Aid to digestion
The digestive system of the horse is of crucial importance to the equine practitioner. Health and optimum function of this system are necessary for almost every aspect of a horse's life. Complaints of diarrhea, weight loss, poor weight gain, lack of performance and colic, among others, often accompany equine digestive disease.
The digestive system begins in the horse's mouth. While attention to the care, maintenance and problems of the teeth has greatly increased, another key part of this system has been largely overlooked.
The equine salivary glands are an important component of the total digestive system in the horse. These three paired glands — the parotid, the mandibular and the sublingual — represent a primary step in the digestive process and are responsible for the production of saliva.Saliva is a viscous fluid that is 99.5 percent water and 0.5 percent salts and organic material — mainly enzymes and mucin.
While saliva is produced almost non-stop in the horse, its flow is greatly increased with the stimulation of chewing or mastication. Saliva production can reach 50ml/minute in adult horses and the composition of this fluid varies with the type of stimulus initiating its secretion. Variations in the sodium, potassium, calcium and bicarbonate concentrations in saliva are noted, depending on the nature of the food being chewed and the amount of saliva flow initiated. Qualities such as degree and type of roughage, Ph, taste and texture all influence saliva production.
Saliva provides lubrication during chewing and helps moisten the mouth and lips. It aids in the process of swallowing and provides a means of rinsing the mouth clean of debris that otherwise might create a culture medium for bacterial growth.
Salivary enzymes also help break down starch into simple sugars (maltose) as the first enzymatic step of the entire digestive process.
The parotid glands are the largest salivary glands, and are found on both sides of the head extending ventrally from the base of the ear and wing of the atlas to the convergence of the maxillary and linguofacial veins and to the caudal border of the mandible.
Each gland is about 20-25cm in length, with an average thickness of 2 cm. The parotid salivary duct begins at the cranioventral angle of the gland and travels rostrally along the medial surface of the mandible. This duct crosses underneath the mandible with the facial vessels and continues along the border of the masseter muscle to empty in the mouth opposite the third upper premolar.
The mandibular salivary glands lie on the inner surface of the lower jaw, extending from the atlantal fossa to the basihyoid bone. The mandibular gland is smaller than the parotid (roughly 19.5 cm by 2.6 cm.) and also has a strong, distinct capsule.
The mandibular duct leaves the gland on the medial surface. It travels over parts of the digastricus and stylo-glossus muscles, entering the mouth at a point referred to as the sublingual caruncle.
The sublingual salivary gland does not have a very distinct capsule, and the sublingual duct is found near the midline below the level of the mucous membranes on the floor of the mouth.
Common diseases, conditions
There are numerous diseases and conditions of these salivary glands and ducts that, although infrequent, should be familiar to equine practitioners.
The most common condition is a foreign-body obstruction of one of the major ducts. Horses present with a large, firm mass palpable over the middle portion of the parotid duct. Because the horse has two glands (right and left), there usually is little swelling or pain or decrease in saliva production following sialothiasis.
Salivary calculi or sialoliths can occur, but F. A. Al-Sobayil and I.M. Ibrahim of the College of Agriculture and Veterinary Medicine at Qassim University in Saudi Arabia feel that these ductal stones (sialolithiaisis) are a fairly rare occurrence, with few documented cases.
The cases of record, however, seem to show that donkeys are more sensitive to stone formation and that stones actually have been found only in the parotid salivary duct.
Analysis of these stones shows them to be multiple layers of calcium carbonate or calcium oxalate that typically develop over months, with the horse's body continually trying to cover the irritating particle much like an oyster creating a pearl.