table surfaces) and smooth mouth (smooth table surfaces); they are characteristi­cally seen in aged horses.

Overgrowth of the incisor teeth in rodent species is readily recognized because the affected animal is unable to close the mouth properly, so that prehension is impaired. An important form of this condition is malocclusion in chinchilla. Anatomical defects, which cause malapposition of the incisor teeth, include prognathia (under­shot jaw) in Hereford, Shorthorn, Jersey, Holstein and Ayrshire calves, and brachygnathia (parrot mouth) of Merino and Rambouillet lambs; both are congenital and hereditary in origin and are readily recognized. Displacement of the lower premolar teeth, in conjunction with shortening and narrowing of the mandible, occurs in calves as an expression of an hereditary defect. In advanced cases of actinomycosis and neoplasia of the jaw bones, tooth displacement is a usual feature.

Supernumerary teeth include wolf teeth in the horse, the more common form of excess incisors (usually a double row is present) and the more rare excess form involving the molar teeth. A dentigerous cyst, which is unique to the horse, and is thought to represent an aberrant dental follicle, is usually located in the vicinity of the mastoid process of the petrous temporal bone, at the base of the ear; the malformation may also occur on the frontal bone or in the paranasal sinuses. The condition becomes obvious, in many cases, during adolescence when normal dental changes are most active.

Tonsils

The tonsils are situated in the fauces, on either side of the root of the tongue; they show ana­tomical variation between species. In the horse there is a series of masses of lymphoid tissue and mucous glands which extend backwards for about 10 cm; the bovine tonsil is bean-shaped and about 1 cm long; in the pig the main mass of tonsillar tissue is situated on each side of the midline of the oral surface of the soft palate, with additional tonsillar tissue situated in the fauces lateral to the root of the tongue; the tonsils of the dog are somewhat similar to those of man. They are about 2-5 cm long, reddish in colour and almost completely concealed by folds of mucous membrane. The tonsils are most easily observed in the dog by opening the mouth and then depressing the tongue at its posterior part. Examination of the tonsils should never be omitted in the dog. The size and colour and the presence of abscesses (often pinhead in size and of varying number), haemorrhages, neoplasia, foreign bodies, etc., are noted. In other species the tonsillar tissues are difficult to inspect on account of their position, relatively small size and anatomical relationships.